NullPointerException异常按钮的OnClickListener [英] NullPointerException on button's OnClickListener
本文介绍了NullPointerException异常按钮的OnClickListener的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
我正在写一个人工智能学期项目中的Android游戏应用程序。我得到的主类的38号线，这是从屏幕上推出新的游戏按钮setOnClickListener一个NullPointerException异常。
I'm writing an Android game app for an Artificial Intelligence semester project. I'm getting a NullPointerException on Line 38 of the main class, which is the setOnClickListener for the new game button from the launch screen.
相关章节类WW3Activity：
Relevant sections Class WW3Activity:
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);*/
}
而LogCat中：
And the 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
startscreen.xml：
startscreen.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>
我本来有一个NullPointerException异常，因为我宣布组按钮之前，我改变了布局视图启动画面，但我固定的，再发生这种情况。从技术上讲，这是功课，但我不认为它值得作业标签，因为这不是一门功课-Y型的问题，因为这个问题没有任何关系与实际分配本身。
I originally had a NullPointerException because I declared the set of buttons before I changed the layout view to startscreen, but I fixed that, and then this happened. Technically this IS homework, but I don't think that it warrants the homework tag because this isn't a homework-y type of problem, because the problem has nothing to do with the actual assignment itself.
推荐答案
的唯一原因 findViewById（）将返回空是因为你正在寻找的视图不属于当前视图。 （您的setContentView设置的视图（））。如果 R.id.buttonNewGame 是 R.layout.startscreen ，尝试清理和重建项目。
The only reason why findViewById() would return null is because the View you are looking for does not belong to the current View. (The View you set with setContentView()). If R.id.buttonNewGame is in R.layout.startscreen, try to clean and rebuild your project.
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