Class in Java

In Java, a class is a blueprint or a template that defines the structure and behavior of objects. It serves as a blueprint for creating objects, which are instances of that class. A class encapsulates data (in the form of fields or variables) and methods (functions) that operate on that data.

In Java, there are several types of classes, including:

  1. Concrete Class: This is the most common type of class in Java. It provides a complete implementation of all the methods defined in its class hierarchy.

  2. Abstract Class: An abstract class is a class that cannot be instantiated on its own but can be subclassed. It may contain abstract methods (methods without an implementation) that must be overridden by its subclasses.

  3. Interface: An interface defines a contract or a set of methods that a class must implement. It specifies a behavior that a class should adhere to. Unlike classes, interfaces cannot be instantiated, but a class can implement multiple interfaces.

  4. Nested Class: Java allows you to define classes within other classes. These nested classes can be further classified into four types: static nested classes, non-static nested classes (also known as inner classes), local classes, and anonymous classes.

  5. Enum Class: An enum class is a special type of class that represents a group of constants. It provides a way to define fixed values that can be used in a program.

These are the main types of classes in Java. Each type has its own purpose and usage, allowing developers to structure their code and define the relationships between objects in a meaningful way.

Category of Class in Java

In Java, classes can be categorized into two main types: user-defined classes and predefined classes.

1. User-Defined Classes

User-defined classes are classes that are created by the programmer to fulfill specific requirements in a program. These classes are written by the programmer and provide custom functionality. When you create a new class using the class keyword, you are defining a user-defined class.

For example, let's say you want to create a program to manage a library. You might create a Book class that represents a book object with properties like title, author, and ISBN. This Book class would be a user-defined class.

2. Predefined Classes

Predefined classes, also known as built-in or standard classes, are classes that are provided by the Java language and are readily available for use. These classes are part of the Java API (Application Programming Interface) and are included in the Java Development Kit (JDK).

Predefined classes provide a wide range of functionality and cover various aspects such as input/output operations, string manipulation, mathematical operations, data structures, and more. Examples of predefined classes include String, Math, ArrayList, Scanner, File, and System.

You can use predefined classes directly in your program by importing them or using the fully qualified class name. These classes save you from reinventing the wheel by providing commonly needed functionality, allowing you to focus on solving the specific problems in your program.

It's important to note that both user-defined classes and predefined classes can be used together in Java programs to create robust and feature-rich applications.

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