Activity onCreate 上的 NullPointerException

Question

我收到这个错误，我不知道为什么。以前使用过相同的代码；在某个时候更新后问题开始出现。

堆栈跟踪：

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.domain.appname, PID: 17964
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.domain.appname/com.domain.appname.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.domain.appname.NavigationDrawerFragment.setUp(int, android.support.v4.widget.DrawerLayout)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2379)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2441)
    at android.app.ActivityThread.access0(ActivityThread.java:162)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1349)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:135)
    at android.app.ActivityThread.main(ActivityThread.java:5431)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:914)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:707)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.domain.appname.NavigationDrawerFragment.setUp(int, android.support.v4.widget.DrawerLayout)' on a null object reference
    at com.domain.appname.MainActivity.onCreate(MainActivity.java:40)
    at android.app.Activity.performCreate(Activity.java:6056)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2332)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2441) 
    at android.app.ActivityThread.access0(ActivityThread.java:162) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1349) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:135) 
    at android.app.ActivityThread.main(ActivityThread.java:5431) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at java.lang.reflect.Method.invoke(Method.java:372) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:914) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:707)

这是 MainActivity.java 中的违规代码：

...

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    if (BuildConfig.DEBUG){ Log.d(TAG, "Created in debug mode"); }

    NavigationDrawerFragment mNavigationDrawerFragment = (NavigationDrawerFragment)
        getSupportFragmentManager().findFragmentById(R.id.navigation_drawer);

    // Set up the drawer.
    Log.d(TAG, "Setting up drawer");
    mNavigationDrawerFragment.setUp( // Line 40
        R.id.navigation_drawer,
        (DrawerLayout) findViewById(R.id.drawer_layout)
    );

...

我发现两个问题似乎有类似的问题。（这里是 the first and ）第一个似乎不相关，第二个没有答案。

我很困惑。 drawer_layout 和 navigation_drawer 做 而不是 抛出 "symbol unresolved" 错误。就像我说的，这一切过去都有效......

Answer 1

您永远不会打电话给 Activity.setContentView(int)。应该是这样的：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.layout_containing_drawer);

    // Rest.
}

Activity onCreate 上的 NullPointerException

NullPointerException on Activity onCreate

java

android

navigation-drawer

drawerlayout