Java中的参数化方法：try-catch无法捕获ClassCastException（在Android上）

Question

考虑创建一个新的Object并将其强制转换为捕获异常的try-catch块中的String。 一切似乎都还好。 但是，如果我们在参数化函数的类似try-catch块中将Object强制转换为T，会发生什么情况呢？ 未捕获到异常，程序崩溃，引发ClassCastException。

更新：感谢您的回答和评论。 只是为了澄清一下：我理解为什么会引发异常（因为显然将Object转换为String）。

问题是 ：为什么在运行时方法主体中没有捕获ClassCastException？

class test
{
    public void doTests()
    {
        Log.i("", "====== CLASS CAST TEST ======");
        Log.i("", "====== REGULAR CAST TEST ");
        final String test1 = test.regularCastTest(); // the exception thrown is catched within the     method
        Log.i("", "====== PARAMETRIZED CAST TEST ");
        final String test2 = test.<String>parametrizedCastTest(); // fails here with     ClassCastException without catching it
        Log.i("", "------ CLASS CAST TEST END --");
    }   
    static <T> T parametrizedCastTest()
    {
        Object obj = new Object();
        try
        {
            final T ret = (T) obj;
            Log.d("", String.format("%s; %s",     obj.getClass().getSimpleName(), ret.getClass().getSimpleName())); // does not fail and prints 'Object; Object'
            return ret;
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }

        return null;
    }

    static String regularCastTest()
    {
        Object obj = new Object();
        try
        {
            final String ret = (String) obj;
            return ret;
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
        return "";
    }
}

PS：参数化方法不会失败，只要我们简单地调用它而不会得到如下结果：

test.<String>parametrizedCastTest();

LOGCAT

11-26 19:13:37.692: I/(13018): ====== CLASS CAST TEST ======
11-26 19:13:37.692: I/(13018): ====== REGULAR CAST TEST 
11-26 19:13:37.692: W/System.err(13018): java.lang.ClassCastException: java.lang.Object cannot be cast to java.lang.String
11-26 19:13:37.692: W/System.err(13018):    at com.kaypu.goods.test.regularCastTest(test.java:57)
11-26 19:13:37.692: W/System.err(13018):    at com.kaypu.goods.test.go1(test.java:30)
11-26 19:13:37.692: W/System.err(13018):    at com.kaypu.goods.AppGoods.doTests(AppGoods.java:285)
11-26 19:13:37.692: W/System.err(13018):    at com.kaypu.goods.AppGoods.onCreate(AppGoods.java:188)
11-26 19:13:37.692: W/System.err(13018):    at android.app.Instrumentation.callApplicationOnCreate(Instrumentation.java:1007)
11-26 19:13:37.692: W/System.err(13018):    at android.app.ActivityThread.handleBindApplication(ActivityThread.java:4473)
11-26 19:13:37.692: W/System.err(13018):    at android.app.ActivityThread.access$1500(ActivityThread.java:144)
11-26 19:13:37.692: W/System.err(13018):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1265)
11-26 19:13:37.692: W/System.err(13018):    at android.os.Handler.dispatchMessage(Handler.java:102)
11-26 19:13:37.692: W/System.err(13018):    at android.os.Looper.loop(Looper.java:136)
11-26 19:13:37.692: W/System.err(13018):    at android.app.ActivityThread.main(ActivityThread.java:5146)
11-26 19:13:37.692: W/System.err(13018):    at java.lang.reflect.Method.invokeNative(Native Method)
11-26 19:13:37.692: W/System.err(13018):    at java.lang.reflect.Method.invoke(Method.java:515)
11-26 19:13:37.692: W/System.err(13018):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:732)
11-26 19:13:37.692: W/System.err(13018):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:566)
11-26 19:13:37.692: W/System.err(13018):    at dalvik.system.NativeStart.main(Native Method)
11-26 19:13:37.692: I/(13018): ====== PARAMETRIZED CAST TEST 
11-26 19:13:37.692: D/(13018): Object; Object
11-26 19:13:37.692: D/AndroidRuntime(13018): Shutting down VM
11-26 19:13:37.692: W/dalvikvm(13018): threadid=1: thread exiting with uncaught exception (group=0x415a2ce0)
11-26 19:13:37.692: E/AndroidRuntime(13018): FATAL EXCEPTION: main
11-26 19:13:37.692: E/AndroidRuntime(13018): Process: com.kaypu.goods, PID: 13018
11-26 19:13:37.692: E/AndroidRuntime(13018): java.lang.RuntimeException: Unable to create application com.kaypu.goods.AppGoods: java.lang.ClassCastException: java.lang.Object cannot be cast to java.lang.String
11-26 19:13:37.692: E/AndroidRuntime(13018):    at android.app.ActivityThread.handleBindApplication(ActivityThread.java:4476)
11-26 19:13:37.692: E/AndroidRuntime(13018):    at android.app.ActivityThread.access$1500(ActivityThread.java:144)
11-26 19:13:37.692: E/AndroidRuntime(13018):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1265)
11-26 19:13:37.692: E/AndroidRuntime(13018):    at android.os.Handler.dispatchMessage(Handler.java:102)
11-26 19:13:37.692: E/AndroidRuntime(13018):    at android.os.Looper.loop(Looper.java:136)
11-26 19:13:37.692: E/AndroidRuntime(13018):    at android.app.ActivityThread.main(ActivityThread.java:5146)
11-26 19:13:37.692: E/AndroidRuntime(13018):    at java.lang.reflect.Method.invokeNative(Native Method)
11-26 19:13:37.692: E/AndroidRuntime(13018):    at java.lang.reflect.Method.invoke(Method.java:515)
11-26 19:13:37.692: E/AndroidRuntime(13018):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:732)
11-26 19:13:37.692: E/AndroidRuntime(13018):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:566)
11-26 19:13:37.692: E/AndroidRuntime(13018):    at dalvik.system.NativeStart.main(Native Method)
11-26 19:13:37.692: E/AndroidRuntime(13018): Caused by: java.lang.ClassCastException: java.lang.Object cannot be cast to java.lang.String
11-26 19:13:37.692: E/AndroidRuntime(13018):    at com.kaypu.goods.test.go1(test.java:32)
11-26 19:13:37.692: E/AndroidRuntime(13018):    at com.kaypu.goods.AppGoods.doTests(AppGoods.java:285)
11-26 19:13:37.692: E/AndroidRuntime(13018):    at com.kaypu.goods.AppGoods.onCreate(AppGoods.java:188)
11-26 19:13:37.692: E/AndroidRuntime(13018):    at android.app.Instrumentation.callApplicationOnCreate(Instrumentation.java:1007)
11-26 19:13:37.692: E/AndroidRuntime(13018):    at android.app.ActivityThread.handleBindApplication(ActivityThread.java:4473)
11-26 19:13:37.692: E/AndroidRuntime(13018):    ... 10 more

Answer 1

由于类型擦除的实现方式，

final T ret = (T) obj;

实际上被翻译成

final Object ret = (Object) obj;

并且parametrizedCastTest的实际实现返回一个Object ，但是在任何被调用的地方，强制转换都在此处完成：

final String test2 = Test.<String> parametrizedCastTest();

变成

final String test2 = (String) Test.parametrizedCastTest();

这就是为什么CCE发生在与您预期不同的地方。

通常，通用方法是通过用类型变量的上限（在本例中为Object）替换类型变量来实现的，然后它们的调用站点会秘密地将实际类型强制转换为您应该退出的类型。

Answer 2

你应该在这里得到警告

final T ret = (T) obj;

关于不安全的演员表

在泛型方法中，您所知道的关于声明的类型参数的全部是其界限。 您已将type参数声明为

static <T> T parametrizedCastTest() {

没有界限。 换句话说，它简化为Object 。 演员在这里

final T ret = (T) obj;

这是不安全的，因为用作返回类型的T实际上可以是其他任何值。 在您的示例中是。

final String test2 = Test.<String> parametrizedCastTest(); // fails here with ClassCastException without catching it

您正在尝试将Object类型的Object分配给String类型的变量。 这将永远行不通。