懒汉单例模式

package oop;
//使用饿汉单例实现单例类
//构造器一定要私有
public class SingleInstance {
//私有化构造器
private SingleInstance(){
}
//定义一个静态成员变量储存对象，只加载一份
private static SingleInstance singleInstance;
//提供一个方法，对外返回单例
public static SingleInstance getInstance(){
if(singleInstance == null){
singleInstance = new SingleInstance();
}
return singleInstance;
}
}

(测试)
package oop;

public class Test {
public static void main(String[] args) {
SingleInstance s1 = SingleInstance.getInstance();
SingleInstance s2 = SingleInstance.getInstance();
System.out.println(s1 == s2);
}

}