java - Intent Extras 上的 NullPointerException

Question

我正在尝试使用Intent从MainActivity调用新的Activity。我正在尝试通过 Intent Extras 传递一些数据。下面是我调用新的 Activity 的代码。

public void tabSelector2(View v) {
    switch (v.getId()) {
        case R.id.veg_tab:
            currentDB = "veg";
            break;
        case R.id.meat_tab :
            currentDB = "meat";
            break;
        default:
            break;
    }

    Intent tabSelect = new Intent(this, TabSelector.class);
    tabSelect.putExtra("db_passed", currentDB);
    startActivity(tabSelect);
}

在Activity2的onCreate内部，我尝试使用我传递的变量currentDB。变量已正确传递。我使用 System.out.println() 打印了该值，并且 logcat 生成了正确的输出。但是当我尝试使用该值时，我收到了 NullPointerException 。 onCreate 的代码 fragment 如下:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    setTheme(android.R.style.Theme_Holo); // (for Android Built In Theme)

    mydb = new DBHelper(this);

    Bundle extras = getIntent().getExtras();
    if (extras != null) {
        currentDB = extras.getString("db_passed");
    }

    System.out.println(currentDB);    // Printing here, correct value

    currentList2 = mydb.getAllItemsAsCollection(currentDB);    // Works Perfectly
    itemAdder2.setDbName(currentDB);    // Produces an error :(
    itemAdder2.updateData2(currentList2);

    switch (currentDB) {
        case "veg" :
            setContentView(R.layout.vegetable);
            currentListView = (ListView) findViewById(R.id.veg_list);
            break;
        case "meat" :
            setContentView(R.layout.meat_drawer);
            currentListView = (ListView) findViewById(R.id.meat_list);
            break;
        default:
            break;
    }

    currentListView.setAdapter(itemAdder2);

    this.getWindow().setSoftInputMode(WindowManager.LayoutParams.SOFT_INPUT_STATE_ALWAYS_HIDDEN);
}

相应的setDBName方法非常简单。

public void setDbName(String dbName){
    this.dbName = dbName;
}

运行时出现以下错误...

08-02 13:20:23.757    2641-2701/com.example.user.reggaeshark I/OpenGLRenderer? Initialized EGL, version 1.4
08-02 13:20:23.838    2641-2701/com.example.user.reggaeshark D/OpenGLRenderer? Enabling debug mode 0
08-02 13:20:24.150    2641-2701/com.example.user.reggaeshark V/RenderScript? Application requested CPU execution
08-02 13:20:24.163    2641-2701/com.example.user.reggaeshark V/RenderScript? 0xb857f848 Launching thread(s), CPUs 4
08-02 13:20:25.817    2641-2659/com.example.user.reggaeshark W/art? Suspending all threads took: 8.192ms
08-02 13:20:25.843    2641-2641/com.example.user.reggaeshark I/System.out? meat
08-02 13:20:25.861    2641-2641/com.example.user.reggaeshark D/AndroidRuntime? Shutting down VM
08-02 13:20:25.868    2641-2641/com.example.user.reggaeshark E/AndroidRuntime? FATAL EXCEPTION: main
Process: com.example.user.reggaeshark, PID: 2641
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.user.reggaeshark/com.example.user.reggaeshark.TabSelector}: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.user.reggaeshark.ListViewAdapter.setDbName(java.lang.String)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2314)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2388)
        at android.app.ActivityThread.access$800(ActivityThread.java:148)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1292)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5312)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:901)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:696)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.user.reggaeshark.ListViewAdapter.setDbName(java.lang.String)' on a null object reference
        at com.example.user.reggaeshark.TabSelector.onCreate(TabSelector.java:43)
        at android.app.Activity.performCreate(Activity.java:5953)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1128)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2267)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2388)
        at android.app.ActivityThread.access$800(ActivityThread.java:148)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1292)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5312)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:901)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:696)

请帮忙。如果你们需要任何额外的东西，请告诉我。

Answer 1

您的itemAdder2变量未初始化。尝试对空引用调用方法会导致您看到 NPE。