instanceof

I recently fumbled into a problem with an API and an implementation where the following type of code appeared: The API is an abstract class: public abstract class A { public A sum(A a) { System.out.println("A.sum(A) called"); return null; } } The implementation is a simple class: public class B extends A { public B sum(B b) { System.out.println("B.sum(B) called"); return null; } } When it comes to using it I write: public class Main { public static void main(String args[]) { B b = new B(); A basa = new B(); b.sum(b); basa.sum(b); basa.sum(basa); } } Which results in: % java Main B.sum(B)

OK this is my class, it encapsulates an object, and delegates equals and to String to this object, why I can´t use instance of??? public class Leaf<L> { private L object; /** * @return the object */ public L getObject() { return object; } /** * @param object the object to set */ public void setObject(L object) { this.object = object; } public boolean equals(Object other) { if(other instanceof Leaf<L>) //--->ERROR ON THIS LINE { Leaf<L> o = (Leaf<L>) other; return this.getObject().equals(o.getObject()); } return false; } public String toString() { return object.toString(); } } how can I get

I coded in NetBeans something like this: public class Grafo<V, E> { class Par { int a, b; Par(int a, int b) { this.a = a; this.b = b; } @Override public boolean equals(Object ob) { if(ob instanceof Par) { Par p = (Par)ob; return this.a==p.a && this.b==p.b; } return false; } } //stuff... } //end of class Grafo The error is in the method equals() from inner class "Par". NetBeans says that the error is "illegal generic type of instanceof". The error is in the line below. if(ob instanceof Par) { What is the cause of the error ? Try ob instanceof Grafo<?,?>.Par I think that the compiler thinks that

来源： https://www.jb51.net/article/74409.htm PHP5的另一个新成员是instdnceof关键字。使用这个关键字可以确定一个对象是类的实例、类的子类，还是实现了某个特定接口，并进行相应的操作。在某些情况下，我们希望确定某个类是否特定的类型，或者是否实现了特定的接口。instanceof操作符非常适合完成这个任务。instanceof操作符检查三件事情：实例是否某个特定的类型，实例是否从某个特定的类型继承，实例或者他的任何祖先类是否实现了特定的接口。例如，假设希望了解名为manager的对象是否为类Employee的实例： $manager = new Employee(); … if ($manager instanceof Employee) echo "Yes"; 有两点值得注意。首先，类名没有任何定界符（引号）。使用定界符将导致语法错误。其次，如果比较失败，脚本将退出执行。instanceof关键字在同时处理多个对象时特别有用。例如，你可能要重复地调用某个函数，但希望根据对象类型调整函数的行为。可以使用case语句和instanceof关键字来实现这个目标。 来源： https://www.cnblogs.com/laijinquan/p/11925440.html

in： 案例1、遍历对象： for(key in obj) { console.info( key+":"+obj[key]; ) }; 案例2、判断对象中是否有属性： "name" in obj 案例3、判断数组是否有此下标： 3 in ['a','b','c','d'] typeof： typeof可以判断一个值类型，对变量或值调用typeof运算符将返回下列字符串： Undefined："undefined" 布尔："boolean" 整数、浮点："number" 字符串："string" 函数："function"：如果变量是个函数 ( 判断事件是否被注册： if(typeof window.onload=="function"){alert("yes")} ) 除了基本类型和function，其他类型都返回："object" instanceof： typeof可以用来检测基础类型的值，但是在检测引用类型的值时用处不大。可以使用instanceof。 arr instanceof Array 判断一个对象是不是某个类型(只能判断引用类型) delete： 删除对象属性、数组元素、变量 例：delete a.name;(delete a["name"]) 删除数组中的值：delete arr[0];(arr[0]=undefined) 删除变量：delete a; 来源：

The idea I need to create Commands. Commands can be configured with parameters. Not every command can receive the same parameters. So some have to be ignored. I have an abstract class Command in which I've defined a Builder. By default every append parameter throws 'UnsupportedOperationException' public abstract class Command { public static abstract class CommandBuilder { // TODO instanceof. How to do this better? public CommandBuilder append(Parameter p) throws UnsupportedOperationException { if (p instanceof URLParameter) return append((URLParameter) p); if (p instanceof ActionParameter)