Java isInstance vs instanceOf运算符 [英] Java isInstance vs instanceOf operator

问题描述

整个泛型的东西有点像扔我一个循环，更多的是RTT。

The whole generics thing is kinda throwing me for a loop, and more so the RTT.

Specialis？好吧，这里是要点：

Specificis? Ah well here's the gist:

enum QueryHelper {
  query1,
  query2;
  static <T> QueryHelper getQueryHelper (Class<T> expectedReturn) {
    if (expectedReturn.isInstance (SomeRelatedClass.class))
      return query1;
    else
      return query2;
  }
}

然后我会这样称呼：

...
QueryHelper helper = QueryHelper.getQueryHelper(SomeRelatedClass.class);
...

这样我才能真正灵活地分配查询返回类型实际的帮手。它做了一些铸造和对象创建。我所看到的是，没有比赛，我应该以其他方式做到这一点吗？或者整个想法是不是很糟糕？

This is so that I can really flexibly assign the query return type in the actual helper. It does some casting and object creation. What I am seeing is that there is no match, should I be doing this some other way? Or is the whole idea just bad?

真正的核心是我不明白class.isInstance和instanceOf运算符之间的区别？我应该使用后者吗？

And the real heart of this is that I don't understand the difference between class.isInstance and the instanceOf operator? Should I be using the latter?

推荐答案

这样我才能真正灵活地在实际帮助器中分配查询返回类型。

This is so that I can really flexibly assign the query return type in the actual helper.

此方法的返回类型没有任何灵活性

There is nothing flexible about the return type of this method

static <T> QueryHelper getQueryHelper (Class<T> expectedReturn) {
    if (expectedReturn.isInstance (SomeRelatedClass.class))
      return query1;
    else
      return query2;
}

它将始终返回 QueryHelper的实例。如果您希望返回类型具有灵活性，则需要将其定义为：

It will always return an instance of QueryHelper. If you want the return type to be flexible you would need to define it as something like:

static <T> T getQueryHelper (Class<T> expectedReturn) {
}

现在返回类型很灵活，因为它将取决于参数的类型

Now the return type is flexible, because it will depend on the type of the argument

真正的核心是我不明白类之间的区别.isInstance和instanceOf运算符？

And the real heart of this is that I don't understand the difference between class.isInstance and the instanceOf operator?

不同之处在于instanceof执行在编译时修复的类型检查，例如：

The difference is that instanceof does a type-check that is fixed at compile-time, for example:

static boolean isInstance(Object myVar) {
    return (myVar instanceof Foo);
}

将始终检查myVar是否为Foo的实例，而

will always check that myVar is an instance of Foo, whereas

static <T> boolean isInstance(Object myVar, Class<T> expectedType) {
    return expectedType.isInstance(myVar);
}

将检查myVar是expectedType的实例，但expectedType可以是不同的每次调用方法时键入

will check that myVar is an instance of expectedType, but expectedType can be a different type each time the method is called

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