怎么做网站教程++用的工具,零基础月做网站多久,如何让wordpress百度霸屏,中小企业解决方案背景故事《曾经最美》是朱铭捷演唱的一首歌曲#xff0c;由陈佳明填词#xff0c;叶良俊谱曲#xff0c;是电视剧《水晶之恋》的主题曲。歌曲时长4分28秒。 歌曲歌词#xff1a;看不穿你的眼睛藏有多少悲和喜像冰雪细腻又如此透明仿佛片刻就要老去整个城市的孤寂不止一个你…背景故事《曾经最美》是朱铭捷演唱的一首歌曲由陈佳明填词叶良俊谱曲是电视剧《水晶之恋》的主题曲。歌曲时长4分28秒。 歌曲歌词看不穿你的眼睛藏有多少悲和喜像冰雪细腻又如此透明仿佛片刻就要老去整个城市的孤寂不止一个你只能远远的想像慰藉我们之间的距离我又不是你的谁不能带给你安慰忍心你枯萎凋零的玫瑰仿佛希望化成灰要不是痛彻心扉谁又记得谁只是云和月相互以为是彼此的盈缺不能哭喊已破碎曾经的最美独自一个人熟悉的街别问你在想谁不去追悔已憔悴爱过的机会真实已粉碎人事已非还有什么最可贵我又不是你的谁不能带给你安慰忍心你枯萎凋零的玫瑰仿佛希望化成灰要不是痛彻心扉谁又记得谁只是云和月相互以为是彼此的盈缺不能哭喊已破碎曾经的最美独自一个人熟悉的街别问你在想谁不去追悔已粉碎爱过的机会真实已粉碎人事已非还有什么最可贵不能哭喊已破碎曾经的最美独自一个人熟悉的街别问你在想谁不去追悔已憔悴爱过的机会真实已粉碎人事已非还有什么最可贵牵线之牛刀小试如何判断是不是谁的谁java有一个instanceof操作符(关系操作符)可以做这件事。public static voidmain(String[] args) {String s Hello World!;System.out.println(sinstanceofString);}打印出结果true可是如果你的哪个谁不存在呢请看代码public static voidmain(String[] args) {String s null;System.out.println(sinstanceofString);}很多人都会异口同声的说false你答对了。JSL-15.20.2规定At run time, the result of the instanceof operator is true if the value of the RelationalExpression is not null and the reference could be cast to the ReferenceType without raising a ClassCastException. Otherwise the result is false.牵线之乱点鸳鸯谱如果没有任何关系的两个类使用instanceof会如何class Point { intx, y; }class Element { intatomicNumber; }public classInstanceofTest {public static voidmain(String[] args) {Point p newPoint();Element e newElement();if (e instanceof Point) {System.out.println(匹配成功!);}else{System.out.println(匹配不成功);}}}不少人会说“匹配不成功”抱歉你又掉进坑里了这个会报编译错误JSL-15.20.2规定The type of the RelationalExpression operand of the instanceof operator must be a reference type or the null type, or a compile-time error occurs.It is a compile-time error if the ReferenceType mentioned after the instanceof operator does not denote a reference type that is reifiable (§4.7).If a cast of the RelationalExpression to the ReferenceType would be rejected as a compile-time error (§15.16), then the instanceof relational expression likewise produces a compile-time error. In such a situation, the result of the instanceof expression could never be true.当然cast也会是编译错误class Point { intx, y; }class Element { intatomicNumber; }public classInstanceofTest {public static voidmain(String[] args) {Point p newPoint();Element e newElement();p (Point)e; //compile-time error}}牵线之暗藏玄机编译器并不是万能的并不能检测出所有问题看下面class Point { intx, y; }class Element { intatomicNumber; }public classInstanceofTest {public static voidmain(String[] args) {Point p newPoint();//Element e new Element();p (Point) newObject();System.out.println(pinstanceofPoint);}}猛一看没事问题编译也没有问题可是运行时报错Exception in thread main java.lang.ClassCastException: java.lang.Object cannot be cast to Point上面的程序展示了当要被转型的表达式的静态类型是转型类型的超类时转型操作符的行为。与instanceof 操作相同如果在一个转型操作中的两种类型都是类那么其中一个必须是另一个的子类型。尽管对我们来说这个转型很显然会失败但是类型系统还没有强大到能够洞悉表达式new Object()的运行期类型不可能是Point的一个子类型。因此该程序将在运行期抛出ClassCastException 异常。牵线之竞争激烈关系操作符instanceof可不是市场上唯一的选择另外一个背靠大山的家伙要注意了Class 的方法booleanisInstance(Object obj)Determines if the specified Object is assignment-compatible with the object represented by this Class.那么什么时候该用instanceof 什么时候该用isInstance呢我的理解是instanceof偏向于比较class之间isInstance偏向于比较instance和class之间stackoverflow也有此问题的解答I take that to mean that isInstance() is primarily intended for use in code dealing with type reflection at runtime. In particular, I would say that it exists to handle cases where you might not know in advance the type(s) of class(es) that you want to check for membership of in advance (rare though those cases probably are).For instance, you can use it to write a method that checks to see if two arbitrarily typed objects are assignment-compatible, like:public booleanareObjectsAssignable(Object left, Object right) {returnleft.getClass().isInstance(right);}In general, Id say that using instanceof should be preferred whenever you know the kind of class you want to check against in advance. In those very rare cases where you do not, use isInstance() instead.参考资料【1】https://docs.oracle.com/javase/specs/jls/se12/html/jls-15.html#jls-15.20.2【2】java解惑【3】https://stackoverflow.com/questions/8692214/when-to-use-class-isinstance-when-to-use-instanceof-operator
