JAVA FULL NOTES

 

JAVA

Written By

G. Ravi Kumar Sarma

where we use java

Java is a versatile programming language that can be used in a variety of different contexts and for a wide range of applications. Some common areas where Java is used include:

• Web development: Java is a popular choice for developing web applications, both on the server side (using technologies such as Java Servlets and JavaServer Faces) and on the client side (using technologies such as Java applets and JavaFX).

• Enterprise applications: Java is widely used for developing enterprise-level applications, such as business systems and e-commerce platforms. The Java Enterprise Edition (Java EE) provides a set of libraries and components for building such applications.

• Mobile development: Java is the primary language used for developing Android mobile applications. The Java Micro Edition (Java ME) can be used for developing applications for other mobile platforms as well.

• Embedded systems: Java can be used for developing applications for embedded systems, such as routers, televisions, and other devices.

• Desktop applications: Java can be used to develop standalone desktop applications, using technologies such as the JavaFX platform.

• Big data and Machine Learning: Java is used in Big data technologies such as Hadoop and in Machine learning libraries such as Weka, Mahout, and Deeplearning4j.

• Gaming: Java can be used to develop games, both 2D and 3D, using libraries such as libGDX and JavaFX.

Overall Java is used in a wide range of applications in different domains such as finance, healthcare, retail, and many more.

Java language futures:-

Java is a mature programming language that has been in active development for over 25 years. Over the years, the language has evolved to include new features and capabilities, while also maintaining backward compatibility. Some of the key features of the Java language include:

• Object-oriented programming: Java is a class-based, object-oriented language, which means that it uses the concepts of classes and objects to model real-world entities.

• Platform independence: Java programs are compiled to an intermediate format called bytecode, which can be run on any platform that has a Java Virtual Machine (JVM) installed. This allows Java code to be written once and run on a variety of different systems without modification.

• Garbage collection: Java automatically manages memory by periodically cleaning up and freeing resources that are no longer in use through a garbage collector.

• Multi-threading: Java has built-in support for multi-threading, allowing multiple threads of execution to run concurrently within a single program.

• Exception handling: Java provides a mechanism for handling exceptional situations (such as runtime errors) through the use of exceptions.

• Standard libraries: Java comes with a large set of standard libraries that provide a wide range of functionality, such as collections, networking, and I/O.

• Dynamic: Java is a dynamic language, which means that it can adapt to changing requirements and environments, and it supports features such as dynamic class loading and runtime type checking.

• Modularity: Java 9 and above versions introduced the Java Platform Module System (JPMS) which allows for more scalable and maintainable Java applications by specifying dependencies between modules and encapsulating internal implementation details.

• Records: Java 14 and above versions introduce the Records feature which enables developers to create lightweight and immutable data classes with minimal boilerplate. Java language is still evolving and new features are added with every release, the latest version being Java 17 which was released in September 2021.

what are job roles in the Java language?

Java is a widely-used programming language, and as such, there are many different job roles that involve working with it. Some examples of job roles that utilize Java include:

• Java Developer: This is a general term for a software developer who specializes in using the Java programming language to build and maintain various types of applications.

• Java Software Engineer: Similar to a Java developer, but with more focus on software engineering principles, design patterns, and best practices.

• Java Web Developer: This role focuses on the development of web-based applications using Java-based frameworks such as Spring, Struts, and JavaServer Faces.

• Java Enterprise Developer: This role focuses on the development of enterprise-level applications using Java Enterprise Edition (Java EE) technologies such as Java Servlets, JavaServer Faces, and Enterprise JavaBeans.

• Android Developer: This role focuses on the development of mobile applications for the Android operating system using the Java programming language and the Android SDK.

• Big Data Engineer or Data Engineer: This role involves using Java to write code for big data systems such as Hadoop, Hive, and Spark.

• Machine Learning Engineer: This role involves using Java to develop machine learning models, and implementing them in production using libraries such as Weka, Mahout, and Deeplearning4j.

• Java Architect: This role is focused on the design and implementation of software architecture, and in particular, the design of Java-based systems and applications.

• Java Technical Lead: This role involves leading a team of Java developers, and providing guidance and mentorship to help them deliver high-quality software.

What are the specifications in Java language?

In Java, specifications refer to the formal documentation that describes the functionality and behavior of a particular feature or component of the language. These specifications are typically defined by the Java Community Process (JCP), an organization that manages the evolution of Java technology through the contributions of industry experts and developers.

Some examples of Java specifications include:

• Java Language Specification: This document defines the syntax and semantics of the Java programming language. It is intended for use by compiler writers, implementers of the Java Virtual Machine, and other developers.

• Java Virtual Machine Specification: This document describes the Java Virtual Machine, the abstract computer on which all Java programs run. It defines the instruction set, memory model, and other features of the JVM.

• Java SE Platform Specification: This document describes the Java SE (Standard Edition) Platform, which includes the Java language and the JVM, as well as a set of core libraries and other components.

• Java Servlet Specification: This document describes the Java Servlet API, which is used to create web applications.

• JavaServer Faces Specification: This document describes the JavaServer Faces (JSF) API, which is used to create web applications with a component-based architecture.

All of these specifications are published and maintained by Oracle (which owns the Java trademark) and are freely available for developers to use and implement.

Where to download java

Java can be downloaded from the official website of Oracle, the company that owns the Java trademark.

You can download the Java Development Kit (JDK), which includes the Java Runtime Environment (JRE) and other tools needed for developing and running Java applications, from the following link:

https://www.oracle.com/java/technologies/javase-downloads.html

You can also download the Java Runtime Environment (JRE) which is only needed to run Java applications and not for developing, from the following link:

https://www.oracle.com/java/technologies/javase-jre-downloads.html

It is important to make sure you are downloading the correct version of Java for your operating system and for the specific needs of your project. Once you have downloaded and installed Java, you can check the version by running the command "java -version" in the command prompt or terminal.

Additionally, you may also find the Java development kit in package managers of different Operating Systems like Ubuntu, Debian, Fedora, and others.

It is important to note that the Java development kit and runtime environments can also be obtained through other distributors like OpenJDK.

How to install Java on a computer

Installing Java on a computer typically involves the following steps:

1. Download the Java Development Kit (JDK) or the Java Runtime Environment (JRE) from the Oracle website. https://www.oracle.com/java/technologies/jacase-downloads.com

Or distributors link OpenJDK.

2. Run the downloaded installer: Once the download is complete, run the installer by double-clicking on the downloaded file. The installer will guide you through the installation process.

3. Follow the prompts: The installer will ask you to accept the terms of the license agreement and choose the destination folder for the installation. You can accept the defaults or choose a different location.

4. Set the JAVA_HOME environment variable: After the installation is complete, you may need to set the JAVA_HOME environment variable to the location where Java was installed. This step is optional but it's needed if you want to use certain tools that rely on the JAVA_HOME variable.

5. Update the PATH environment variable: You may also need to update the PATH environment variable to include the location of the Java binaries. This step is also optional.

6. Verify the installation: After the installation is complete, you can verify that Java is installed correctly by opening a command prompt or terminal and running the command "java -version". This should display the version of Java that you just installed.

Java programs developing software

There are many software tools available for developing Java programs, some of the most popular ones include:

Eclipse: Eclipse is a popular open-source integrated development environment (IDE) for Java and other languages. It provides features such as code completion, debugging, and a built-in Java development environment.

IntelliJ IDEA: IntelliJ IDEA is another popular Java IDE, developed by JetBrains. It offers advanced code navigation and refactoring, built-in support for version control systems, and a wide range of plugins for various frameworks and technologies.

NetBeans: NetBeans is an open-source IDE, with support for Java, C, C++, PHP, HTML, and JavaScript. It provides a drag-and-drop interface for GUI development and has a wide range of plugins available.

BlueJ: BlueJ is a free and open-source IDE, specifically designed for learning and teaching Java. It features an interactive interface that allows users to explore the structure of their code and the relationships between classes.

jGRASP: jGRASP is a lightweight IDE, that provides a visual representation of the structure of a program, and supports multiple languages including Java.

Apache Maven: Maven is a build automation tool that is primarily used for Java projects. It is used to manage dependencies, and build, test, and deploy projects.

Apache Ant: Ant is another build automation tool, that is similar to Maven, but uses an XML file to define the build process.

Gradle: Gradle is an open-source build automation tool that is similar to Maven, but uses a domain-specific language (DSL) based on the Groovy programming language.

Java Variables

In Java, a variable is a named storage location in memory that can hold a value of a specific data type. Variables are used to store and manipulate data in a program.

Java supports different types of variables, including

Primitive variables: These are the basic data types in Java, such as int, float, double, char, and boolean. Primitive variables hold the actual value of the data type in memory.

Reference variables: These are variables that hold a reference to an object in memory. A reference variable does not hold the actual value of an object, but rather a reference to the location in memory where the object is stored.

Java also has a few rules for naming variables, which include:

Variable names must begin with a letter, dollar signs ($), or underscore (_).

Variable names can contain letters, digits, dollar signs ($), and underscore (_).

Variable names are case-sensitive.

Java also has a few rules for declaring variables:

Variables must be declared with a specific data type.

Variables must be initialized with a value before they can be used.

Variables can be declared and initialized at the same time.

Variables can be re-assigned to new values as needed.

For example, here is a simple Java program that declares and initializes a variable:

In this example, the variable "age" is of type int and is assigned the value 30. The value of the variable can be accessed and used throughout the program as needed.

It's worth noting that in Java, variables have a scope, which means that they are only accessible within a specific block of code or method, and there are two types of variables: instance variables, and local variables. Instance variables are defined in a class and are accessible within the class, while local variables are defined within a method and are only accessible within the method.

Java Operators

Java operators are symbols that are used to perform operations on variables and values. They can be used to perform mathematical calculations, assign values, compare values, and perform logical operations.

Java supports several types of operators, including

Arithmetic operators: These operators are used to perform mathematical operations such as addition (+), subtraction (-), multiplication (*), division (/), modulus (%), and increment (++) and decrement (--) operators.

Comparison operators: These operators are used to compare two values and return a boolean result. They include equal to (==), not equal to (!=), greater than (>), less than (<), greater than or equal to (>=), and less than or equal to (<=).

Logical operators: These operators are used to perform logical operations such as and (&&), or (||), and not (!). They are used to combine or negate Boolean expressions.

Assignment operators: These operators are used to assign a value to a variable. The assignment operator is the equal sign (=), but there are also compound assignment operators that perform an operation and an assignment in one step such as (+=) and (-=)

Conditional operator: This operator is also known as the ternary operator (? :), it is used to evaluate a condition and return one value if the condition is true and another value if it is false.

Bitwise operators: These operators are used to perform bit-level operations on integers. They include bitwise AND (&), bitwise OR (|), bitwise XOR (^), bitwise complement (~), and left shift (1)

Here's an example of using the different types of operators in a Java program:

Condition statements In Java

In Java, conditional statements are used to control the flow of a program by executing certain blocks of code based on certain conditions. Some of the most commonly used conditional statements in Java include

• if-else: This statement is used to execute a block of code if a certain condition is true, and another block of code if the condition is false. The basic syntax of the if-else statement is as follows:

• if-else if-else: This statement is used to check multiple conditions and execute a block of code based on the first condition that is true. The basic syntax of the if-else if-else statement is as follows:

• switch-case: This statement is used to perform different actions based on multiple cases or conditions. The basic syntax of the switch-case statement is as follows:

It's worth noting that the switch statement only works with certain.

Looping statements In Java

In Java, there are several types of looping statements:

1. for loop: is used to execute a set of statements repeatedly a fixed number of times.

2. while loop: used to execute a set of statements repeatedly as long as a given condition is true.

3. do-while loop: used to execute a set of statements repeatedly until a given condition is true.

4. enhanced for loop or for-each loop: used to iterate over elements of an array or a collection.

Each of these looping statements has its own specific use cases and can be used to accomplish different tasks. It is important to choose the appropriate looping statement for a given task to make the code more readable and efficient.

Jump statement In Java

In Java, there are several types of jump statements:

1. ‘break’ statement: used to exit a loop or a switch statement.

2. ‘continue’ statement: used to skip over one iteration of a loop.

3. ‘return’ statement: used to exit a method and return a value (if the method returns a value).

4. ‘throw’ statement: used to throw an exception.

5. ‘throws’ statement: used to indicate that a method throws an exception.

It is important to use jump statements in a proper manner to make the code more readable and to avoid infinite loops or unexpected behavior.

Functions In Java

In Java, a function is known as a method. A method is a block of code that performs a specific task and may return a value or not. Methods are used to organize the code and make it more readable.

Java methods have the following components:

1. Method signature: includes the method name, return type, and the parameter list.

2. Method body: contains the statements that are executed when the method is called.

3. Return type: specifies the type of value that the method returns. If the method doesn't return any value, its return type is void.

4. Parameter list: contains the variables that are passed to the method when it is called.

For example

This is a sample of a method named ‘add’ which takes two integers as input parameters and returns their sum as an integer.

Java also supports method overloading, which means that you can have multiple methods with the same name but different parameter lists. This allows you to write methods that have similar functionality but work with different types of data.

‘Oops’ concept In Java

OOP (Object-Oriented Programming) is a programming paradigm that is based on the concept of objects, which have properties (also known as attributes) and methods. Java is an object-oriented programming language, which means that it supports all the concepts of OOP such as encapsulation, inheritance, polymorphism, and abstraction.

Encapsulation: The process of hiding the implementation details of an object and exposing only the necessary information to the outside world is called encapsulation. In Java, this can be achieved by using access modifiers such as private, protected, and public.

Inheritance: The process of inheriting properties and methods from one class to another is called inheritance. In Java, this can be achieved by using the extends keyword.

Polymorphism: The ability of an object to behave in multiple ways is called polymorphism. In Java, this can be achieved by using interfaces and overriding methods.

Abstraction: The process of hiding the complexity of an object and exposing only the essential features is called abstraction. In Java, this can be achieved by using abstract classes and interfaces.

Java also has several built-in classes, such as String and ArrayList, which are implemented using these concepts of OOP. Additionally, there are several design patterns, such as the Singleton, Factory, and Observer patterns, that are commonly used in Java development to solve common programming problems in an object-oriented way.

Oops, program In Java

Here is a simple example of OOP in Java:

In this example, we have a superclass 'Animal' with properties 'name' and 'age', encapsulated using private access modifiers and accessed using setters and getters. Additionally, it also has a method 'makeSound' which is overridden in the subclass 'Dog' to produce a dog-specific sound.

This example illustrates the concepts of inheritance, encapsulation, and polymorphism in Java. The Dog class inherited the properties and methods from the Animal class and also has its own properties and methods. The 'makeSound' method in the Dog class overrides the method of the same name in the Animal class, which is an example of polymorphism.

Local variable In Java

In Java, a local variable is a variable that is declared within a method or a block of code. A local variable can only be accessed within the method or block in which it is declared and it's not visible to other methods or blocks.

For example:

In this example, the variables 'x', 'y', and 'sum' are all local variables that are declared and used within the 'main()' method. They can't be accessed outside of the main method.

It's important to note that local variables are not initialized by default, they must be initialized before they can be used.

Additionally, Local variables should be declared as close as possible to where they are used.

Also, the Local variable should have a meaningful name, that describes the variable's purpose and scope of the variable.

Instance variables & Static variables In Java

1. Instance variables: These are variables that are declared inside a class, but outside of any method, constructor or block. Each object of the class has its own copy of the instance variables and they are also known as non-static variables. They are defined using the keyword "non-static".

For example:

2. Static variables: These are variables that are shared among all the objects of the class and they are also known as class variables. They are defined using the keyword "static".

For example

It is important to note that, Static variables are initialized by default, they are set to their default values when the class is loaded.

Additionally, a common use case for static variables is as counters, for example, to keep track of how many objects of a class have been instantiated.

Also, you can access the static variables directly with the class name, you don't need to create an instance of the class to access it.

Methods In Java

In Java, a method is a block of code that performs a specific task and may return a value or not. Methods are used to organize the code and make it more readable.

Java methods have the following components:

1. Method signature: which includes the method name, return type, and the parameter list.

2. Method body: contains the statements that are executed when the method is called.

3. Return type: specifies the type of value that the method returns. If the method doesn't return any value, its return type is void.

4. Parameter list: contains the variables that are passed to the method when it is called.

For example:

This is a sample of a method named 'add' which takes two integers as input parameters and returns their sum as an integer.

Java also supports method overloading, which means that you can have multiple methods with the same name but different parameter lists. This allows you to write methods that have similar functionality but work with different types of data.

Java also has several built-in methods such as 'length()', 'substring()', 'toLowerCase()' etc. that can be used for various purposes.

Additionally, Java has two types of methods:

1. Instance methods: methods that operate on the instance of a class and require an object of the class to be created before they can be called.

2. Static methods: methods that operate on the class itself, and can be called directly from the class, without creating an object of the class.

It's also important to note that, the 'this' keyword can be used inside an instance method to refer to the current object.

Call by value & Call by reference In Java

In Java, there are two ways to pass arguments to a method: by value and by reference.

1. Call by value: When a primitive data type (such as int, float, double, etc.) is passed as an argument to a method, a copy of the value is passed to the method. Any changes made to the argument inside the method do not affect the original value outside the method. This is known as call by value.

For example:

2. Call by reference: When an object (such as a class instance or an array) is passed as an argument to a method, the reference to the object is passed to the method. Any changes made to the object's properties or elements inside the method are reflected outside the method. This is known as call by reference.

For example:

In Java, all objects are passed by reference, and all primitive types are passed by value.

It is important to understand the difference between call by value and call by reference because they can have different effects on the behavior of the program.

Also, it's worth noting that, String is a special case in Java, String objects are immutable which means that, when you try to change the value of a String object, it creates a new object with the new value.

Method overloading In Java

Method overloading in Java is the ability of a class to have multiple methods with the same name but different method signatures. A method signature is made up of the method name and the number and type of its parameters.

When a method is called, the Java compiler uses the method signature to determine which version of the method to execute. This allows for methods with the same name to perform different operations depending on the input they receive.

For example, consider a class with two methods named "add".

In this case, the first method takes two integers as arguments and returns the sum of them, while the second method takes two doubles as arguments and returns their sum.

When calling the add method, the Java compiler will automatically select the correct version of the method based on the types of the arguments passed in.

It's important to note that in method overloading, the return type of the method doesn't play a role in method overloading, it's only the parameter's type and number that matter.

In summary, Method overloading in Java is a feature that allows a class to have multiple methods with the same name but different method signatures. This allows methods with the same name to perform different operations based on the input they receive, and the Java compiler automatically selects the correct version of the method to execute based on the method signature.

Constructor In Java

In Java, a constructor is a special method that is used to initialize an object's state when it is created. Constructors are defined using the keyword "constructor" and have the same name as the class. Constructors do not have any return type, not even void. They are automatically called when an object is created and can be used to set initial values for the object's attributes.

A class can have multiple constructors that differ in the number or type of arguments they take. This is known as constructor overloading. For example, a class can have a constructor that takes no arguments and another constructor that takes one or more arguments. This allows objects to be initialized in different ways depending on the situation.

The "new" keyword is used to create an object and call its constructor. For example:

This creates an object of type MyClass and calls the constructor of MyClass to initialize its state.

It's important to note that if a class does not have any constructors defined, the compiler will automatically provide a default constructor with no arguments.

A constructor also has a special method called a copy constructor, it creates a new object with the same values as an existing object.

In summary, Constructors are special methods in Java that are used to initialize the state of an object when it is created. They have the same name as the class, do not have a return type and are automatically called when an object is created. They can be overloaded and can take different number and types of arguments.

This Keyword In Java

In Java, the keyword "this" is used to refer to the current instance of an object. It can be used in several ways:

1. To refer to instance variables: When a local variable has the same name as an instance variable, the local variable shadows the instance variable. To access the instance variable, you can use "this" keyword.

2. To invoke other constructors of the same class: The "this" keyword can be used to call another constructor of the same class, using the 'this(...)' notation.

3. To refer to the current object: The "this" keyword can be used to pass the current object as an argument to a method or constructor.

4. To refer to the current class: The "this" keyword can be used to refer to the current class. It can be used to refer to the current class in a static context.

It's important to note that the use of the "this" keyword is optional, and it is mostly used to resolve ambiguity. If a local variable has the same name as an instance variable, you can use "this" keyword to refer to the instance variable without any issues.

In general, the 'this' keyword is not used as frequently as other keywords in Java, but it can be useful in certain situations where there is name conflict or when passing the current object as an argument.

Inheritance In Java

Inheritance in Java is a mechanism by which one class can inherit properties and methods from another class. It is a fundamental concept in object-oriented programming and allows for code reuse and the creation of a hierarchy of classes.

A class that inherits from another class is called a subclass or derived class, and the class from which it inherits is called the superclass or base class. The subclass inherits all the non-private properties and methods of the superclass, and can also add its own properties and methods.

The keyword "extends" is used to indicate that one class is a subclass of another. For example:

In this example, the Dog class inherits all the properties and methods of the Animal class.

Inheritance allows for the creation of a hierarchy of classes. For example, a class "Mammal" can be a superclass of "Dog" and "Cat", and "Dog" and "Cat" can be subclasses of "Mammal", "Dog" and "Cat" class have their own properties and methods, but also inherit properties and methods from "Mammal" class.

It's also important to note that Java supports only single inheritance, meaning that a class can inherit from only one superclass but can implement multiple interfaces.

In summary, Inheritance in Java is the mechanism by which one class can inherit properties and methods from another class. It allows for code reuse, the creation of a hierarchy of classes, and makes it possible to extend the functionality of existing classes. A subclass inherits all the non-private properties and methods of the superclass, and can also add its own properties and methods. Java supports single inheritance.

Super keyword in Java

The "super" keyword in Java is used to refer to the immediate parent class or superclass of a subclass. It is often used to call methods or constructors from the superclass or to access the superclass's member variables.

The "super" keyword can be used in several ways:

1. To call a constructor from the superclass:

Here, the "super()" call invokes the default constructor of the superclass, Animal.

2. To call a method from the superclass:

Here, the "super.move()" call invokes the move method of the superclass, Animal.

3. To access a variable from the superclass:

Here, "super.name" access the variable name of the superclass, Animal.

It's important to note that the "super" keyword can only be used in a subclass and should not be used in a class that does not have a superclass.

In summary, The "super" keyword in Java is used to refer to the immediate parent class or superclass of a subclass. It is often used to call methods or constructors from the superclass or to access the superclass's member variables. It can be used in the constructor, methods, or variable access of the subclass, and it should not be used in a class that does not have a superclass.

Polymorphism In Java

Polymorphism in Java is the ability of an object to take on many forms. It is one of the fundamental concepts in object-oriented programming and enables Java to support method overriding and the use of interfaces.

There are two types of polymorphism in Java:

1. Compile-time polymorphism, also known as method overloading, is when a class has multiple methods with the same name but different method signatures. The Java compiler selects the correct method to call at compile-time based on the method signature and the arguments passed to the method.

2. Run-time polymorphism, also known as method overriding, is when a subclass provides a different implementation of a method that is already defined in its superclass. The Java Virtual Machine (JVM) selects the correct method to call at runtime based on the actual type of the object.

For example, consider the following code:

Here, the move method is overridden in the Dog class. If we create an instance of the Dog class and call the move method, the JVM will call the overridden version of the method, printing "Dogs can walk and run".

Polymorphism is a powerful feature of Java that allows for code reuse, flexibility, and the ability to use objects of different types in the same way.

In summary, Polymorphism in Java is the ability of an object to take on many forms. It enables Java to support method overriding and the use of interfaces. There are two types of polymorphism in Java: compile-time polymorphism, also known as method overloading, and run-time polymorphism, also known as method overriding. Polymorphism allows for code reuse, flexibility, and the ability to use objects of different types in the same way.

Hiding vs overriding In Java

In Java, hiding and overriding are two related but distinct concepts that allow a subclass to change the behavior of a method defined in a superclass.

Hiding, also known as shadowing, occurs when a subclass defines a method with the same name and signature as a method in its superclass, but does not use the "override" keyword. In this case, the subclass method will hide the superclass method and the superclass method will not be accessible within the subclass.

For example, consider the following code:

Here, the move method in the Dog class is hiding the move method in the Animal class. If we create an instance of the Dog class and call the move method, the JVM will call the method in the Dog class, printing "Dogs can walk and run".

Overriding, on the other hand, occurs when a subclass defines a method with the same name and signature as a method in its superclass and uses the "override" keyword. In this case, the subclass method will override the superclass method and the superclass method will be overridden in the subclass.

For example, consider the following code:

Here, the move method in the Dog class is overriding the move method in the Animal class. If we create an instance of the Dog class and call the move method, the JVM will call the method in the Dog class, printing "Dogs can walk and run".

It's important to note that, in terms of functionality, hiding and overriding are similar, but in terms of code design, overriding is more desirable, because it preserves the polymorphic behavior of the superclass methods.

In summary, Hiding and Overriding in Java are related but distinct concepts that allow a subclass to change the behavior of a method defined in a superclass. Hiding occurs when a subclass defines a method with the same name and signature as a method in its superclass but does not use the "override" keyword. Overriding, on the other hand, occurs when a subclass defines a method with the same name and signature as a method in its superclass and uses the "override" keyword. Hiding and Overriding are similar in functionality but Overriding is more desirable because it preserves the polymorphic behavior of the superclass methods.

Final Keyword In Java

In Java, the "final" keyword can be used in several ways to indicate that something cannot be changed.

1. Final Variables: A final variable is a variable that can only be assigned a value once. Once a final variable is assigned a value, its value cannot be changed.

For example:

2. Final Methods: A final method is a method that cannot be overridden by subclasses.

For example:

3. Final Classes: A final class is a class that cannot be subclassed. A final class is typically used when a class is considered complete and should not be extended.

For example:

It's important to note that final variables, methods, and classes are typically used to prevent unwanted changes and to enforce design constraints.

In summary, the "final" keyword in Java can be used to indicate that something cannot be changed. A final variable is a variable that can only be assigned a value once, a final method is a method that cannot be overridden by subclasses, and a final class is a class that cannot be subclassed. These are typically used to prevent unwanted changes and to enforce design constraints.

Abstraction in Java

Abstraction in Java is the process of hiding the implementation details of a class or method and exposing only the necessary information to the user. It is one of the fundamental concepts in object-oriented programming and is implemented using abstract classes and interfaces.

1. Abstract classes: An abstract class is a class that cannot be instantiated, but can be subclassed. An abstract class can have abstract and non-abstract methods. Abstract methods are methods that do not have an implementation and must be overridden by subclasses.

2. Interfaces: An interface is a collection of abstract methods that a class must implement. An interface cannot be instantiated and is typically used to define a set of methods that a class must implement.

Abstraction is useful for hiding the complexity of a class or method and allowing the user to work with the class or method in a simplified way. It also allows for code reuse and flexibility by allowing the implementation of a class or method to change without affecting the code that uses it.

In summary, Abstraction in Java is the process of hiding the implementation details of a class or method and exposing only the necessary information to the user. It is implemented using abstract classes and interfaces. Abstract classes are classes that cannot be instantiated, but can be subclassed and have abstract and non-abstract methods. Interfaces are collections of abstract methods that a class must implement. Abstraction is useful for hiding the complexity of a class or method and allowing the user to work with the class or method in a simplified way, also it allows for code reuse and flexibility.

Interface In Java

In Java, an interface is a collection of abstract methods that a class must implement. An interface defines a set of methods and their signatures, but does not provide an implementation. It is a blueprint for classes and is used to define a contract that classes must adhere to.

An interface is defined using the "interface" keyword, and its methods are defined using the "abstract" keyword. For example:

In this example, the Shape interface defines a single method called "draw" without an implementation.

A class can implement an interface using the "implements" keyword. For example:

In this example, the Square class implements the Shape interface and provides an implementation for the "draw" method.

It's important to note that an interface can only contain abstract methods and final variables, and all methods in an interface are implicitly public and abstract. Also, a class can implement multiple interfaces, but can only extend one class.

Interfaces are useful for creating a contract between different classes, allowing them to work together in a predictable way. They are also useful for providing a common interface for classes that have different implementations.

In summary, an interface in Java is a collection of abstract methods that a class must implement. It defines a set of methods and their signatures, but does not provide an implementation. A class can implement an interface using the "implements" keyword. Interfaces are useful for creating a contract between different classes, allowing them to work together in a predictable way and also for providing a common interface for classes that have different implementations. They can only contain abstract methods and final variables, and all methods in an interface are implicitly public and abstract.

Multiple classes In Java

In Java, a class is a blueprint for creating objects (a particular data structure), providing initial values for state (member variables or attributes), and implementations of behavior (member functions or methods).

A Java program can have multiple classes defined in it. In fact, most Java programs have multiple classes defined in them. The main class is usually the class that contains the main method which is the entry point for the program and other classes are used to provide additional functionality.

For example, you might have a 'Car' class that defines the state and behavior of a car object, and a 'Driver' class that defines the state and behavior of a driver object. The 'Car' class might have a method called 'drive()' that is used to move the car, while the 'Driver' class might have a method called 'steer()' that is used to change the direction of the car.

Each class is defined in its own file with the same name as the class name followed by .java .

The main class is executed by the JVM (Java Virtual Machine) and it will use other classes as per requirement.

Package In Java

In Java, a package is a way to organize related classes and interfaces into a single unit. Packages are used to group related classes and interfaces together, and to provide a level of access protection for the classes and interfaces within a package.

A package is a collection of related classes and interfaces. It is used to group classes and interfaces together based on their functionality, and to provide a level of access protection for the classes and interfaces within the package. Packages can be nested, which means that a package can contain other packages.

 v hgFor example, the Java Standard Library contains a package called 'java.util' which contains classes and interfaces for working with collections, dates, and other utility classes.

To create a package, you use the 'package' keyword at the beginning of the source file, followed by the package name. For example, if you wanted to create a package called 'com.example.mypackage', you would write the following at the top of your source file:

To use a class or interface from a package, you need to import it using the 'import' statement. For example, if you wanted to use the 'ArrayList' class from the 'java.util' package, you would write the following at the top of your source file:

When you run your java program, the package name is used to determine where to find the class files on the file system. The package name is also used to prevent naming conflicts between classes with the same name that exist in different packages.

It's a best practice to use package names in reverse order of domain name. It will help to avoid naming conflict and provides a unique package name.

Access mod In Java

In Java, access modifiers are used to control the visibility and accessibility of class members (fields and methods) and constructors. Java provides four access modifiers: public, protected, private, and the default (no modifier).

public: A class, method, or field that is declared as public can be accessed from anywhere. A public class or interface can be used by any class, regardless of which package the class is in. A public method can be called by any object of any class, and a public field can be read and modified by any object.

protected: A class, method, or field that is declared as protected can be accessed within its own package and any subclasses (even if they are in a different package).

private: A class, method, or field that is declared as private can only be accessed within the same class in which it is declared. Private members are not accessible from outside of the class.

default: If no access modifier is specified, the default access level is used. A class, method, or field that has default access can only be accessed within the same package.

It's a good practice to use access modifiers to limit the scope of a class, method, or field to only those parts of the code that need to access it. This helps to reduce the potential for bugs and makes the code more maintainable.

For example, if you have a class called Car and you want to make the speed field private so that it can only be accessed and modified by methods within the Car class, you would write the following:

Additionally, you can use access modifiers on constructors, methods, and nested classes as well.

Encapsulation In Java

Encapsulation is a fundamental concept in object-oriented programming and is one of the four pillars of OOP (the other three being inheritance, polymorphism, and abstraction). It refers to the practice of hiding the internal details of an object from the outside world, and instead providing a public interface that can be used to access and modify the object's state.

In Java, encapsulation is implemented by using access modifiers (public, protected, private, and the default) to control the visibility and accessibility of class members (fields and methods). By making fields private and providing public methods to access and modify them, the internal state of the object is hidden from the outside world and can only be accessed and modified through the methods provided by the class.

This has several benefits:

• It allows the implementation of a class to change without affecting the code that uses the class.

• It prevents unauthorized access to the internal state of the object.

• It promotes the use of the Single Responsibility Principle, where each class has a single responsibility and is not concerned with the implementation details of other classes.

For example, if you have a class called 'Car' and you want to encapsulate the internal state of the car object, you would write the following:

In this example, the 'speed' field is private, so it can only be accessed and modified by methods within the 'Car' class. The 'getSpeed()' method is public, so it can be called by any object to get the current speed of the car, and the 'setSpeed(int speed)' method is also public, so it can be called by any object to set the speed of the car.

Encapsulation is a fundamental concept in Java and allows you to create more robust, maintainable, and scalable code.

exception handling In Java

In Java, exception handling is a mechanism to handle runtime errors that may occur in a program. An exception is an abnormal event that occurs during the execution of a program and disrupts the normal flow of instructions. When an exception occurs, it generates an exception object that contains information about the exception, such as its type and a description of the error.

Java provides a built-in mechanism for handling exceptions, which allows you to handle exceptions in a structured way and to separate the error-handling code from the normal code.

The basic structure of exception handling in Java is as follows:

1. Use the 'try' block to enclose the code that might throw an exception.

2. Use one or more 'catch' blocks to handle the exception.

3. Use the 'finally' block to enclose code that should always be executed, regardless of whether an exception is thrown.

Here is an example of how to handle an exception in Java:

In this example, the code in the 'try' block attempts to divide 10 by 0, which will throw an 'ArithmeticException'. The 'catch' block catches this exception and prints an error message. The 'finally' block contains code that is always executed, regardless of whether an exception is thrown or not.

Java also provides a 'throws' keyword that can be used to indicate that a method might throw an exception. This allows the caller of the method to handle the exception or to pass the responsibility for handling the exception to the caller's caller, and so on.

It's important to handle the exceptions properly to avoid the abnormal termination of the program, to give a proper message to the user and to perform the recovery actions.

try-catch statement In Java

In Java, the try-catch statement is used to handle exceptions that may occur during the execution of a program. The try block encloses the code that might throw an exception, and the catch block contains the code that will handle the exception if one is thrown.

The basic structure of a try-catch statement in Java is as follows:

For example, consider the following code snippet:

In this example, the code in the try block attempts to divide 10 by 0, which will throw an 'ArithmeticException'. The catch block catches this exception and prints an error message.

It's important to note that only the exception type that is specified in the catch block will be caught, if the exception thrown is not of the same type or of the subtype of the exception type specified in the catch block, the exception will not be caught.

You can have multiple catch blocks for a single try block, each catch block is designed to handle a specific exception type.

It's a good practice to catch the specific exception type as much as possible instead of a general exception.

Additionally, you can use a finally block, which will execute after the try-catch block, regardless of whether an exception was thrown or not. This can be used to clean up resources, such as closing files or releasing memory, that were acquired in the try block.

Overall, the try-catch statement allows you to handle exceptions in a structured way and to separate the error-handling code from the normal code. It's a fundamental concept in Java and allows you to create more robust, maintainable, and scalable code.

Throw Throws In Java

In Java, the 'throw' and 'throws' keywords are used to handle exceptions. The 'throw' keyword is used to throw an exception explicitly, while the 'throws' keyword is used to indicate that a method might throw an exception.

The 'throw' keyword is used to throw an exception explicitly. When you throw an exception, the normal flow of the program is disrupted and the exception is propagated up the call stack until it is caught by an appropriate catch block or until the program terminates. Here is an example of how to throw an exception in Java:

In this example, if the value of x is less than 0, an 'IllegalArgumentException' is thrown with the message "x should be positive".

The 'throws' keyword is used to indicate that a method might throw an exception. When a method is declared with the 'throws' keyword, it indicates that the method might throw one or more exceptions of the specified types. The caller of the method is responsible for either catching the exception or passing the responsibility for handling the exception to its own caller. Here is an example of how to indicate that a method might throw an exception in Java:

In this example, the 'myMethod' might throw an 'IOException'. The caller of the method is responsible for either catching the exception or passing the responsibility for handling the exception to its own caller.

It's important to handle the exceptions properly. If the methods are not designed to handle the exception then it should be propagated to the calling method using throws keyword. This allows the calling method to handle the exception or to pass the responsibility for handling the exception to its own caller, and so on. It's important to note that if a method is declared with the 'throws' keyword, the caller must either catch the exception or propagate it to its own caller using the 'throws' keyword.

Using the 'throw' and 'throws' keywords in Java allows you to handle exceptions in a structured way and to separate the error-handling code from the normal code. It also allows for better readability of the code and helps to identify the potential errors that might occur during the execution of the program.

Arrays In Java

In Java, an array is a data structure that stores a fixed-size, homogeneous collection of elements. The elements can be of any type, such as integers, strings, or objects. The size of an array is set when it is created and cannot be modified afterwards.

Arrays are created using the "new" keyword, followed by the data type of the elements and the size of the array in square brackets. For example, to create an array of integers with a size of 10, you would use the following code:

You can also initialize an array with values at the time of creation by listing the values in curly braces {} after the size:

You can access and modify elements of an array by using the array name followed by the index of the element in square brackets. For example, to set the first element of the array to 5, you would use the following code:

You can also use loops to iterate through all elements of an array. One way to do this is using the "for" loop, like this:

Java also provides a class called java.util.Arrays which has many utility methods for arrays such as sorting, searching and comparing.

Note that arrays are objects in java and the size of array is fixed and cannot be changed once created.

Types of arrays In Java

In Java, there are two main types of arrays: one-dimensional arrays and multi-dimensional arrays.

1. One-dimensional arrays: A one-dimensional array is a linear collection of elements, where each element is identified by a single index. For example, an array of integers or an array of strings.

2. Multi-dimensional arrays: A multi-dimensional array is an array that contains one or more arrays as its elements. The most common type of multi-dimensional array is the two-dimensional array, also known as a matrix. A two-dimensional array is an array of arrays, where each element is identified by two indices.

Java also provides an array of arrays called jagged arrays. A jagged array is an array whose elements are arrays. The size of the sub-arrays can be different.

Additionally, arrays can be of any type, including objects of custom classes.

It's important to note that the size of the array must be specified at the time of its creation and cannot be changed afterwards. Also, the index of arrays starts from 0 in Java.

String In Java

In Java, a String is an object that represents a sequence of characters. String objects are immutable, which means that the contents of a String object cannot be modified after it is created. Instead, a new String object is created whenever the contents of a String are modified.

You can create a String object in several ways:

1. Using double quotes:

2. Using the new keyword:

3. Using the string literal:

You can perform various operations on strings such as concatenation, substring, length, replacing characters, and comparing strings.

1. Concatenation: You can concatenate two or more strings using the + operator or the concat() method.

2. Substring: You can extract a portion of a string using the substring() method.

3. Length: You can find the number of characters in a string using the length() method.

4. Replacing characters: You can replace a character or a substring in a string using the replace() method.

5. Comparing Strings: You can compare two strings to check if they are equal or not using the equals() method or the == operator.

Java also provides many other useful methods to manipulate strings such as split(), trim(), indexOf(), lastIndexOf(), toLowerCase(), toUpperCase(), etc.

It's important to note that Strings are objects in Java, and the == operator compares the memory addresses of the objects, not their contents. Therefore, when comparing strings, it's generally recommended to use the equals() method or the equalsIgnoreCase() method instead.

String methods In Java

In Java, the String class provides many useful methods for manipulating and working with strings. Some of the most commonly used String methods include:

1. length(): This method returns the number of characters in a string.

2. charAt(int index): This method returns the character at the specified index in the string. The index of the first character is 0.

3. substring(int beginIndex, int endIndex): This method returns a new string that is a substring of the original string. The beginIndex is the starting index of the substring (inclusive), and the endIndex is the ending index of the substring (exclusive).

4. indexOf(String str): This method returns the index of the first occurrence of the specified substring in the string. If the substring is not found, it returns -1.

5. replace(char oldChar, char newChar): This method replaces all occurrences of a specified character with a new character in the string.

6. toUpperCase(): This method returns a new string in which all the characters are in uppercase.

7. toLowerCase(): This method returns a new string in which all the characters are in lowercase.

8. trim(): This method returns a new string after removing the leading and trailing white spaces.

9. split(String regex): This method splits the string into an array of substrings based on a specified delimiter.

10. equals(Object obj): This method compares the contents of the string with the contents of the specified object. It returns true if the contents are the same, and false otherwise.

These are just a few examples of the many useful methods provided by the String class in Java. It's worth exploring the Java documentation to learn more about the other methods available, and what they can be used for.

Java Interview Questions ?

1. Can you explain the difference between an interface and an abstract class in Java?

2. What is the purpose of the static keyword in Java?

3. Can you explain the difference between a constructor and a method in Java?

4. How do you handle exceptions in Java?

5. Can you explain the concept of inheritance in Java?

6. Can you explain the difference between a HashMap and a Hashtable in Java?

7. Can you explain the concept of polymorphism in Java?

8. Can you explain the difference between a String, StringBuilder, and StringBuffer in Java?

9. Can you explain the concept of encapsulation in Java?

10. Can you explain the difference between a for loop and a while loop in Java?

11. Can you explain the difference between a break statement and a continue statement in Java?

12. Can you explain the difference between the == operator and the equals() method in Java?

13. Can you explain the concept of an anonymous inner class in Java?

14. Can you explain the difference between a final variable, a final method, and a final class in Java?

15. Can you explain the concept of garbage collection in Java?

16. Can you explain the difference between a public, protected, and private access modifier in Java?

17. Can you explain the concept of a JavaFX application?

18. Can you explain the difference between a static method and an instance method in Java?

19. Can you explain the difference between a try-catch block and a try-catch-finally block in Java?

20. Can you explain the concept of a Java Applet?

These are some examples of common interview questions that may be asked about Java. It's important to note that the questions may vary depending on the specific job or company.

Answers

(1).(a)

 An interface in Java defines a set of method signatures that must be implemented by a class that implements the interface. An abstract class, on the other hand, can define both method signatures and implementations, and is intended to be extended by other classes. An interface can't have any method implementation, whereas an abstract class can have some method implementation.

(2).(b)

The static keyword in Java is used to define a class-level variable or method. A variable or method that is declared as static is shared across all instances of the class, rather than being unique to each instance. A static variable is also called a class variable, and a static method is also called a class method.

(3).(c)

A constructor in Java is a special method that is called when an object of a class is created. It is used to initialize the state of an object. A method, on the other hand, is a general block of code that can be invoked by an object to perform a specific task. A constructor doesn't have return type, whereas a method has a return type.

(4).(d)

In Java, exceptions are events that occur during the execution of a program that disrupt the normal flow of instructions. They can be handled using try-catch blocks, where the code that may throw an exception is placed in a try block and the exception handling code is placed in a catch block. It's also possible to use the throws keyword to specify that a method might throw an exception, or to use a finally block to include code that will always be executed regardless of whether an exception is thrown.

(5).(e)

Inheritance in Java is a mechanism that allows a class to inherit the properties and methods of another class. This allows for code reuse and a more organized class hierarchy. A subclass can inherit all or some of the properties and methods of its superclass, and can also add new properties and methods or override those inherited from the superclass.

(6).(f)

A HashMap in Java is an implementation of the Map interface that stores key-value pairs in a hash table. A Hashtable, on the other hand, is an older class that also implements the Map interface. The main difference between the two is that Hashtable is synchronized, which means that only one thread can access it at a time, whereas HashMap is not synchronized.

(7).(g)

Polymorphism in Java is the ability of an object to take on multiple forms. It allows for a single variable, method, or object to refer to multiple types or classes, and to use the appropriate implementation based on the context. This can be achieved through inheritance, interfaces, and overriding methods.

(8).(h)

A String in Java is an immutable object that represents a sequence of characters. A StringBuilder is a mutable object that can be used to create and manipulate strings, and it's more efficient for situations where many strings are concatenated together. A StringBuffer is similar to StringBuilder, but it is synchronized, which means that only one thread can access it at a time.

(9).(i)

Encapsulation in Java is the process of hiding the implementation details of a class from other classes and providing a public interface for interacting with the class. This allows for the implementation to change without affecting the rest of the program, and it also improves security by preventing unauthorized access to the internal state of an object.

(10).(j)

A for loop in Java is used to iterate through a collection of elements or a range of numbers. It is typically used for situations where the number of iterations is known in advance, and it has a specific syntax that includes the initialization, termination, and increment/decrement of the loop variable. A while loop, on the other hand, is used to execute a block of code repeatedly as long as a particular condition is true. It's typically used for situations where the number of iterations is not known in advance, and it has a simpler syntax that only includes the condition to be checked.

(11).(k)

A break statement in Java is used to exit a loop or a switch statement immediately. When a break statement is encountered, the control is transferred out of the loop or switch statement, and the next statement after the loop or switch statement is executed. A continue statement, on the other hand, is used to skip the current iteration of a loop and continue with the next iteration.

(12).(l)

The == operator in Java is used to compare the memory addresses of two objects. The equals() method, on the other hand, is used to compare the contents of two objects. It's generally recommended to use the equals() method when comparing strings, as the == operator may not always give the expected result.

(13).(m)

An anonymous inner class in Java is a class that is defined without a name and is used only once. It is typically used as a shorthand way to create an object that implements an interface or extends a class, without the need to create a separate class definition.

(14).(n)

A final variable in Java is a variable that cannot be reassigned after it is initialized. A final method in Java is a method that cannot be overridden by subclasses. A final class in Java is a class that cannot be inherited by other classes. Using the final keyword with a variable, method or class, prevents them from being modified, thus ensuring their state or behavior is constant.

(15).(o)

Garbage collection in Java is a mechanism used by the Java Virtual Machine (JVM) to automatically reclaim memory that is no longer being used by the program. The JVM periodically scans the heap for objects that are no longer reachable by the program and frees up the memory occupied by those objects. This allows Java programmers to focus on their application logic without having to worry about manually managing memory.

(16).(p)

In Java, access modifiers are used to control the visibility and accessibility of class members (fields, methods, and constructors). Public members can be accessed from anywhere, protected members can be accessed by classes in the same package and by subclasses in different packages, and private members can only be accessed within the same class.

(17),(q)

JavaFX is a Java library used for developing rich client applications, such as desktop and mobile applications, web applications, and interactive media. It provides a set of APIs for creating and manipulating graphical elements, handling events, and creating animations and multimedia effects.

(18).(r)

A static method in Java is a method that can be called on a class, rather than on an instance of the class. A static method is associated with the class, rather than with any particular object of the class, so it can be invoked without creating an instance of the class. An instance method, on the other hand, is associated with an instance of the class, and can be invoked on an object of the class.

(19).(s)

A try-catch block in Java is used to handle exceptions that may occur in the try block. The try block contains the code that may throw an exception, and the catch block contains the code that will be executed if an exception is thrown. A try-catch-finally block is similar, but it also includes a finally block, which contains code that will always be executed regardless of whether an exception is thrown. The finally block is useful for cleaning up resources or performing other tasks that must be done regardless of whether an exception occurs.

(20).(t)

A Java Applet is a type of Java program that is designed to be run within a web browser. Applets are typically used to create interactive or dynamic elements on a web page, such as animations or games. They are executed on the client-side and have limited access to the client's system resources. With the emergence of other technology such as HTML5 and javascript, the use of applets have been limited.