Question

考虑创建一个新的Object并将其转换为捕获异常的try-catch块中的String。一切似乎都没问题。但是如果我们在参数化函数中的类似try-catch块中将Object转换为T会发生什么？没有捕获异常，程序崩溃引发ClassCastException。

更新：感谢您的回答和评论。但只是为了澄清：我理解为什么异常上升（因为显然将Object转换为String）。

问题是：为什么它在方法体中的运行时没有捕获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：如果我们只是调用它而没有得到如下结果，那么参数化方法不会失败：

test.<String>parametrizedCastTest();

logcat的

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

由于实现了类型擦除的方式，

final T ret = (T) obj;

实际上已翻译为

final Object ret = (Object) obj;

并且parametrizedCastTest的实际实现会返回Object，但无论在哪里调用，都会在那里完成转换：

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

变为

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

这就是为什么CCE发生在与你预期不同的地方。

作为一般规则，泛型方法是通过用它们的上限（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秘密地将实际的转换为你的类型应该离开。

Answer 2

你应该在这里发出警告

final T ret = (T) obj;

关于一个不安全的演员。

在泛型方法中，您对声明的类型参数的所有了解都是它的边界。您已将类型参数声明为

static <T> T parametrizedCastTest() {

没有界限。换句话说，它减少到Object。演员在这里

final T ret = (T) obj;

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东西。在你的例子中它是。

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

您尝试将Object类型的对象分配给String类型的变量。这将永远不会奏效。

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

2 个答案: