我正在为人工智能学期项目编写 Android 游戏应用程序。我在主类的第 38 行收到 NullPointerException，它是启动屏幕中新游戏按钮的 setOnClickListener。
相关部分 WW3Activity 类：
public class WW3Activity extends Activity
{
public boolean DebugMode = false;
private String buildMenuEmplacement = "";
//[Initialization] Called when the app is first launched
@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.startscreen);
//[Initialization] Makes the button resources available to the class
final Button newGame = (Button) findViewById(R.id.buttonNewGame);
final Button loadGame = (Button) findViewById(R.id.buttonLoadGame);
final Button exitGame = (Button) findViewById(R.id.buttonExit);
final Button about = (Button) findViewById(R.id.buttonAbout);
final Button troll = (Button) findViewById(R.id.buttonTroll);
final Button debug = (Button) findViewById(R.id.buttonDebug);
//[Action] When user pushes the New Game button
newGame.setOnClickListener(new View.OnClickListener()
{
public void onClick(View v)
{
// Perform action on click
setContentView(R.layout.main);
buttonInitialize();
}
});
//[Action] When user pushes the Exit Game button
exitGame.setOnClickListener(new View.OnClickListener()
{
public void onClick(View v)
{
// Perform action on click
finish();
}
});
//[Action] When user pushes the Debug Mode button
exitGame.setOnClickListener(new View.OnClickListener()
{
public void onClick(View v)
{
// Perform action on click
DebugMode = true;
}
});
}
//[Initialization] Gets the emplacements ready to be pushed
public void buttonInitialize()
{
final ImageView pe1 = (ImageView) findViewById(R.id.playerEmplacement1);
final ImageView pe2 = (ImageView) findViewById(R.id.playerEmplacement2);
final ImageView pe3 = (ImageView) findViewById(R.id.playerEmplacement3);
final ImageView pc = (ImageView) findViewById(R.id.playerCity);
final ImageView ee1 = (ImageView) findViewById(R.id.enemyEmplacement1);
final ImageView ee2 = (ImageView) findViewById(R.id.enemyEmplacement2);
final ImageView ee3 = (ImageView) findViewById(R.id.enemyEmplacement3);
final ImageView ec = (ImageView) findViewById(R.id.enemyCity);
registerForContextMenu(findViewById(R.id.playerEmplacement1));
registerForContextMenu(findViewById(R.id.playerEmplacement2));
registerForContextMenu(findViewById(R.id.playerEmplacement3));
registerForContextMenu(findViewById(R.id.enemyEmplacement1));
registerForContextMenu(findViewById(R.id.enemyEmplacement2));
registerForContextMenu(findViewById(R.id.enemyEmplacement3));
/*empAction(pe1);
empAction(pe2);
empAction(pe3);
empAction(ee1);
empAction(ee2);
empAction(ee3);
empAction(ec);
empAction(pc);*/
}
还有 LogCat：
10-24 13:29:12.297: D/dalvikvm(440): GC_FOR_ALLOC freed 40K, 4% free 6356K/6595K, paused 177ms
10-24 13:29:12.303: I/dalvikvm-heap(440): Grow heap (frag case) to 6.653MB for 409616-byte allocation
10-24 13:29:12.503: D/dalvikvm(440): GC_FOR_ALLOC freed <1K, 5% free 6755K/7047K, paused 128ms
10-24 13:29:12.614: D/dalvikvm(440): GC_FOR_ALLOC freed 400K, 8% free 6762K/7303K, paused 57ms
10-24 13:29:12.663: D/AndroidRuntime(440): Shutting down VM
10-24 13:29:12.663: W/dalvikvm(440): threadid=1: thread exiting with uncaught exception (group=0x40014760)
10-24 13:29:12.683: E/AndroidRuntime(440): FATAL EXCEPTION: main
10-24 13:29:12.683: E/AndroidRuntime(440): java.lang.RuntimeException: Unable to start activity ComponentInfo{edu.mbijou.cosc473.ww3/edu.mbijou.cosc473.ww3.WW3Activity}: java.lang.NullPointerException
10-24 13:29:12.683: E/AndroidRuntime(440): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1815)
10-24 13:29:12.683: E/AndroidRuntime(440): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1831)
10-24 13:29:12.683: E/AndroidRuntime(440): at android.app.ActivityThread.access$500(ActivityThread.java:122)
10-24 13:29:12.683: E/AndroidRuntime(440): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1024)
10-24 13:29:12.683: E/AndroidRuntime(440): at android.os.Handler.dispatchMessage(Handler.java:99)
10-24 13:29:12.683: E/AndroidRuntime(440): at android.os.Looper.loop(Looper.java:132)
10-24 13:29:12.683: E/AndroidRuntime(440): at android.app.ActivityThread.main(ActivityThread.java:4123)
10-24 13:29:12.683: E/AndroidRuntime(440): at java.lang.reflect.Method.invokeNative(Native Method)
10-24 13:29:12.683: E/AndroidRuntime(440): at java.lang.reflect.Method.invoke(Method.java:491)
10-24 13:29:12.683: E/AndroidRuntime(440): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841)
10-24 13:29:12.683: E/AndroidRuntime(440): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599)
10-24 13:29:12.683: E/AndroidRuntime(440): at dalvik.system.NativeStart.main(Native Method)
10-24 13:29:12.683: E/AndroidRuntime(440): Caused by: java.lang.NullPointerException
10-24 13:29:12.683: E/AndroidRuntime(440): at edu.mbijou.cosc473.ww3.WW3Activity.onCreate(WW3Activity.java:38)
10-24 13:29:12.683: E/AndroidRuntime(440): at android.app.Activity.performCreate(Activity.java:4397)
10-24 13:29:12.683: E/AndroidRuntime(440): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1048)
10-24 13:29:12.683: E/AndroidRuntime(440): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1779)
10-24 13:29:12.683: E/AndroidRuntime(440): ... 11 more
启动屏幕.xml：
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<ImageView
android:id="@+id/logo"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_alignParentTop="true"
android:layout_marginLeft="493dp"
android:layout_marginTop="217dp"
android:src="@drawable/logo" >
</ImageView>
<Button
android:id="@+id/buttonNewGame"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_below="@+id/logo"
android:layout_marginLeft="200dp"
android:layout_marginTop="133dp"
android:clickable="true"
android:text="@string/bNewGame" >
</Button>
<Button
android:id="@+id/buttonLoadGame"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBottom="@+id/buttonNewGame"
android:layout_alignLeft="@+id/buttonTroll"
android:layout_alignRight="@+id/buttonTroll"
android:clickable="true"
android:text="@string/bLoadGame" >
</Button> <Button
android:id="@+id/buttonExit"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBaseline="@+id/buttonLoadGame"
android:layout_alignBottom="@+id/buttonLoadGame"
android:layout_alignLeft="@+id/buttonDebug"
android:layout_alignRight="@+id/buttonDebug"
android:layout_toRightOf="@+id/buttonLoadGame"
android:clickable="true"
android:text="@string/bExit" >
</Button>
<Button
android:id="@+id/buttonAbout"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/buttonNewGame"
android:layout_alignRight="@+id/buttonNewGame"
android:layout_below="@+id/buttonNewGame"
android:layout_marginTop="50dp"
android:text="@string/bAbout" />
<Button
android:id="@+id/buttonTroll"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBaseline="@+id/buttonAbout"
android:layout_alignBottom="@+id/buttonAbout"
android:layout_centerHorizontal="true"
android:text="@string/bTrollface" />
<Button
android:id="@+id/buttonDebug"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBottom="@+id/buttonTroll"
android:layout_toRightOf="@+id/buttonTroll"
android:layout_marginLeft="200dp"
android:text="@string/bDebug" />
</RelativeLayout>
我最初遇到了 NullPointerException，因为我在将布局视图更改为开始屏幕之前声明了一组按钮，但我修复了该问题，然后就发生了这种情况。从技术上讲，这是家庭作业，但我不认为它有必要贴上家庭作业标签，因为这不是家庭作业类型的问题，因为这个问题与实际作业本身无关。
唯一的原因是findViewById()会回来null是因为你要找的View不属于当前View。 （您设置的视图setContentView()). If R.id.buttonNewGame is in R.layout.startscreen，尝试清理并重建您的项目。
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