这是我的主要活动：
package com.UAS.mathgame;
import android.app.Activity;
import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.app.AlertDialog;
import android.content.DialogInterface;
import android.content.Intent;
public class MainActivity extends Activity implements OnClickListener {
private Button playBtn, helpBtn, highBtn;
private String[] levelNames = {"Easy", "Medium", "Hard"};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
if (savedInstanceState == null) {
getFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment()).commit();
}
playBtn = (Button)findViewById(R.id.play_btn);
helpBtn = (Button)findViewById(R.id.help_btn);
highBtn = (Button)findViewById(R.id.high_btn);
playBtn.setOnClickListener(this);
helpBtn.setOnClickListener(this);
highBtn.setOnClickListener(this);
}
@Override
public void onClick(View view) {
if(view.getId()==R.id.play_btn){
AlertDialog.Builder builder = new AlertDialog.Builder(this);
builder.setTitle("Choose a level")
.setSingleChoiceItems(levelNames, 0, new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int which) {
dialog.dismiss();
//start gameplay
startPlay(which);
}
});
AlertDialog ad = builder.create();
ad.show();
}
else if(view.getId()==R.id.help_btn){
//how to play button
Intent helpIntent = new Intent(this, HowToPlay.class);
this.startActivity(helpIntent);
}
else if(view.getId()==R.id.high_btn){
//high scores button
Intent highIntent = new Intent(this, HighScores.class);
this.startActivity(highIntent);
}
}
private void startPlay(int chosenLevel)
{
//start gameplay
Intent playIntent = new Intent(this, PlayGame.class);
playIntent.putExtra("level", chosenLevel);
this.startActivity(playIntent);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, 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_main, container,
false);
return rootView;
}
}
}
这是我的logcat：
06-24 06:15:08.057: E/AndroidRuntime(1701): FATAL EXCEPTION: main
06-24 06:15:08.057: E/AndroidRuntime(1701): Process: com.UAS.mathgame, PID: 1701
06-24 06:15:08.057: E/AndroidRuntime(1701): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.UAS.mathgame/com.UAS.mathgame.MainActivity}: java.lang.NullPointerException
06-24 06:15:08.057: E/AndroidRuntime(1701): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
06-24 06:15:08.057: E/AndroidRuntime(1701): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
06-24 06:15:08.057: E/AndroidRuntime(1701): at android.app.ActivityThread.access$800(ActivityThread.java:135)
06-24 06:15:08.057: E/AndroidRuntime(1701): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
06-24 06:15:08.057: E/AndroidRuntime(1701): at android.os.Handler.dispatchMessage(Handler.java:102)
06-24 06:15:08.057: E/AndroidRuntime(1701): at android.os.Looper.loop(Looper.java:136)
06-24 06:15:08.057: E/AndroidRuntime(1701): at android.app.ActivityThread.main(ActivityThread.java:5017)
06-24 06:15:08.057: E/AndroidRuntime(1701): at java.lang.reflect.Method.invokeNative(Native Method)
06-24 06:15:08.057: E/AndroidRuntime(1701): at java.lang.reflect.Method.invoke(Method.java:515)
06-24 06:15:08.057: E/AndroidRuntime(1701): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
06-24 06:15:08.057: E/AndroidRuntime(1701): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
06-24 06:15:08.057: E/AndroidRuntime(1701): at dalvik.system.NativeStart.main(Native Method)
06-24 06:15:08.057: E/AndroidRuntime(1701): Caused by: java.lang.NullPointerException
06-24 06:15:08.057: E/AndroidRuntime(1701): at com.UAS.mathgame.MainActivity.onCreate(MainActivity.java:37)
06-24 06:15:08.057: E/AndroidRuntime(1701): at android.app.Activity.performCreate(Activity.java:5231)
06-24 06:15:08.057: E/AndroidRuntime(1701): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
06-24 06:15:08.057: E/AndroidRuntime(1701): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
06-24 06:15:08.057: E/AndroidRuntime(1701): ... 11 more
06-24 06:15:26.507: I/Process(1701): Sending signal. PID: 1701 SIG: 9
抱歉英语不好，谢谢你们帮助我们......
答案 0 :(得分：0)
改变这个..
setContentView(R.layout.activity_main);
到
setContentView(R.layout.fragment_main);
因为Button位于fragment_main.xml，所以setContentView应该引用fragment_main.xml
和删除
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment()).commit();
}
或者将您的Button初始化和听众移至PlaceholderFragment.class，如下所示
public static class PlaceholderFragment extends Fragment implements OnClickListener{
public PlaceholderFragment() {
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container,
false);
playBtn = (Button) rootView.findViewById(R.id.play_btn);
helpBtn = (Button) rootView.findViewById(R.id.help_btn);
highBtn = (Button) rootView.findViewById(R.id.high_btn);
playBtn.setOnClickListener(getActivity());
helpBtn.setOnClickListener(getActivity());
highBtn.setOnClickListener(getActivity());
return rootView;
}
@Override
public void onClick(View view) {
if(view.getId()==R.id.play_btn){
AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
builder.setTitle("Choose a level")
.setSingleChoiceItems(levelNames, 0, new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int which) {
dialog.dismiss();
//start gameplay
startPlay(which);
}
});
AlertDialog ad = builder.create();
ad.show();
}
else if(view.getId()==R.id.help_btn){
//how to play button
Intent helpIntent = new Intent(getActivity(), HowToPlay.class);
getActivity().startActivity(helpIntent);
}
else if(view.getId()==R.id.high_btn){
//high scores button
Intent highIntent = new Intent(getActivity(), HighScores.class);
getActivity().startActivity(highIntent);
}
}
}