android - Android:即使在setContentView之后，findViewById返回Null

这是我在LoginActivity中的代码：

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

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

    loginButton = (Button)findViewById(R.id.loginButton);

    loginButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            onLoginButtonClicked();
        }
    });

    // Check if there is a currently logged in user
    // and they are linked to a Facebook account.
    ParseUser currentUser = ParseUser.getCurrentUser();
    if ((currentUser != null) && ParseFacebookUtils.isLinked(currentUser)) {
        // Go to the main activity
        showHomeActivity();
    }
}

在fragment_login.xml中

<Button
    android:id="@+id/loginButton"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"
    android:layout_centerVertical="true"
    android:text="@string/login_button" />

这是我运行应用程序时的日志：

E/AndroidRuntime(6647): FATAL EXCEPTION: main
E/AndroidRuntime(6647): Process: com.moostachestudio.drinkitapp, PID: 6647
E/AndroidRuntime(6647): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.moostachestudio.drinkitapp/com.moostachestudio.drinkitapp.LoginActivity}: java.lang.NullPointerException
E/AndroidRuntime(6647):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
E/AndroidRuntime(6647):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
E/AndroidRuntime(6647):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
E/AndroidRuntime(6647):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
E/AndroidRuntime(6647):     at android.os.Handler.dispatchMessage(Handler.java:102)
E/AndroidRuntime(6647):     at android.os.Looper.loop(Looper.java:136)
E/AndroidRuntime(6647):     at android.app.ActivityThread.main(ActivityThread.java:5017)
E/AndroidRuntime(6647):     at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime(6647):     at java.lang.reflect.Method.invoke(Method.java:515)
E/AndroidRuntime(6647):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
E/AndroidRuntime(6647):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
E/AndroidRuntime(6647):     at dalvik.system.NativeStart.main(Native Method)
E/AndroidRuntime(6647): Caused by: java.lang.NullPointerException
E/AndroidRuntime(6647):     at com.moostachestudio.drinkitapp.LoginActivity.onCreate(LoginActivity.java:45)
E/AndroidRuntime(6647):     at android.app.Activity.performCreate(Activity.java:5231)
E/AndroidRuntime(6647):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
E/AndroidRuntime(6647):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
E/AndroidRuntime(6647):     ... 11 more

问题是“ loginButton”为NULL，但我不知道为什么，因为我在setContentView之后调用了findViewById！

最佳答案

看来您的Button是在片段布局中，所以on click方法可能在您的片段（PlaceholderFragment）中，而不是您的活动中。您需要声明另一个使用Fragment扩展的类。

首先，创建一个名为PlaceHolderFragment的新类。用Fragment扩展它，并在下面添加以下代码：

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
    Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.fragment_login, container, false);
    // ...
    loginButton = (Button) rootView.findViewById(R.id.loginButton);
    loginButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            onLoginButtonClicked();
        }
    });
    // ...
    return rootView;
}

// put your method onLoginButtonClicked here.

然后，您的MainActivity（命名为activity_main）的布局活动可能具有一个ID容器为FrameLayout，如下所示：

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity" >

    <!-- other views -->

</FrameLayout>

最后，您的MainActivity仅具有：

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

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

    // ... without loginButton
    // and some more stuff
}