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单例模式

单例(Singleton)：只允许创建一个该类的对象。

方式1：饿汉式(类加载时创建，天生线程安全)

class Teacher{
    private static final Teacher t=new Teacher();
    private Teacher(){}
    public static Teacher getInstance(){
        return t;
    }
}

方式2：懒汉式(使用时创建，线程不安全，加同步)

class Teacher{
    private static Teacher t=null;
    private Teacher(){}
    public static synchronized Teacher getInstance(){
        if (t == null) {
            t=new Teacher();
        }
        return t;
    }
}

方式3：懒汉式(使用时创建，线程安全)

class Teacher{
    private Teacher(){}
    private static class Holder{
        static Teacher t=new Teacher();
    }
    public static Teacher getInstance(){
        return Holder.t;
    }
}