设计模式4-单例模式

public class Singleton{
    private volatile static Singleton uniqueInstance;

    private singleton(){}

    public static Singleton getInstance(){
        if(uniqueInstance == null){
            synchronized (Singleton.class){
                if(uniqueInstance == null){
                    uniqueInstance = new Singleton();
                }
            }
        }
        return uniqueInstance;
    }
}