启动应用程序时出现 NullPointerException

我厌倦了启动我的 Activity （在 AndroidManifest 中声明），但后来我看到了错误：

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

这意味着错误出现在第 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);
    }

我不知道为什么我有这个错误。该方法的调用方式如下：

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

我斗胆猜测你正在初始化pm在外面onCreate()。这会在你实际拥有之前尝试初始化它Context它会返回null。你需要在里面执行此操作onCreate()在调用使用它的函数之前。

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

    pm = getPackageManager();

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

任何需要一个Context里面一个Activity最早需要初始化onCreate()自从Activity和它的Context在此之前未初始化。