// 問題1：為什么加 final
// 問題2：如果實(shí)現(xiàn)了序列化接口, 還要做什么來防止反序列化破壞單例
public final class Singleton_hungry implements Serializable {

    // 問題3：為什么設(shè)置為私有? 是否能防止反射創(chuàng)建新的實(shí)例?
    private Singleton_hungry(){}

    // 問題4：這樣初始化是否能保證單例對象創(chuàng)建時(shí)的線程安全?
    private static Singleton_hungry INSTANCE = new Singleton_hungry();

    // 問題5：為什么提供靜態(tài)方法而不是直接將 INSTANCE 設(shè)置為 public, 說出你知道的理由
    public static Singleton_hungry getInstance() {
        return INSTANCE;
    }
    public Object readResolve(){  // 防止反射創(chuàng)建新的實(shí)例?
        return INSTANCE;
    }
}

enum Singleton { 
   INSTANCE; 
}

public final class Singleton_lazy {
    private Singleton_lazy(){}
    private static Singleton_lazy INSTANCE = null;
    // 缺點(diǎn)
    public static synchronized Singleton_lazy getInstance() {
        if(INSTANCE != null) {
            return INSTANCE;
        }
        INSTANCE = new Singleton_lazy();
        return INSTANCE;
    }
}

public final class Singleton_DCL {

    private Singleton_DCL() {}

    // 問題1：解釋為什么要加 volatile ?
    private static volatile Singleton_DCL INSTANCE= null;

    // 問題2：對比實(shí)現(xiàn)3, 說出這樣做的意義
    public static Singleton_DCL getInstance() {
        if(INSTANCE != null) {
            return INSTANCE;
        }
        synchronized (Singleton_DCL.class) {
          
            // 問題3：為什么還要在這里加為空判斷, 之前不是判斷過了嗎
            if(INSTANCE != null) {
                return INSTANCE;
            }
            INSTANCE = new Singleton_DCL();
            return INSTANCE;
        }
    }
}

public final class Singleton_LazyHolder {
    private Singleton_LazyHolder(){}

    // 問題1：屬于懶漢式還是餓漢式
    private static class LazyHolder{
        static final Singleton_LazyHolder INSTANCE = new Singleton_LazyHolder();
    }

    // 問題2：在創(chuàng)建時(shí)是否有并發(fā)問題
    public static Singleton_LazyHolder getInstance() {
        return LazyHolder.INSTANCE;
    }
}