早上随便翻到单例的写法，那我也随手写几个吧

public class SingleClassTest {
    //定义一个private的构造方法，就是防止外部通过new SingleClassTest去实例化
    private SingleClassTest(){}

    //普通饿汉写法
    private static final SingleClassTest INSTANCE = new SingleClassTest();

    private static SingleClassTest getInstance(){
        return INSTANCE;
    }

    //普通懒汉写法
    private static SingleClassTest INSTANCE = null;

    private static SingleClassTest getInstance(){
        if (INSTANCE == null){
            INSTANCE = new SingleClassTest();
        }
        return INSTANCE;
    }

    //禁止重排序，double-check写法
    private static volatile SingleClassTest INSTANCE = null;

    private static SingleClassTest getInstance(){
        if (INSTANCE == null){
            synchronized (SingleClassTest.class){
                if (INSTANCE == null){
                    INSTANCE = new SingleClassTest();
                }
            }
        }
        return INSTANCE;
    }

    //静态内部类写法
    private static SingleClassTest getInstance(){
       return InnerSingleClass.INSTANCE;
    }

    private static class InnerSingleClass{
        private static final SingleClassTest INSTANCE = new SingleClassTest();
    }

}

除了以下几种，需要知道枚举也是属于一种实现方式，并且它可以防止序列化/反序列化、反射

public enum SingleClassTest {
    INSTANCE;
    public static SingleClassTest getInstance(){
        return INSTANCE;
    }
}