Design Pattern Study Notes 4 - Singleton

Problem:
only one instance in the system.

Solution:
Singleton pattern

1) singleton class
===================== Singleton.java ====================
package test.pattern.singleton;

public class Singleton {

private static Singleton instance = null;

/*
* the constructor must be private
*/
private Singleton() {
System.out.println("private constructor");
}

/*
* just in case running in multi-thread env,need to add synchronized
*/
public static synchronized Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}

return instance;
}
}

2) test class

==================== Client.java ====================

package test.pattern.singleton;

public class Client {

public static void main(String[] args) {

Singleton s1 = Singleton.getInstance();

Singleton s2 = Singleton.getInstance();

System.out.println(s1==s2);

}

}

This is the output:

private constructor

true