对于单例模式面试官会怎样提问呢？你又该如何回答呢？

// 饿汉式单例
public class HungrySingleton{
    // 私有静态实例引用，创建私有静态实例，并将引用所指向的实例
    private static HungrySingleton singleton = new HungrySingleton();
    // 私有的构造方法
    private HungrySingleton(){}
    //返回静态实例的静态公有方法，静态工厂方法
    public static HungrySingleton getSingleton(){
        return singleton;
    }
}
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// 懒汉式单例
public class LazySingleton {
    // 私有静态实例引用
    private static LazySingleton singleton;
    // 私有的构造方法
    private LazySingleton(){}
    // 返回静态实例的静态公有方法，静态工厂方法
    public static LazySingleton getSingleton(){
        //当需要创建类的时候创建单例类，并将引用所指向的实例
        if (singleton == null) {
            singleton = new LazySingleton();
        }
        return singleton;
    }
}
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// 线程安全的懒汉式单例
public class SynchronizedSingleton {
    private static SynchronizedSingleton synchronizedSingleton;
    private SynchronizedSingleton(){}
    // 使用 synchronized 修饰，临界资源的同步互斥访问
    public static synchronized SynchronizedSingleton getSingleton(){
        if (synchronizedSingleton == null) {
            synchronizedSingleton = new SynchronizedSingleton();
        }
        return synchronizedSingleton;
    }
}
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// 线程安全的懒汉式单例
public class BlockSingleton {
    private static BlockSingleton singleton;
    private BlockSingleton(){}
    public static BlockSingleton getSingleton2(){
        synchronized(BlockSingleton.class){  // 使用 synchronized 块，临界资源的同步互斥访问
            if (singleton == null) { 
                singleton = new BlockSingleton();
            }
        }
        return singleton;
    }
}
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// 线程安全的懒汉式单例
public class InsideSingleton {
    // 私有内部类，按需加载，用时加载，也就是延迟加载
    private static class Holder {
        private static InsideSingleton insideSingleton = new InsideSingleton();
    }
    private InsideSingleton() {
    }
    public static InsideSingleton getSingleton() {
        return Holder.insideSingleton;
    }
}
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// 线程安全的懒汉式单例
public class DoubleCheckSingleton {
    //使用volatile关键字防止重排序，因为 new Instance()是一个非原子操作，可能创建一个不完整的实例
    private static volatile DoubleCheckSingleton singleton;
    private DoubleCheckSingleton() {
    }
    public static DoubleCheckSingleton getSingleton() {
        // Double-Check idiom
        if (singleton == null) {
            synchronized (DoubleCheckSingleton.class) {       
                // 只需在第一次创建实例时才同步
                if (singleton == null) {      
                    singleton = new DoubleCheckSingleton();      
                }
            }
        }
        return singleton;
    }
}
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memory = allocate();        //1:分配对象的内存空间
ctorInstance(memory);       //2:初始化对象
singleton = memory;        //3:使singleton3指向刚分配的内存地址
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memory = allocate();        //1:分配对象的内存空间
singleton3 = memory;        //3:使singleton3指向刚分配的内存地址
ctorInstance(memory);       //2:初始化对象
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// 线程安全的懒汉式单例
public class ThreadLocalSingleton 
    // ThreadLocal 线程局部变量
    private static ThreadLocal<ThreadLocalSingleton> threadLocal = new ThreadLocal<ThreadLocalSingleton>();
    private static ThreadLocalSingleton singleton = null;
    private ThreadLocalSingleton(){}
    public static ThreadLocalSingleton getSingleton(){
        if (threadLocal.get() == null) {        // 第一次检查：该线程是否第一次访问
            createSingleton();
        }
        return singleton;
    }
    public static void createSingleton(){
        synchronized (ThreadLocalSingleton.class) {
            if (singleton == null) {          // 第二次检查：该单例是否被创建
                singleton = new ThreadLocalSingleton();   // 只执行一次
            }
        }
        threadLocal.set(singleton);      // 将单例放入当前线程的局部变量中 
    }
}
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public enum EnumSingleton {
    instance;
    public void whateverMethod(){
        //dosomething
    }
}
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public class Test {
    public static void main(String[] args) {
        Thread[] threads = new Thread[10];
        for (int i = 0; i < threads.length; i++) {
            threads[i] = new TestThread();
        }
        for (int i = 0; i < threads.length; i++) {
            threads[i].start()；
        }
    }
}
class TestThread extends Thread {
    @Override
    public void run() {
        // 对于不同单例模式的实现，只需更改相应的单例类名及其公有静态工厂方法名即可
        int hash = Singleton5.getSingleton5().hashCode();  
        System.out.println(hash);
    }
}
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