我刚刚开始学习 Android 编程，遇到了麻烦。我正在使用《Android 编程大 Nerd 牧场指南》一书。我的 IDE 是 Eclipse 和 Genymotion 来模拟该应用程序。这是日志猫:
04-13 00:19:48.065: D/dalvikvm(1256): Late-enabling CheckJNI
04-13 00:19:48.781: D/AndroidRuntime(1256): Shutting down VM
04-13 00:19:48.785: W/dalvikvm(1256): threadid=1: thread exiting with uncaught exception (group=0xa4d8ab20)
04-13 00:19:48.785: E/AndroidRuntime(1256): FATAL EXCEPTION: main
04-13 00:19:48.785: E/AndroidRuntime(1256): Process: com.bignerdranch.android.geoquiz, PID: 1256
04-13 00:19:48.785: E/AndroidRuntime(1256): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.bignerdranch.android.geoquiz/com.bignerdranch.android.geoquiz.QuizActivity}: java.lang.NullPointerException
04-13 00:19:48.785: E/AndroidRuntime(1256): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
04-13 00:19:48.785: E/AndroidRuntime(1256): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
04-13 00:19:48.785: E/AndroidRuntime(1256): at android.app.ActivityThread.access$800(ActivityThread.java:135)
04-13 00:19:48.785: E/AndroidRuntime(1256): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
04-13 00:19:48.785: E/AndroidRuntime(1256): at android.os.Handler.dispatchMessage(Handler.java:102)
04-13 00:19:48.785: E/AndroidRuntime(1256): at android.os.Looper.loop(Looper.java:136)
04-13 00:19:48.785: E/AndroidRuntime(1256): at android.app.ActivityThread.main(ActivityThread.java:5017)
04-13 00:19:48.785: E/AndroidRuntime(1256): at java.lang.reflect.Method.invokeNative(Native Method)
04-13 00:19:48.785: E/AndroidRuntime(1256): at java.lang.reflect.Method.invoke(Method.java:515)
04-13 00:19:48.785: E/AndroidRuntime(1256): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
04-13 00:19:48.785: E/AndroidRuntime(1256): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
04-13 00:19:48.785: E/AndroidRuntime(1256): at dalvik.system.NativeStart.main(Native Method)
04-13 00:19:48.785: E/AndroidRuntime(1256): Caused by: java.lang.NullPointerException
04-13 00:19:48.785: E/AndroidRuntime(1256): at com.bignerdranch.android.geoquiz.QuizActivity.onCreate(QuizActivity.java:30)
04-13 00:19:48.785: E/AndroidRuntime(1256): at android.app.Activity.performCreate(Activity.java:5231)
04-13 00:19:48.785: E/AndroidRuntime(1256): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
04-13 00:19:48.785: E/AndroidRuntime(1256): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
04-13 00:19:48.785: E/AndroidRuntime(1256): ... 11 more
04-13 00:19:56.345: I/Process(1256): Sending signal. PID: 1256 SIG: 9
这是fragment_quiz.xml:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center"
android:orientation="vertical" >
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:padding="24dp"
android:text="@string/question_text" />
<LinearLayout
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="horizontal" >
<Button
android:id="@+id/true_button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/true_button" />
<Button
android:id="@+id/false_button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/false_button" />
</LinearLayout>
</LinearLayout>
QuizActivity.java 文件(大部分代码由 Eclipse 导入):
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v7.app.ActionBarActivity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.Toast;
public class QuizActivity extends ActionBarActivity {
private Button mTrueButton;
private Button mFalseButton;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_quiz);
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment()).commit();
}
mTrueButton = (Button) findViewById(R.id.true_button);
mTrueButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Toast.makeText(QuizActivity.this, R.string.incorrect_toast, Toast.LENGTH_SHORT)
.show();
}
});
mFalseButton = (Button) findViewById(R.id.false_button);
mFalseButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Toast.makeText(QuizActivity.this, R.string.correct_toast, Toast.LENGTH_SHORT)
.show();
}
});
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.quiz, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
/**
* A placeholder fragment containing a simple view.
*/
public static class PlaceholderFragment extends Fragment {
public PlaceholderFragment() {
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_quiz, container,
false);
return rootView;
}
}
}
我做错了什么？
最佳答案
将布局定义放入 PlaceholderFragment 类中。对于 findViewById，请使用 rootView.findViewById。对于上下文，请使用 getActivity()。
为什么这有效？实际上，屏幕上显示了 2 个独立的布局。主图显示了一切，包括一个 fragment 。 Fragment 基本上是一个独立的迷你 Activity ，在平板电脑开发中非常有用，甚至对于手机类型设备也有一定的用处。您的布局位于 fragment_quiz xml 文件中，该文件仅在 onCreateView 语句中的 fragment 内部膨胀。只要您在 findViewById 膨胀后使用它，就可以了。
关于java - 第一个程序导致大量错误，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/23037856/