Singleton Pattern: Object Creational Pattern

Intent

Ensure a class has only one instance and provide a global point of access to it.

Structure

Here’s the breakdown:

• Singleton: Defines an Instance operation that lets clients access its unique instance.
• Instance is a class operation; it might be responsible for creating its own unique instance.

Applicability

Use the Singleton pattern when:

• A class must have exactly one instance, and clients can access it from a well-known access point.
• The sole instance should be extensible by subclassing, and clients should be able to use an extended instance without changing their code.

Example

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public class main{
    // s1 and s2 are the same instance (same memory address)
    Singleton s1 = Singleton.getInstance();
    Singleton s2 = Singleton.getInstance();
}

class Singleton{
    private int num = 2023;

    public void setNum(int num){
        this.num = num;
    }
    public int getNum(){
        return this.num;
    }

    private Singleton(){}

    private static Singleton instance = new Singleton();

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