懒汉单例模式

Crow.java

public class Crow
{
	public Crow(){
		System.out.println("crow");
	}
};

CrowFactory.java

public class CrowFactory
{
	static Crow crow = null;

	public static Crow getInstance(){
		if(crow == null){
			crow = new Crow();
		}
		return crow;
	}
};

Main.java

public class Main
{
	public static void main(String[] args){
		Crow c = CrowFactory.getInstance();
		Crow c1 = CrowFactory.getInstance();
	}
};