题目：设计一个类，我们只能生成该类的一个实例
public class Test02 {/*** 单例模式，，懒汉式，线程安全*/public static class Singleton {private final static Singleton INSTANCE = new Singleton();private Singleton() {}public static Singleton getInstance() {return INSTANCE;}}/*** 单例模式，饿汉式，线程不安全*/public static class Singleton2 {private static Singleton2 instance = null;private Singleton2() {}public static Singleton2 getInstance() {if (instance == null) {instance = new Singleton2();}return instance;}}/*** 单例模式，饿汉式，线程安全，多线程环境下效率不高*/public static class Singleton3 {private static Singleton3 instance = null;private Singleton3() {}public static synchronized Singleton3 getInstance() {if (instance == null) {instance = new Singleton3();}return instance;}}/*** 单例模式，懒汉式，变种，线程安全*/public static class Singleton4 {private static Singleton4 instance = null;static {instance = new Singleton4();}private Singleton4() {}public static Singleton4 getInstance() {return instance;}}/*** 单例模式，使用静态内部类，线程安全【推荐】*/public static class Singleton5 {private final static class SingletonHolder {private static final Singleton5 INSTANCE = new Singleton5();}private Singleton5() {}public static Singleton5 getInstance() {return SingletonHolder.INSTANCE;}}/*** 静态内部类，使用枚举方式，线程安全【推荐】*/public enum Singleton6 {INSTANCE;public void whateverMethod() {}}/*** 静态内部类，使用双重校验锁，线程安全【推荐】*/public static class Singleton7 {private volatile static Singleton7 instance = null;private Singleton7() {}public static Singleton7 getInstance() {if (instance == null) {synchronized (Singleton7.class) {if (instance == null) {instance = new Singleton7();}}}return instance;}}public static void main(String[] args) {System.out.println(Singleton.getInstance() == Singleton.getInstance());System.out.println(Singleton2.getInstance() == Singleton2.getInstance());System.out.println(Singleton3.getInstance() == Singleton3.getInstance());System.out.println(Singleton4.getInstance() == Singleton4.getInstance());System.out.println(Singleton5.getInstance() == Singleton5.getInstance());System.out.println(Singleton6.INSTANCE == Singleton6.INSTANCE);System.out.println(Singleton7.getInstance() == Singleton7.getInstance());}}运行结果：
突然之间失去了语言。那才是真正的寂寞，
