检查对象是否属于 Java 中的类
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Check if an object belongs to a class in Java
提问by chimeracoder
Is there an easy way to verify that an object belongs to a given class? For example, I could do
有没有一种简单的方法来验证一个对象是否属于给定的类？例如，我可以做
if(a.getClass() = (new MyClass()).getClass())
{
//do something
}
but this requires instantiating a new object on the fly each time, only to discard it. Is there a better way to check that "a" belongs to the class "MyClass"?
但这需要每次都动态实例化一个新对象，然后丢弃它。有没有更好的方法来检查“a”是否属于“MyClass”类？
采纳答案by dhm
The instanceofkeyword, as described by the other answers, is usually what you would want.
Keep in mind that instanceofwill return truefor superclasses as well.
如instanceof其他答案所述，关键字通常是您想要的。请记住，超类instanceof也会返回true。
If you want to see if an object is a direct instance of a class, you could compare the class. You can get the class object of an instance via getClass(). And you can statically access a specific class via ClassName.class.
如果你想查看一个对象是否是一个类的直接实例，你可以比较这个类。您可以通过 获取实例的类对象getClass()。您可以通过静态访问特定类ClassName.class。
So for example:
例如：
if (a.getClass() == X.class) {
// do something
}
In the above example, the condition is true if ais an instance of X, but not if ais an instance of a subclass of X.
在上面的例子中，如果a是 的实例，则条件为真X，但如果a是 的子类的实例，则条件不成立X。
In comparison:
相比下：
if (a instanceof X) {
// do something
}
In the instanceofexample, the condition is true if ais an instance of X, or if ais an instance of a subclassof X.
在instanceof示例中，如果a是 的实例X，或者如果a是 的子类的实例，则条件为真X。
Most of the time, instanceofis right.
大多数时候，instanceof是对的。
回答by Kos
Try operator instanceof.
试试运营商instanceof。
回答by Jacob Relkin
回答by gdejohn
回答by Justin Muller
I agree with the use of instanceofalready mentioned.
我同意使用instanceof已经提到的。
An additional benefit of using instanceofis that when used with a nullreference instanceofof will return false, while a.getClass()would throw a NullPointerException.
using 的另一个好处instanceof是，当与of的null引用一起使用时instanceof将返回false，而a.getClass()将抛出一个NullPointerException。
回答by Martin Konicek
The usual way would be:
通常的方法是：
if (a instanceof A)
However, there are cases when you can't do this, such as when Ain a generic argument.
但是，有些情况下您不能这样做，例如A在泛型参数中时。
Due to Java's type erasure, the following won't compile:
由于 Java 的类型擦除，以下将无法编译：
<A> boolean someMethod(Object a) {
if (a instanceof A)
...
}
and the following won't work (and will produce an unchecked castwarning):
并且以下将不起作用（并且会产生unchecked cast警告）：
<A> void someMethod(Object a) {
try {
A casted = (A)a;
} catch (ClassCastException e) {
...
}
}
You can't cast to Aat runtime, because at runtime, Ais essentially Object.
您不能A在运行时强制转换为，因为在运行时，A本质上是Object.
The solutions to such cases is to use a Classinstead of the generic argument:
这种情况的解决方案是使用 aClass而不是泛型参数：
void someMethod(Object a, Class<A> aClass) {
if (aClass.isInstance(a)) {
A casted = aClass.cast(a);
...
}
}
You can then call the method as:
然后，您可以调用该方法：
someMethod(myInstance, MyClass.class);
someMethod(myInstance, OtherClass.class);
