安卓 java.lang.RuntimeException : Unable to instantiate activity ComponentInfo . .. java.lang.NullPointerException

Question

出于某种原因，每次尝试启动我的应用程序时，我都会收到以下错误:

Unable to instantiate activity ComponentInfo{com.example.lifeahead/com.example.lifeahead.MainActivity}:java.lang.NullPointerException

我检查了我的 list 文件，每个 Activity 都已添加。

启动应用程序时我使用的唯一方法是:

private void logIn(){
    Button logIn = (Button) findViewById(R.id.login);
    logIn.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            Intent loginIntent = new Intent(MainActivity.this, HomeActivity.class);
            loginIntent.putExtra("cmUN", cmUN);
            loginIntent.putExtra("cmPW", cmPW);
            startActivity(loginIntent);
        }
    });
}

这是 onCreate:

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment()).commit();
    }

    logIn();
}

完整日志:

E

/AndroidRuntime(1607): FATAL EXCEPTION: main
E/AndroidRuntime(1607): Process: com.example.lifeahead, PID: 1607
E/AndroidRuntime(1607): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.lifeahead/com.example.lifeahead.MainActivity}: java.lang.NullPointerException
E/AndroidRuntime(1607):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2121)
E/AndroidRuntime(1607):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
E/AndroidRuntime(1607):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
E/AndroidRuntime(1607):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
E/AndroidRuntime(1607):     at android.os.Handler.dispatchMessage(Handler.java:102)
E/AndroidRuntime(1607):     at android.os.Looper.loop(Looper.java:136)
E/AndroidRuntime(1607):     at android.app.ActivityThread.main(ActivityThread.java:5017)
E/AndroidRuntime(1607):     at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime(1607):     at java.lang.reflect.Method.invoke(Method.java:515)
E/AndroidRuntime(1607):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
E/AndroidRuntime(1607):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
E/AndroidRuntime(1607):     at dalvik.system.NativeStart.main(Native Method)
E/AndroidRuntime(1607): Caused by: java.lang.NullPointerException
E/AndroidRuntime(1607):     at android.app.Activity.findViewById(Activity.java:1884)
E/AndroidRuntime(1607):     at com.example.lifeahead.MainActivity.<init>(MainActivity.java:22)
E/AndroidRuntime(1607):     at java.lang.Class.newInstanceImpl(Native Method)
E/AndroidRuntime(1607):     at java.lang.Class.newInstance(Class.java:1208)
E/AndroidRuntime(1607):     at android.app.Instrumentation.newActivity(Instrumentation.java:1061)
E/AndroidRuntime(1607):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2112)
E/AndroidRuntime(1607):     ... 11 more

我很感激能得到的所有帮助!

Answer 1

堆栈跟踪告诉你

  Button logIn = (Button) findViewById(R.id.login);

findViewById 返回的对象为 null，因为您拥有的唯一方法是 logIn()。 findViewById 返回 null 的唯一原因是因为您正在寻找不属于当前 Activity 的 View 层次结构的 View