用Java写一个单例类

饿汉式单例

public class Singleton {

    private Singleton(){}

    private static Singleton instance = new Singleton();

    public static Singleton getInstance(){

        return instance;

    }

}

懒汉式单例

public class Singleton {

    private static Singleton instance = null;

    private Singleton() {}

    public static synchronized Singleton getInstance(){

        if (instance == null) instance ＝ new Singleton();

        return instance;

    }

}