Java: getInstance vs Static

Question

What is the purpose of getInstance() in Java?

During my research I keep reading that getInstance() helps achieve a Singleton design pattern (w

Answer 1

Singleton

A singleton allows you to use a single reference to a java Object. For example, here is a singleton which contains a number;

public class MySingleton {

    private int myNumber;
    private static MySingleton instance;

    public static MySingleton getInstance() {
        if (instance == null) {
             instance = new MySingleton();
        }
        return instance;
    }

    private MySingleton() {}

    public void setMyNumber(int myNumber) {
        this.myNumber = myNumber;
    }

    public int getMyNumber() {
       return myNumber;
    }
}

Now we are going to set the value of this number in the A class:

public class A {
    /*...*/
    MySingleton mySingleton = MySingleton.getInstance();
    mySingleton.setMyNumber(42);
    /*...*/
}

Then, you can access this value from another class:

public class B {
    /*...*/
    MySingleton mySingleton = MySingleton.getInstance();
    int number = mySingleton.getMyNumber();
    /*...*/
}

In this class the number variable will have the value 42. This is the advantage of a singleton over a simple object:

All the values stored in the singleton will be accessible from "everywhere".

---

Static class

The purpose is different, here the advantage is to use an object without having to create it.

For example:

public static class MyStaticClass {
    public static void sayHello() {
        System.out.println("Hello");
    }
}

Now you can use the sayHello() method from any classes by calling:

MyStaticClass.sayHello();