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

Question

Consider creating a new Object and casting it to String within a try-catch block that catches an Exception. Everything seems to be ok. But what does happen if we cast an Object to T within similar try-catch block in a parametrized function? The exception is not catched and the program crashes raising a ClassCastException.

Upd: thanks for answers and comments. But just to clarify: I understand why the exception is risen (because of casting Object to String obviously).

The question is : why does it NOT catch the ClassCastException during runtime in the method body?

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: the parametrized method does not fail if we simply call it without getting the result like:

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

Because of the way type erasure is implemented,

final T ret = (T) obj;

is actually translated to

final Object ret = (Object) obj;

and the actual implementation of parametrizedCastTest returns an Object , but anywhere it's called , the cast is done there:

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

becomes

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

which is why the CCE happens in a different place than you expected.

As a general rule, generic methods are implemented by replacing type variables by their upper bound (Object, in this case), and then their call sites secretly do the actual casting to the type you're supposed to get out.

Answer 2

You should have gotten a warning here

final T ret = (T) obj;

about an unsafe cast.

Within a generic method, all you know about a declared type parameter is its bound(s). You've declared the type parameter as

static <T> T parametrizedCastTest() {

with no bounds. In other words, it reduces to Object . The cast here

final T ret = (T) obj;

is unsafe because T , which is used as a return type, can actually be anything else. And in your example it is.

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

You're trying to assign an object of type Object to a variable of type String . This will never work.