考虑创建一个新对象并将其转换为捕获异常的 try-catch block 中的字符串。一切似乎都很好。但是,如果我们在参数化函数的类似 try-catch block 中将 Object 转换为 T 会发生什么?未捕获异常,程序崩溃并引发 ClassCastException。
更新:感谢您的回答和评论。但只是为了澄清:我理解为什么会出现异常(显然是因为将 Object 转换为 String)。
问题是:为什么它在运行时不在方法体中捕获 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:如果我们简单地调用它而没有得到如下结果,参数化方法不会失败:
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
最佳答案
由于类型删除的实现方式,
final T ret = (T) obj;
实际翻译成
final Object ret = (Object) obj;
parametrizedCastTest
的实际实现返回一个对象
,但在调用 的任何地方,转换都在那里完成:
final String test2 = Test.<String> parametrizedCastTest();
成为
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。
作为一般规则,泛型方法是通过将类型变量替换为它们的上限(在本例中为 Object)来实现的,然后它们的调用站点 secret 地进行实际转换为您的类型我们应该出去。
关于java - Java 中的参数化方法 : try-catch does not catch ClassCastException (on Android),我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/27153731/