Java单例模式实现方式

public class LazyNoSafe {
    private static LazyNoSafe instance;
    public static LazyNoSafe getInstance() {
        if (instance == null) {
            instance = new LazyNoSafe();
        }
        return instance;
    }
    private LazyNoSafe() {}
}

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

public class LazyUseInner {
    private static class InnerClass {
        private static LazyUseInner instance = new LazyUseInner();
    }
    public static LazyUseInner getInstance() {
        return InnerClass.instance;
    }
    private LazyUseInner() {}
}

public class DoubleLock {
    private volatile static DoubleLock instance;
    public static DoubleLock getInstance() {
        if (instance == null) { //第一次检查
            synchronized(DoubleLock.class) {
                if (instance == null) {
                    instance = new DoubleLock();
                }
            }
        }
        return instance;
    }
    private DoubleLock() {}
}

public class HungrySafe {
    private static HungrySafe instance = new HungrySafe();
    public static HungrySafe getInstance() {
        return instance;
    }
    private HungrySafe() {}
}

public enum UseEnum {
    INSTANCE;
    public void otherMethod() {
        // do something...
    }
}