java面试——单例模式

在进公司java面试时，容易考到的题目，singleton模式的特点并写一个singleton。

单例模式有以下的特点：
1.单例类只可有一个实例。
2.单例类必须自己创建自己这惟一的实例。
3.单例类必须给所有其他对象提供这一实例。

package com.xyq.demo;

public class Singleton {
	private static Singleton singleton;
	private Singleton() {
	};
	public static Singleton getInstance() {
		if (singleton == null)
			singleton = new Singleton();
		return singleton;
	}
}

// 测试类
class singletonTest {
	public static void main(String[] args) {
		Singleton s1 = Singleton.getInstance();
		Singleton s2 = Singleton.getInstance();
		if (s1 == s2)
			System.out.println("s1 is the same instance with s2");
		else
			System.out.println("s1 is not the same instance with s2");
	}
}

 输出:  s1 is the same instance with s2

 

恶汉式单例类

package com.xyq.demo;

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

// 测试类
class singletonTest {
	public static void main(String[] args) {
		Singleton s1 = Singleton.getInstance();
		Singleton s2 = Singleton.getInstance();
		if (s1 == s2)
			System.out.println("s1 is the same instance with s2");
		else
			System.out.println("s1 is not the same instance with s2");
	}
}

 输出:  s1 is the same instance with s2

 

懒汉式单例类

package com.xyq.demo;

public class Singleton {
	private static Singleton instance = null;

	/**
	 * 私有的默认构造子，保证外界无法直接实例化
	 */
	private Singleton() {
	}

	/**
	 * 静态工厂方法，返还此类的惟一实例
	 */
	synchronized public static Singleton getInstance() {
		if (instance == null) {
			instance = new Singleton();
		}
		return instance;
	}
}

// 测试类
class singletonTest {
	public static void main(String[] args) {
		Singleton s1 = Singleton.getInstance();
		Singleton s2 = Singleton.getInstance();
		if (s1 == s2)
			System.out.println("s1 is the same instance with s2");
		else
			System.out.println("s1 is not the same instance with s2");
	}
}

 输出:  s1 is the same instance with s2

我写的这些仅适用于公司面试，如果要想好好学习设计模式，得全面的看看这方面的资料