UI部分单独工作正常，但后来当我编写OnClickListener并运行代码时，我收到一条错误消息，指出“遗憾的是应用停止了工作”。
我再次尝试了一个新项目我收到了这些错误。我已经附加了我的XML，MainActivity类和日志。请帮助我。我一直坚持这个简单的程序一整个星期。 ：（
日志：
05-18 01:54:31.015: D/Android :(1436): The onCreate() event
05-18 01:54:31.015: D/AndroidRuntime(1436): Shutting down VM
05-18 01:54:31.015: W/dalvikvm(1436): threadid=1: thread exiting with uncaught exception (group=0xb2afaba8)
05-18 01:54:31.055: E/AndroidRuntime(1436): FATAL EXCEPTION: main
05-18 01:54:31.055: E/AndroidRuntime(1436): Process: com.example.test3, PID: 1436
05-18 01:54:31.055: E/AndroidRuntime(1436): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.test3/com.example.test3.MainActivity}: java.lang.NullPointerException
05-18 01:54:31.055: E/AndroidRuntime(1436): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
05-18 01:54:31.055: E/AndroidRuntime(1436): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
05-18 01:54:31.055: E/AndroidRuntime(1436): at android.app.ActivityThread.access$800(ActivityThread.java:135)
05-18 01:54:31.055: E/AndroidRuntime(1436): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
05-18 01:54:31.055: E/AndroidRuntime(1436): at android.os.Handler.dispatchMessage(Handler.java:102)
05-18 01:54:31.055: E/AndroidRuntime(1436): at android.os.Looper.loop(Looper.java:136)
05-18 01:54:31.055: E/AndroidRuntime(1436): at android.app.ActivityThread.main(ActivityThread.java:5017)
05-18 01:54:31.055: E/AndroidRuntime(1436): at java.lang.reflect.Method.invokeNative(Native Method)
05-18 01:54:31.055: E/AndroidRuntime(1436): at java.lang.reflect.Method.invoke(Method.java:515)
05-18 01:54:31.055: E/AndroidRuntime(1436): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
05-18 01:54:31.055: E/AndroidRuntime(1436): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
05-18 01:54:31.055: E/AndroidRuntime(1436): at dalvik.system.NativeStart.main(Native Method)
05-18 01:54:31.055: E/AndroidRuntime(1436): Caused by: java.lang.NullPointerException
05-18 01:54:31.055: E/AndroidRuntime(1436): at com.example.test3.MainActivity.onCreate(MainActivity.java:25)
05-18 01:54:31.055: E/AndroidRuntime(1436): at android.app.Activity.performCreate(Activity.java:5231)
05-18 01:54:31.055: E/AndroidRuntime(1436): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
05-18 01:54:31.055: E/AndroidRuntime(1436): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
05-18 01:54:31.055: E/AndroidRuntime(1436): ... 11 more
---
我不确定我的MainActivity.java是否有任何问题。我在这里附上我的XMl文件和MainActivity.java以获得清晰的图片。
MainActivity.java：
package com.example.test3;
import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
public class MainActivity extends Activity {
String msg = "Android : ";
int counter;
Button add,sub;
TextView display;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Log.d(msg, "The onCreate() event");
counter=0;
add=(Button) findViewById(R.id.addButton);
sub=(Button) findViewById(R.id.subButton);
display=(TextView) findViewById(R.id.disp);
add.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
counter++;
display.setText("Count is "+counter);
}
});
sub.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
counter--;
display.setText("Count is "+counter);
}
});
}
}
XML文件：
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.test3.MainActivity$PlaceholderFragment" >
<Button
android:id="@+id/subButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_above="@+id/disp"
android:layout_toRightOf="@+id/addButton"
android:text="Sub 1" />
<Button
android:id="@+id/addButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_alignParentTop="true"
android:layout_marginLeft="58dp"
android:layout_marginTop="14dp"
android:text="Add 1" />
<TextView
android:id="@+id/disp"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/addButton"
android:layout_centerHorizontal="true"
android:layout_marginTop="56dp"
android:text="Count is 0"
android:textAppearance="?android:attr/textAppearanceLarge" />
</RelativeLayout>
答案 0 :(得分：2)
可能原因（猜测）视图属于片段布局。因此，应该在片段onCreateView中初始化视图，如下所示。
我注意到的另一件事是你在点击两次按钮时都counter++。点击子按钮
count--
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container,
false);
counter=0;
add=(Button) rootView.findViewById(R.id.Add);
sub=(Button) rootView.findViewById(R.id.Sub);
disp=(TextView) rootView.findViewById(R.id.Display);
add.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
counter++;
disp.setText("counter is "+counter);
}
});
sub.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
counter++;
disp.setText("counter is "+counter);
}
});
return rootView;
}