单例模式

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

public class Singleton {
    private static Singleton instance = new Singleton();

    private Singleton() {}

    public static Singleton getInstance() {
        return instance;
    }
}

public class Singleton {
    private static volatile Singleton instance;//volatile 保证多线程能正确处理instance变量

    private Singleton() {
    }

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

public class Singleton {
    private static class SingletonHolder {
        private static final Singleton INSTANCE = new Singleton();
    }

    private Singleton() {
    }

    public static final Singleton getInstance() {
        return SingletonHolder.INSTANCE;
    }
}

public enum SingletonEnum {
    INSTANCE;
    void someMethod(){
        System.out.println("这里是单例对象的方法!");
    }
}