使用“instanceof"在 Java 中 [英] Use of "instanceof" in Java

问题描述

'instanceof' 运算符的用途是什么?

我了解到 Java 有 instanceof 运算符.能否详细说明它的用途和优点是什么?

I learned that Java has the instanceof operator. Can you elaborate where it is used and what are its advantages?

推荐答案

基本上，您检查对象是否是特定类的实例.通常，当您拥有对超类或接口类型的对象的引用或参数，并且需要知道实际对象是否具有其他类型(通常更具体)时，通常会使用它.

Basically, you check if an object is an instance of a specific class. You normally use it, when you have a reference or parameter to an object that is of a super class or interface type and need to know whether the actual object has some other type (normally more concrete).

示例:

public void doSomething(Number param) {
  if( param instanceof Double) {
    System.out.println("param is a Double");
  }
  else if( param instanceof Integer) {
    System.out.println("param is an Integer");
  }

  if( param instanceof Comparable) {
    //subclasses of Number like Double etc. implement Comparable
    //other subclasses might not -> you could pass Number instances that don't implement that interface
    System.out.println("param is comparable"); 
  }
}

请注意，如果您必须经常使用该运算符，通常表明您的设计存在一些缺陷.因此，在设计良好的应用程序中，您应该尽可能少地使用该运算符(当然，该一般规则也有例外).

Note that if you have to use that operator very often it is generally a hint that your design has some flaws. So in a well designed application you should have to use that operator as little as possible (of course there are exceptions to that general rule).

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