Java 泛型：比较 Object o 和 <E> 的类
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Java Generics: Comparing the class of Object o to <E>
提问by SCdF
Let's say I have the following class:
假设我有以下课程：
public class Test<E> {
public boolean sameClassAs(Object o) {
// TODO help!
}
}
How would I check that ois the same class as E?
我将如何检查o与E?
Test<String> test = new Test<String>();
test.sameClassAs("a string"); // returns true;
test.sameClassAs(4); // returns false;
I can't change the method signature from (Object o)as I'm overridding a superclass and so don't get to choose my method signature.
我无法更改方法签名，(Object o)因为我正在覆盖超类，因此无法选择我的方法签名。
I would also rather not go down the road of attempting a cast and then catching the resulting exception if it fails.
我也宁愿不尝试强制转换，然后在失败时捕获由此产生的异常。
采纳答案by Tom Hawtin - tackline
An instance of Testhas no information as to what Eis at runtime. So, you need to pass a Class<E>to the constructor of Test.
的实例Test没有关于E运行时什么的信息。因此，您需要将 a 传递Class<E>给 Test 的构造函数。
public class Test<E> {
private final Class<E> clazz;
public Test(Class<E> clazz) {
if (clazz == null) {
throw new NullPointerException();
}
this.clazz = clazz;
}
// To make things easier on clients:
public static <T> Test<T> create(Class<T> clazz) {
return new Test<T>(clazz);
}
public boolean sameClassAs(Object o) {
return o != null && o.getClass() == clazz;
}
}
If you want an "instanceof" relationship, use Class.isAssignableFrominstead of the Classcomparison. Note, Ewill need to be a non-generic type, for the same reason Testneeds the Classobject.
如果您想要“instanceof”关系，请使用Class.isAssignableFrom而不是Class比较。注意，E将需要是非泛型类型，出于同样的原因Test需要Class对象。
For examples in the Java API, see java.util.Collections.checkedSetand similar.
有关 Java API 中的示例，请参阅java.util.Collections.checkedSet和类似内容。
回答by Slartibartfast
I could only make it working like this:
我只能让它像这样工作：
public class Test<E> {
private E e;
public void setE(E e) {
this.e = e;
}
public boolean sameClassAs(Object o) {
return (o.getClass().equals(e.getClass()));
}
public boolean sameClassAs2(Object o) {
return e.getClass().isInstance(o);
}
}
回答by Nick Fortescue
The method I've always used is below. It is a pain and a bit ugly, but I haven't found a better one. You have to pass the class type through on construction, as when Generics are compiled class information is lost.
我一直使用的方法如下。这是一种痛苦，有点难看，但我还没有找到更好的。您必须在构造时传递类类型，因为编译泛型时类信息会丢失。
public class Test<E> {
private Class<E> clazz;
public Test(Class<E> clazz) {
this.clazz = clazz;
}
public boolean sameClassAs(Object o) {
return this.clazz.isInstance(o);
}
}
回答by Ayman
I was just trying to do the same thing, and one neat trick i just realized is that you can can try a cast, and if the cast fails, ClassCastException will be thrown. You can can catch that, and do whatever.
我只是想做同样的事情，我刚刚意识到的一个巧妙技巧是您可以尝试强制转换，如果强制转换失败，将抛出 ClassCastException。你可以抓住它，做任何事情。
so your sameClassAs method should look like:
所以你的 sameClassAs 方法应该是这样的：
public boolean sameClassAs(Object o) {
boolean same = false;
try {
E t = (E)o;
same = true;
} catch (ClassCastException e) {
// same is false, nothing else to do
} finally {
return same;
}
}
