Design Pattern - Singleton Pattern

 This pattern involves a single class which is responsible to create an object while making sure that only single object gets created. This class provides a way to access its only object which can be accessed directly without need to instantiate the object of the class.

package Main;

public class Test2 {

    public int a;

    private static Test2 instance = new Test2();

    private Test2(){

    }

    public static Test2 getInstance(){
        return instance;
    }
}

package Main;

public class Test {
    public int a;

    private static Test t = null;

    private Test(){

    }

    public static Test instance(){
        if(t == null){
            t = new Test();
        }
        return t;
    }
}
package Main;

public class Main {

    public static void main(String[] args) {
       Test t1 = Test.instance();
       t1.a = 5;
        System.out.println(t1.a);

        Test t2 = Test.instance();
        t2.a = 10;
        System.out.println(t1.a);
        System.out.println(t2.a);
        System.out.println("----------------------------");
        Test2 tt2 = Test2.getInstance();
        tt2.a = 10;
        System.out.println(tt2.a);
        Test2 tt3 = Test2.getInstance();
        tt3.a = 20;
        System.out.println(tt2.a);
        System.out.println(tt3.a);
    }
}
The outcome will be:
5
10
10
----------------------------
10
20
20

package com.company;

public class UserConfig {

    private String username;
    private String password;

    private static UserConfig userConfig = null;

    private UserConfig(String username, String password) {
        this.username = username;
        this.password = password;
    }

    public static UserConfig getInstance(String username, String password) {
        if (userConfig == null) {
            userConfig = new UserConfig(username, password);
        }
        return userConfig;
    }

}
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package main;

public class Car {
    public int a;

    private static Car instance = null;

    private Car() {
    }

    public static Car getInstance() {
        if (instance == null) {
            synchronized (Car.class) {
                if (instance == null)
                    instance = new Car();
            }
        }
        return instance;
    }
}