Types of Classes in Java

A class is a blueprint in the Java programming language from which an individual object can be built. In Java, we may declare a class by using the class keyword. Class members and functions are declared simply within the class. Classes are required for the creation of Java programs. The object-oriented paradigm (OOP) allows users to describe real-world objects. Also, a class is sometimes known as a user-defined data type. The following components make up a class declaration:

• Modifiers
• Class name
• Keywords
• The class body within curly brackets {}.

Class in Java  

1. Class is a set of object which shares common characteristics/ behavior and common properties/ attributes.
2. Class is not a real world entity. It is just a template or blueprint or prototype from which objects are created.
3. Class does not occupy memory.
4. Class is a group of variables of different data types and group of methods.

A class in java can contain:
• data member
• method
• constructor
• nested class and 
• interface

Syntax to declare a class:

access_modifier class<class_name>
{  
    data member;  
    method;  
    constructor;
    nested class;
    interface;
}

Eg: 
• Animal
• Student
• Bird
• Vehicle
• Company   

Java

class Student
{  
    int id;//data member (also instance variable)  
    String name; //data member (also instance variable)  

    public static void main(String args[])
    {  
        Student s1=new Student();//creating an object of Student  
        System.out.println(s1.id);  
        System.out.println(s1.name);  
    }  
}  

Output

0
null

Types of Classes

1. Final Class
2. Static Class
3. Abstract Class
4. Concrete Class
5. POJO Class
6. Singleton Class
7. Inner Class

1. Final Class

When a variable, function, or class is declared final, its value persists throughout the program. Declaring a method with the final keyword indicates that the method cannot be overridden by subclasses. That is a class that is marked final cannot be subclasses. This is very useful when creating immutable classes such as  String classes. A class cannot be mutated unless it is declared final.  If only all the members of the class is final then it can’t be mutated otherwise it can be mutated

Java

import java.io.*;
import java.lang.*;
import java.util.*;

final class Base {

    void Display()
    {
        System.out.print("Method for Base class.");
    }
}

class Extended extends Base {

    void Display()
    {
        System.out.print("Method of Extended class.");
    }
}

class GFG {

    public static void main(String[] arg)
    {
        Extended d = new Extended();
        d.Display();
    }
}

2. Static Class

Static is a Java word that explains how objects are kept in memory. A static object belongs to that class rather than instances of that class. The primary function of the class is to provide blueprints for the inherited classes. A static class has only static members. An object cannot be created for a static class.

Java

import java.io.*;
import java.lang.*;
import java.util.*;

class staticclasses {
    static int s;
    static void met(int x, int y)
    {
        System.out.println(
            "static method to calculate sum");
        s = x + y;
        System.out.println(x + "+" + y);
    }
    static class MyNestedClass {
        static
        {
            System.out.println(
                "static block inside a static class");
        }
        public void disp()
        {
            int x1;
            int y1;
            Scanner sc = new Scanner(System.in);
            System.out.println("Enter two numbers");
            x1 = sc.nextInt();
            y1 = sc.nextInt();
            met(x1, y1);
            System.out.println("Sum of the 2 numbers-" + s);
        }
    }
}
public class GFG {
    public static void main(String args[])
    {
        staticclasses.MyNestedClass nestedclass
            = new staticclasses.MyNestedClass();
        nestedclass.disp();
    }
}

3. Abstract Class

A class that has zero or more abstract methods and is specified with the abstract keyword is called an abstract class. We must rigorously extend the abstract classes to a concrete class in order to use them because they are incomplete classes. Constructors and static methods can also be included. It can have final methods, which force the subclass to keep the body of the method unhung.

Java

import java.io.*;
import java.lang.*;
import java.util.*;

interface X {
    int product(int x, int y);
}
abstract class Product implements X {

    public int product(int x, int y) { return x * y; }
}
class GFG extends Product {
    public static void main(String args[])
    {
        Main ob = new Main();
        int p = ob.product(20, 10);
        System.out.println("Product: " + p);
    }
}

4. Concrete Class

A normal class with an implementation for all of its methods and no abstract methods is called a concrete class. They are not permitted to have any processes that are not in use at the time. If it implements all of its methods, a concrete class can extend its parent, an abstract class, or an interface. It’s a fully working, instantiable class.

Java

import java.io.*;
import java.lang.*;
import java.util.*;

public class GFG {
    static int sum(int x, int y) { return x + y; }

    public static void main(String args[])
    {
        int p = sum(10, 8);
        System.out.println("Sum: " + p);
    }
}

5. POJO Class

“Plain Old Java Object” is an abbreviation for “Plain Old Java Object.” A POJO class has only private variables with setter and getter methods to access them. It’s a pure data structure with fields that can override some Object methods (e.g. equals) or other interfaces (e.g. serializable), but it has no behavior of its own.

POJO class properties:

• When writing a POJO class, public setter and getter methods are required.
• Private variables should be used for all instance variables.
• It should not extend classes that have already been defined.
• It should not implement interfaces that have been pre-defined.
• There should be no pre-defined annotations.
• It might not have a function Object() { [native code] } that takes no arguments.

Java

import java.io.*;
import java.lang.*;
import java.util.*;

class POJO {
    private int value = 365;
    public int getValue() { return value; }
    public void setValue(int value) { this.value = value; }
}
public class GFG {
    public static void main(String args[])
    {
        POJO p = new POJO();
        System.out.println(p.getValue());
    }
}

6. Singleton Class

A singleton class is one that has just one object at any one moment. Even yet, if we try to create an instance again, the newly created instance refers to the previous one. Any modification we make to the class through any instance impacts the variables in that specific instance as well. It’s commonly used to manage access while working with database connections and socket programming.

The following is used to make Singleton Class:

• Make a function Object() { [native code] } that is only available to you.
• Create a static function that returns the singleton class’s object (using lazy initialization).

Java

import java.io.*;
import java.lang.*;
import java.util.*;

class Singleton {

    private static Singleton single_instance = null;

    public String s;

    private Singleton()
    {
        s = "This is a string part of Singleton class";
    }
    // here a private constructor is used

    // Method
    public static Singleton Singleton()
    {
        if (single_instance == null) {
            single_instance = new Singleton();
        }
        return single_instance;
    }
}

// Main class
class GFG {
    public static void main(String args[])
    {
        Singleton x = Singleton.Singleton();
        Singleton y = Singleton.Singleton();

        // change var of x
        x.s = (x.s).toUpperCase();

        System.out.println("String from x is -->" + x.s);
        System.out.println("String from y is -->" + y.s);
        System.out.println("\n");

        y.s = (y.s).toLowerCase();

        System.out.println("String from x is -->" + x.s);
        System.out.println("String from y is -->" + y.s);
    }
}

7. Inner Class

We can define a class within a class in Java, and these classes are referred to as nested classes. It’s used to logically arrange classes and achieve encapsulation. The outer class members (including private) can be accessed by the inner class. 

Syntax:

Java

import java.io.*;

class OuterClass {
    // Write the code
    class NestedClass {
        // Write the code
    }
}

There are 4 types of inner classes:

• Nested Inner class
• Anonymous inner classes
• Static nested classes
• Method Local inner classes

A. Nested Inner Class:

It has access to an outer class’s private instance variables. The access modifiers private, protected, public, and default can be applied to any instance variable.

Java

import java.io.*;
import java.lang.*;
import java.util.*;

class Outer {
    class Inner {
        public void show()
        {
            System.out.println("Inside a nested class");
        }
    }
}

class GFG {
    public static void main(String[] args)
    {
        Outer.Inner in = new Outer().new Inner();
        in.show();
    }
}

B. Anonymous Inner Class:

Basically, these classes are declared without any name.

Example 1: Using Subclass

Java

import java.io.*;
import java.lang.*;
import java.util.*;

class Outer {
    void show()
    {
        System.out.println("Show method of super class");
    }
}

class GFG {

    static Outer o = new Outer() {
        void show()
        {
            super.show();
            System.out.println("Demo class");
        }
    };

    public static void main(String[] args) { o.show(); }
}

Example 2: Using Interface

Java

import java.io.*;
import java.lang.*;
import java.util.*;

class GFG {

    static Hello h = new Hello() {
        public void show()
        {
            System.out.println(
                "This is an anonymous class");
        }
    };

    public static void main(String[] args) { h.show(); }
}

interface Hello {
    void show();
}

C. Static Nested Class:

These classes are like static members of the outer class.

Java

import java.io.*;
import java.lang.*;
import java.util.*;

class GFG {
    static int data = 100;
    static class Inner {

        void msg()
        {
            System.out.println("data is " + data);
        }
    }

    public static void main(String args[])
    {
        GFG.Inner obj = new GFG.Inner();
        obj.msg();
    }
}

D. Method Local inner Class:

An inner class can be declared within a method of an outer class.

Java

import java.io.*;
import java.lang.*;
import java.util.*;

class Outer {
    void outerMethod()
    {
        System.out.println("Outer Method");
        class Inner {
            void innerMethod()
            {
                System.out.println("Inner Method");
            }
        }

        Inner y = new Inner();
        y.innerMethod();
    }
}
class GFG {
    public static void main(String[] args)
    {
        Outer x = new Outer();
        x.outerMethod();
    }
}