LogCat中未打印ArrayList值

Question

DBAdapter類的功能：

public void getValues(){

        Cursor mCursor = db.query(DBAdapter.DATABASE_TABLE2, new String[] {
                EDGE_ID,
                SOURCE_NODE_ID,
                DESTINATION_NODE_ID,
                VALUE1}, null, null, null, null, null);
        mCursor.moveToFirst();
        while(!mCursor.isAfterLast()) {
            mArrayList.add(mCursor.getString(mCursor.getColumnIndex(DBAdapter.SOURCE_NODE_ID)));
            mCursor.moveToNext();
        }

        for(String d1:mArrayList) {
            System.out.println(d1);
        }
    }

以下是主要活動類別：

public void onCreate(Bundle icicle) {
        super.onCreate(icicle);
        setContentView(R.layout.main);

    final DBAdapter db = new DBAdapter(this);
    db.open();
    db.getValues();
    db.close();
}

根據上面的代碼，我應該得到一個數組列表，其中包含我的數據庫表的值

SOURCE_NODE_ID

屬性。 我想在LogCat中打印此arraylist。 但是我得到以下錯誤：

05-30 14:51:39.435: E/AndroidRuntime(4954): FATAL EXCEPTION: main
05-30 14:51:39.435: E/AndroidRuntime(4954): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.navigationsystem/com.navigationsystem.NavigationSystemActivity}: java.lang.NullPointerException
05-30 14:51:39.435: E/AndroidRuntime(4954):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1956)
05-30 14:51:39.435: E/AndroidRuntime(4954):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981)
05-30 14:51:39.435: E/AndroidRuntime(4954):     at android.app.ActivityThread.access$600(ActivityThread.java:123)
05-30 14:51:39.435: E/AndroidRuntime(4954):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
05-30 14:51:39.435: E/AndroidRuntime(4954):     at android.os.Handler.dispatchMessage(Handler.java:99)
05-30 14:51:39.435: E/AndroidRuntime(4954):     at android.os.Looper.loop(Looper.java:137)
05-30 14:51:39.435: E/AndroidRuntime(4954):     at android.app.ActivityThread.main(ActivityThread.java:4424)
05-30 14:51:39.435: E/AndroidRuntime(4954):     at java.lang.reflect.Method.invokeNative(Native Method)
05-30 14:51:39.435: E/AndroidRuntime(4954):     at java.lang.reflect.Method.invoke(Method.java:511)
05-30 14:51:39.435: E/AndroidRuntime(4954):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
05-30 14:51:39.435: E/AndroidRuntime(4954):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
05-30 14:51:39.435: E/AndroidRuntime(4954):     at dalvik.system.NativeStart.main(Native Method)
05-30 14:51:39.435: E/AndroidRuntime(4954): Caused by: java.lang.NullPointerException
05-30 14:51:39.435: E/AndroidRuntime(4954):     at com.navigationsystem.DBAdapter.getValues(DBAdapter.java:117)
05-30 14:51:39.435: E/AndroidRuntime(4954):     at com.navigationsystem.NavigationSystemActivity.onCreate(NavigationSystemActivity.java:53)
05-30 14:51:39.435: E/AndroidRuntime(4954):     at android.app.Activity.performCreate(Activity.java:4465)
05-30 14:51:39.435: E/AndroidRuntime(4954):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
05-30 14:51:39.435: E/AndroidRuntime(4954):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1920)
05-30 14:51:39.435: E/AndroidRuntime(4954):     ... 11 more

這段代碼有什么問題。

謝謝。

Answer 1

05-30 14:51:39.435: E/AndroidRuntime(4954): Caused by: java.lang.NullPointerException 
05-30 14:51:39.435: E/AndroidRuntime(4954):     at com.navigationsystem.DBAdapter.getValues(DBAdapter.java:117)

您的代碼中有一個NullPointer 。

我認為您的getValue()方法中mArrayList為null 。 您在哪里創建該列表？

也許DBAdapter的db也為null 。 您是否嘗試過調試？ 你輸出變量了嗎？