【Design Pattern】Singleton Pattern

singleton pattern

Only one object of a class can exist in memory

Note: In order to prevent other programs from creating the object too much, first control and prohibit other programs from creating objects of this class, then customize a private object in the class, and finally provide a method to access the private object.

The code implementation is divided into three steps: (1) Constructor privatization. (2) Create a private object of this class in the class. (3) Provide a method for obtaining the object externally.

Next, let's talk about the classification of singleton mode: hungry mode (recommended), full mode (thread unsafe)

/**
 * Hungry mode
 * Objects are created when the class is created
 */
class SingleDemo1{
	private int a;
	private SingleDemo1(){}
	private static SingleDemo1 s1 = new SingleDemo1();
	public static SingleDemo1 getInstance(){
		return s1;
	}
	public int getA() {
		return a;
	}
	public void setA(int a) {
		this.a = a;
	}
}

/**
 * Lazy mode
 * The object is created when the object is called
 * Due to multi-threading technology, the lazy mode may be unsafe
 */
class SingleDemo2{
	private SingleDemo2(){}
	private static SingleDemo2 s2 = null;
	public static SingleDemo2 getInstance(){
		if(s2==null){
			s2 = new SingleDemo2();
		}
		return s2;
	}
}

Test Results:

public class Test {
	
	public static void main(String[] args) {
		/*
		 * The two variables single1 and single2 refer to the same object
		 */
		SingleDemo1 single1 = SingleDemo1.getInstance();
		SingleDemo1 single2 = SingleDemo1.getInstance();
		single1.setA(10);
		System.out.println(single2.getA()); //10
	}

}