Java Calendar.getTime() 在 Android 但不是 Java 項目中返回空異常消息

Question

我在 Eclipse 中有以下 Java 代碼：

public void onClick(View v) {
        if (v.getId()==R.id.button1){
            System.out.println("start...");
            GregorianCalendar cl = new GregorianCalendar(Locale.US);
            cl.setLenient(false);
            cl.clear();
            cl.set(2000, 10, 33);       
            System.out.println("date set");     
            try {
                cl.getTime();//validating the date
            } catch (Exception e) {
                String err = e.getMessage();
                System.out.println("error:"+err);
                if (err.equals("DAY_OF_MONTH")) System.out.println("wrong day");
            }
            System.out.println("finished");
        }       
    }

當我在 Android 項目中運行上述代碼時，遇到如下奇怪錯誤：

start...
date set
error:null
FATAL EXCEPTION: main
java.lang.NullPointerException
at com.example.tstcal.MainActivity.onClick
... (see complete stack trace at the end)

當我在 Java 項目中運行相同的代碼時，一切都很好：

start...
date set
error:DAY_OF_MONTH
wrong day
finished

很明顯 e.getMessage() 在 Android 中為空，但為什么？

完整的堆棧跟蹤：

09-22 16:52:27.960: I/System.out(1696): start...
09-22 16:52:27.960: I/System.out(1696): date set
09-22 16:52:28.004: I/System.out(1696): error:null
09-22 16:52:28.004: D/AndroidRuntime(1696): Shutting down VM
09-22 16:52:28.010: W/dalvikvm(1696): threadid=1: thread exiting with uncaught exception (group=0x40a13300)
09-22 16:52:28.021: E/AndroidRuntime(1696): FATAL EXCEPTION: main
09-22 16:52:28.021: E/AndroidRuntime(1696): java.lang.NullPointerException
09-22 16:52:28.021: E/AndroidRuntime(1696):     at com.example.tstcal.MainActivity.onClick(MainActivity.java:42)
09-22 16:52:28.021: E/AndroidRuntime(1696):     at android.view.View.performClick(View.java:4084)
09-22 16:52:28.021: E/AndroidRuntime(1696):     at android.view.View$PerformClick.run(View.java:16966)
09-22 16:52:28.021: E/AndroidRuntime(1696):     at android.os.Handler.handleCallback(Handler.java:615)
09-22 16:52:28.021: E/AndroidRuntime(1696):     at android.os.Handler.dispatchMessage(Handler.java:92)
09-22 16:52:28.021: E/AndroidRuntime(1696):     at android.os.Looper.loop(Looper.java:137)
09-22 16:52:28.021: E/AndroidRuntime(1696):     at android.app.ActivityThread.main(ActivityThread.java:4745)
09-22 16:52:28.021: E/AndroidRuntime(1696):     at java.lang.reflect.Method.invokeNative(Native Method)
09-22 16:52:28.021: E/AndroidRuntime(1696):     at java.lang.reflect.Method.invoke(Method.java:511)
09-22 16:52:28.021: E/AndroidRuntime(1696):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
09-22 16:52:28.021: E/AndroidRuntime(1696):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
09-22 16:52:28.021: E/AndroidRuntime(1696):     at dalvik.system.NativeStart.main(Native Method)

e.printStackTrace() 的輸出：

09-22 17:50:19.400: W/System.err(1936): java.lang.IllegalArgumentException
09-22 17:50:19.411: W/System.err(1936):     at java.util.GregorianCalendar.computeTime(GregorianCalendar.java:834)
09-22 17:50:19.411: W/System.err(1936):     at java.util.Calendar.getTimeInMillis(Calendar.java:1099)
09-22 17:50:19.421: W/System.err(1936):     at java.util.Calendar.getTime(Calendar.java:1085)
09-22 17:50:19.421: W/System.err(1936):     at com.example.tstcal.MainActivity.onClick(MainActivity.java:38)
09-22 17:50:19.421: W/System.err(1936):     at android.view.View.performClick(View.java:4084)
09-22 17:50:19.421: W/System.err(1936):     at android.view.View$PerformClick.run(View.java:16966)
09-22 17:50:19.421: W/System.err(1936):     at android.os.Handler.handleCallback(Handler.java:615)
09-22 17:50:19.431: W/System.err(1936):     at android.os.Handler.dispatchMessage(Handler.java:92)
09-22 17:50:19.464: W/System.err(1936):     at android.os.Looper.loop(Looper.java:137)
09-22 17:50:19.464: W/System.err(1936):     at android.app.ActivityThread.main(ActivityThread.java:4745)
09-22 17:50:19.464: W/System.err(1936):     at java.lang.reflect.Method.invokeNative(Native Method)
09-22 17:50:19.464: W/System.err(1936):     at java.lang.reflect.Method.invoke(Method.java:511)
09-22 17:50:19.464: W/System.err(1936):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
09-22 17:50:19.471: W/System.err(1936):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
09-22 17:50:19.471: W/System.err(1936):     at dalvik.system.NativeStart.main(Native Method)

Answer 1

我的猜測是您當前版本的 Android 正在使用替代 Java 源代碼，它與您的 Java 項目中的 Java 源代碼不同。

我正在觀察兩個源代碼（Android java 庫和源 Java），並且有一些不同之處。

Android Java 庫和 Java 庫：

/**
 * Computes {@code time} from the Calendar fields.
 *
 * @throws IllegalArgumentException
 *                if the time cannot be computed from the current field
 *                values.
 */
protected abstract void computeTime();

Android Java 源代碼中的 GregorianCalendar 拋出如下異常：

//If an exception occurs...
throw new IllegalArgumentException();

Java 源代碼中的 GregorianCalendar 拋出如下異常：

//If an exception occurs...
throw new IllegalArgumentException(getFieldName(field));

類 IllegalArgumentException 接收消息的參數，如下所示：

/**
 * Constructs a new {@code IllegalArgumentException} with the current stack
 * trace and the specified detail message.
 *
 * @param detailMessage
 *            the detail message for this exception.
 */
public IllegalArgumentException(String detailMessage) {
    super(detailMessage);
}

所以，使用Android Java源碼並沒有詳細的消息，而Java有。