我有一个活动，其中包含指导下一个活动的按钮。
问题是包含这些按钮的第一个活动给我这个错误。
你能解释一下这个错误是什么吗？谢谢。
08-06 05:16:53.278: E/AndroidRuntime(771): FATAL EXCEPTION: main
08-06 05:16:53.278: E/AndroidRuntime(771): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.lspu.handbook/com.lspu.handbook.apr}: java.lang.NullPointerException
08-06 05:16:53.278: E/AndroidRuntime(771): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
08-06 05:16:53.278: E/AndroidRuntime(771): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
08-06 05:16:53.278: E/AndroidRuntime(771): at android.app.ActivityThread.access$600(ActivityThread.java:141)
08-06 05:16:53.278: E/AndroidRuntime(771): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
08-06 05:16:53.278: E/AndroidRuntime(771): at android.os.Handler.dispatchMessage(Handler.java:99)
08-06 05:16:53.278: E/AndroidRuntime(771): at android.os.Looper.loop(Looper.java:137)
08-06 05:16:53.278: E/AndroidRuntime(771): at android.app.ActivityThread.main(ActivityThread.java:5039)
08-06 05:16:53.278: E/AndroidRuntime(771): at java.lang.reflect.Method.invokeNative(Native Method)
08-06 05:16:53.278: E/AndroidRuntime(771): at java.lang.reflect.Method.invoke(Method.java:511)
08-06 05:16:53.278: E/AndroidRuntime(771): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
08-06 05:16:53.278: E/AndroidRuntime(771): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
08-06 05:16:53.278: E/AndroidRuntime(771): at dalvik.system.NativeStart.main(Native Method)
08-06 05:16:53.278: E/AndroidRuntime(771): Caused by: java.lang.NullPointerException
08-06 05:16:53.278: E/AndroidRuntime(771): at com.lspu.handbook.apr.onCreate(apr.java:35)
08-06 05:16:53.278: E/AndroidRuntime(771): at android.app.Activity.performCreate(Activity.java:5104)
08-06 05:16:53.278: E/AndroidRuntime(771): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
08-06 05:16:53.278: E/AndroidRuntime(771): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
08-06 05:16:53.278: E/AndroidRuntime(771): ... 11 more
这是我的代码：
public class apr extends Activity implements OnClickListener{
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
requestWindowFeature(Window.FEATURE_NO_TITLE);
getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
WindowManager.LayoutParams.FLAG_FULLSCREEN);
this.setContentView(R.layout.apr);
Button ar = (Button)findViewById(R.id.ar);
ar.setOnClickListener(this);
Button reg = (Button)findViewById(R.id.reg);
reg.setOnClickListener(this);
Button ta = (Button)findViewById(R.id.ta);
ta.setOnClickListener(this);
Button vr = (Button)findViewById(R.id.vr);
vr.setOnClickListener(this);
Button foreign = (Button)findViewById(R.id.foreign);
foreign.setOnClickListener(this);
Button non = (Button)findViewById(R.id.nonreg);
non.setOnClickListener(this);
Button cross = (Button)findViewById(R.id.cross);
cross.setOnClickListener(this);
Button scholar = (Button)findViewById(R.id.scholar);
scholar.setOnClickListener(this);
Button nxt = (Button)findViewById(R.id.lspu_nxt_btn);
nxt.setOnClickListener(this);
StartAnimations();
}
private void StartAnimations() {
Animation anim = AnimationUtils.loadAnimation(this, R.anim.alpha);
anim.reset();
LinearLayout l=(LinearLayout) findViewById(R.id.lspulayout);
l.clearAnimation();
l.startAnimation(anim);
//back button
Button backButton = (Button)findViewById(R.id.lspu_bak_btn);
backButton.setOnClickListener(new OnClickListener() {
public void onClick(View v) {
Intent instructionsIntent = new Intent(apr.this,MainBookActivity.class);
startActivity(instructionsIntent);
finish();
}
});
}
@Override
public void onDestroy(){
super.onDestroy();
System.gc();
finish();
}
@Override
public void onClick(View v) {
Intent intent = null;
switch( v.getId() ) {
case R.id.ar:
intent = new Intent(this, Hand_Book_ar.class);
break;
case R.id.reg:
intent = new Intent(this, Hand_Book_reg.class);
break;
case R.id.ta:
intent = new Intent(this, Hand_Book_ta.class);
break;
case R.id.vr:
intent = new Intent(this, Hand_Book_vr.class);
break;
case R.id.foreign:
intent = new Intent(this, Hand_Book_foreign.class);
break;
case R.id.nonreg:
intent = new Intent(this, Hand_Book_nonreg.class);
break;
case R.id.cross:
intent = new Intent(this, Hand_Book_cross.class);
break;
case R.id.scholar:
intent = new Intent(this, Hand_Book_scholar.class);
break;
case R.id.lspu_nxt_btn:
intent = new Intent(this, apr2.class);
break;
default:
Toast toast = Toast.makeText(this, "Do I missed the button you clicked?", Toast.LENGTH_SHORT);
toast.show();
break;
}
// Open images
if (intent != null) {
intent.setAction(intent.ACTION_VIEW);
startActivity(intent);
}
}
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
//Handle the back button
if(keyCode == KeyEvent.KEYCODE_BACK) {
//Ask the user if they want to quit
new AlertDialog.Builder(this)
.setIcon(android.R.drawable.ic_dialog_alert)
.setTitle(R.string.quit)
.setMessage(R.string.really_quit)
.setPositiveButton(R.string.yes, new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
//Stop the activity
apr.this.finish();
}
})
.setNegativeButton(R.string.no, null)
.show();
return true;
}
else {
return super.onKeyDown(keyCode, event);
}
}
}
这是xml：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:background="@drawable/menubg"
android:orientation="vertical" >
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="match_parent" >
<LinearLayout
android:id="@+id/lspulayout"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >
<ImageView
android:id="@+id/tirlw"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="right"
android:src="@drawable/titleapr" />
<Button
android:id="@+id/ar"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:background="@drawable/btnallctr"
android:gravity="top|center"
android:text="Admission Requirements"
android:textSize="17dp" />
<Button
android:id="@+id/reg"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:background="@drawable/btnallctr"
android:gravity="top|center"
android:text="Registration"
android:textSize="17dp" />
<Button
android:id="@+id/ta"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:background="@drawable/btnallctr"
android:gravity="top|center"
android:text="Transfer Admision"
android:textSize="17dp" />
<Button
android:id="@+id/vr"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:background="@drawable/btnallctr"
android:gravity="top|center"
android:text="Validation Requirements"
android:textSize="17dp" />
<Button
android:id="@+id/foreign"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:background="@drawable/btnallctr"
android:gravity="top|center"
android:text="Foreign Student Admission"
android:textSize="16sp" />
<Button
android:id="@+id/nonreg"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:background="@drawable/btnallctr"
android:gravity="top|center"
android:text="Non-Regular Admission"
android:textSize="17sp" />
<Button
android:id="@+id/cross"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:background="@drawable/btnallctr"
android:gravity="top|center"
android:text="Cross Registration"
android:textSize="18sp" />
<Button
android:id="@+id/scholar"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:background="@drawable/btnallctr"
android:gravity="top|center"
android:text="Scholarship"
android:textSize="18sp" />
<RelativeLayout
android:layout_width="fill_parent"
android:layout_height="wrap_content" >
<Button
android:id="@+id/lspu_bak_btn"
android:layout_width="40dp"
android:layout_height="30dp"
android:layout_gravity="bottom|left"
android:layout_marginTop="10dp"
android:background="@drawable/backbtncntrl_hd" />
<Button
android:id="@+id/lspu_nxt_btn"
android:layout_width="40dp"
android:layout_height="30dp"
android:layout_alignParentBottom="true"
android:layout_alignParentRight="true"
android:layout_alignTop="@+id/lspu_bak_btn"
android:background="@drawable/nxtbtncntrl_hd" />
</RelativeLayout>
</LinearLayout>
</RelativeLayout>
</LinearLayout>
给那些帮助的人，谢谢！
答案 0 :(得分：0)
at com.lspu.handbook.apr.onCreate(apr.java:35)指出按钮未初始化或换句话说它为空。在您的代码中，您正在Button方法中实例化onCreate()对象，因此一旦onCreate（）返回，所有这些按钮的范围都是onCreate()的范围，这些按钮超出了范围，所以当您单击任何按钮时，您将获得NullPointerException。简单的解决方案是在方法外部创建所有按钮对象，然后在onCreate()内实例化它们。
Button ar,reg,ta,vr,foreign,non,cross,scholar,nxt;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
requestWindowFeature(Window.FEATURE_NO_TITLE);
getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
WindowManager.LayoutParams.FLAG_FULLSCREEN);
this.setContentView(R.layout.apr);
ar = (Button)findViewById(R.id.ar);
ar.setOnClickListener(this);
reg = (Button)findViewById(R.id.reg);
reg.setOnClickListener(this);
ta = (Button)findViewById(R.id.ta);
ta.setOnClickListener(this);
vr = (Button)findViewById(R.id.vr);
vr.setOnClickListener(this);
foreign = (Button)findViewById(R.id.foreign);
foreign.setOnClickListener(this);
non = (Button)findViewById(R.id.nonreg);
non.setOnClickListener(this);
cross = (Button)findViewById(R.id.cross);
cross.setOnClickListener(this);
scholar = (Button)findViewById(R.id.scholar);
scholar.setOnClickListener(this);
nxt = (Button)findViewById(R.id.lspu_nxt_btn);
nxt.setOnClickListener(this);
StartAnimations();
}