用Java中的泛型类型安全 [英] Type safety with generics in Java

问题描述

我遇到了Java中的泛型行为，我完全无法理解（使用我的.NET背景）。

I've encountered a behaviour of generics in Java that I completely can't understand (with my .NET background).

public class TestGeneric<T>
{
    public void get (Object arg)
    {
        T temp = (T) arg;

        System.out.println(temp.toString());

        return;
    }
}

TestGeneric<Integer> tg = new TestGeneric<Integer>();
tg.get("Crack!!!");

请告诉我为什么我没有得到ClassCastException，而且在Idea中我看到temp为字符串赋值后的值为Crack !!!。另外，我怎么可能会抛出ClassCastException？我在Windows 7 x64上使用JDK 1.7.0_07。

Please tell me why I'm not getting ClassCastException in get, moreover, in Idea I see temp as String after assignment and having value of "Crack!!!". Also, how could I have that ClassCastException throwed? I'm using JDK 1.7.0_07 on Windows 7 x64.

推荐答案

你没有得到类转换异常的原因是Java泛型通过类型擦除来实现。与需要对CLS进行重大更改的.NET泛型不同，Java泛型在编译时完全处理。在运行时，转换为 T 会被忽略。为了在运行时检查类型，您需要存储 Class< T> ，并使用它的方法来检查传入参数的类型：

The reason you are not getting a class cast exception is that Java generics are implemented through type erasure. Unlike .NET generics that required significant changes to CLS, Java generics are processed entirely in compile-time. At runtime, the cast to T is ignored. In order to check type at runtime, you need to store Class<T>, and use its methods to check the type of a parameter passed in:

public class TestGeneric<T>
{
    private Class<T> genClass;
    public TestGeneric(Class<T> t) {genClass = t;}
    public void get (Object arg)
    {
        if (!genClass.isInstance(arg)) {
            throw new ClassCastException();
        }
        T temp = (T) arg;

        System.out.println(temp.toString());

        return;
    }
}

TestGeneric<Integer> tg = new TestGeneric<Integer>(Integer.class);
tg.get("Bang!!!"); // Now that's a real Bang!!!

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