设计模式 --单例模式

package singleton_pattern;

public class SingleTonPattern {
	public static void main(String[] args) {
		SingleTon s1=SingleTon.getInstance();
//		SingleTon s2=SingleTon.getInstance();
		System.out.println(s1);
		SingleTon2 s3=SingleTon2.getInstance();
		System.out.println(s3);
		SingleTon3 s4=SingleTon3.getInstance();
		System.out.println(s4);
		SingleTon4 s5=SingleTon4.getInstance();
		System.out.println(s5);
	}
}

class SingleTon{
	//单例的必要操作--构造方法私有化
	private SingleTon() {}
	//创建静态私有成员变量new一个实例对象
	private static SingleTon st=new SingleTon();
	//公有方法返回
	public static SingleTon getInstance() {
		return st;
	}
}

class SingleTon2{
	//同样的配方，私有构造
	private SingleTon2(){}
	//创建静态私有成员变量赋null
	private static SingleTon2 st=null;
	public static SingleTon2 getInstance() {
		if(st==null)
			st=new SingleTon2();
		return st;
	}
}

class SingleTon3{
	//同样的配方，私有构造
	private SingleTon3(){}
	//创建静态私有成员变量赋null
	private static SingleTon3 st=null;
	public static SingleTon3 getInstance() {
		if(st==null) {
			synchronized (SingleTon3.class) {
				if(st==null)//双重判断，很nice
					st=new SingleTon3();
			}
		}
		return st;
	}
}

class SingleTon4{
	//同样的配方，私有构造
	private SingleTon4(){}
	//静态内部类
	private static class Instance{
		 private static SingleTon4 st=new SingleTon4();
	}
	public static SingleTon4 getInstance() {
		return Instance.st;
	}
}