Question

我厌倦了开始我的Activity（在AndroidManifest中声明），但后来我看到了这个错误：

08-11 16:04:05.952: E/AndroidRuntime(815): FATAL EXCEPTION: main
08-11 16:04:05.952: E/AndroidRuntime(815): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.radzik.devadmin/com.radzik.devadmin.MainActivity}: java.lang.NullPointerException
08-11 16:04:05.952: E/AndroidRuntime(815):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
08-11 16:04:05.952: E/AndroidRuntime(815):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
08-11 16:04:05.952: E/AndroidRuntime(815):  at android.app.ActivityThread.access$600(ActivityThread.java:130)
08-11 16:04:05.952: E/AndroidRuntime(815):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
08-11 16:04:05.952: E/AndroidRuntime(815):  at android.os.Handler.dispatchMessage(Handler.java:99)
08-11 16:04:05.952: E/AndroidRuntime(815):  at android.os.Looper.loop(Looper.java:137)
08-11 16:04:05.952: E/AndroidRuntime(815):  at android.app.ActivityThread.main(ActivityThread.java:4745)
08-11 16:04:05.952: E/AndroidRuntime(815):  at java.lang.reflect.Method.invokeNative(Native Method)
08-11 16:04:05.952: E/AndroidRuntime(815):  at java.lang.reflect.Method.invoke(Method.java:511)
08-11 16:04:05.952: E/AndroidRuntime(815):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
08-11 16:04:05.952: E/AndroidRuntime(815):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
08-11 16:04:05.952: E/AndroidRuntime(815):  at dalvik.system.NativeStart.main(Native Method)
08-11 16:04:05.952: E/AndroidRuntime(815): Caused by: java.lang.NullPointerException
08-11 16:04:05.952: E/AndroidRuntime(815):  at com.radzik.devadmin.MainActivity.ustawWidokMain(MainActivity.java:107)
08-11 16:04:05.952: E/AndroidRuntime(815):  at com.radzik.devadmin.MainActivity.onCreate(MainActivity.java:35)
08-11 16:04:05.952: E/AndroidRuntime(815):  at android.app.Activity.performCreate(Activity.java:5008)
08-11 16:04:05.952: E/AndroidRuntime(815):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
08-11 16:04:05.952: E/AndroidRuntime(815):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
08-11 16:04:05.952: E/AndroidRuntime(815):  ... 11 more

这意味着错误在第107行：

private void ustawWidokMain()
{
    setContentView(R.layout.activity_main);
    Button b01 = (Button)findViewById(R.id.main_pass);
    Button b02 = (Button)findViewById(R.id.main_wipes);
    Button b03 = (Button)findViewById(R.id.main_camera);
    Button b04 = (Button)findViewById(R.id.main_enctpyion);
    Button b05 = (Button)findViewById(R.id.main_b1);
    if(!pm.hasSystemFeature(PackageManager.FEATURE_CAMERA) && !pm.hasSystemFeature(PackageManager.FEATURE_CAMERA_FRONT)) // there's error
    {
        b03.setEnabled(false);
    }

我不知道为什么会有这个错误。该方法被称为：

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        /* Some code has been removed */
        if(dcm.isAdminActive(c))
        ustawWidokMain();

Answer 1

我冒昧地猜测你是在pm之外初始化onCreate()。这会尝试在您实际拥有Context之前初始化它，它会返回null。您需要在onCreate()内部并在调用使用它的函数之前执行此操作。

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

    pm = getPackageManager();

    /* Some code has been removed */
    if(dcm.isAdminActive(c))
    ustawWidokMain();

Context内需要Activity内的onCreate()内的任何内容最早都需要在Activity Context进行初始化，因为{{1}}及其{{1}}在此之前初始化。

Answer 2

您是否在Manifest中声明了权限？

<uses-permission android:name="android.permission.CAMERA" />
<uses-feature android:name="android.hardware.camera" />
<uses-feature android:name="android.hardware.camera.autofocus" />

什么是pm？你在哪里实施它？好像你错过了。

Answer 3

这意味着您没有在此ustawWidokMain方法中声明或初始化该对象。请重新检查或调试哪一行会给您带来错误。

启动应用程序时出现NullPointerException

3 个答案: