Design Pattern

Observer

An application calls an Observable object's notifyObservers method to have all the object's observers notified of the change.
Usage: Weather notification, wechat notification

Singleton

Restrict the instantiation of a class to one object. eg:Login system.

1.Eager Initialization

The instance of Singleton class is created at the time of class loading
Drawback: Instance is created even though client application might not be using it

public class Singleton {
	private static Singleton instance = new Singleton();
	public static Singleton getInstance() {
		return instance;
	}
}

2.Lazy Initialization

Lazy initialization solve the eager's issues, in order to make sure the thread safe, use synchronized and double check here.

public class SingletonLazy {
	//volatile is optional, volatile make sure the variable get value from RAM(hint: JMM)
	private static volatile SingletonLazy instance = null;
	public static SingletonLazy getInstance() {
		if (instance == null) {
			synchronized(SingletonLazy.class) {
				if (instance == null) {
					instance = new SingletonLazy();
				}
			}
		}
		return instance;
	}
}

3.Enum Initialization

Enum is also thread safe and very efficient, recommend.

public enum SingletonEnum {
	instance;
	public void method() {
	}
}