这里的主要活动。
public class MainActivity extends Activity{
Button b;
@Override
public void onCreate(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
b = (Button) findViewById(R.id.button1);
b.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Intent intent = new Intent(MainActivity.this, SecondActivity.class);
startActivity(intent);
}
});
}
}
这里的SecondActivity（当我只运行第二次活动时，它工作正常。）
public class SecondActivity extends ListActivity {
ListView mylist;
ProgressDialog PD;
// JSON Node names
public static final String TAG_ID = "id";
public static final String TAG_NAME = "name";
//public static final String TAG_TYPE = "төрөл";
public static final String TAG_MATH = "Mathematics";
public static final String TAG_SCIE = "Science";
public static final String TAG_COMP = "Computer";
List<HashMap<String, String>> studentList = new ArrayList<HashMap<String, String>>();
/* Called when the activity is first created. */
@SuppressLint("NewApi")
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ListView listview = getListView();
listview.setOnItemClickListener(new OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view,
int position, long id) {
String stud_id = studentList.get(position).get(TAG_ID);
String stud_name = studentList.get(position).get(TAG_NAME);
Intent stud_intent = new Intent(SecondActivity.this,
Student_Detail.class);
stud_intent.putExtra("stud", studentList.get(position));
startActivity(stud_intent);
}
});
new MyTask().execute();
}
class MyTask extends AsyncTask<Void, Void, Void> {
@Override
protected void onPreExecute() {
super.onPreExecute();
PD = new ProgressDialog(SecondActivity.this);
PD.setMessage("Loading...");
PD.setCancelable(false);
PD.show();
}
@Override
protected Void doInBackground(Void... arg0) {
String webdata = new MakeRequest().MakeGetRequest();
String name = null;
try {
JSONObject json = new JSONObject(webdata);
JSONArray employee = json.getJSONArray("students");
for (int i = 0; i < employee.length(); i++) {
JSONObject students = employee.getJSONObject(i);
HashMap student = new HashMap<String, String>();
student.put(TAG_ID, students.get(TAG_ID));
student.put(TAG_NAME, students.get(TAG_NAME));
//student.put(TAG_TYPE, students.get(TAG_TYPE));
JSONObject score = students.getJSONObject("score");
student.put(TAG_MATH, score.get(TAG_MATH));
student.put(TAG_SCIE, score.get(TAG_SCIE));
student.put(TAG_COMP, score.get(TAG_COMP));
studentList.add(student);
}
} catch (JSONException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return null;
}
@Override
protected void onPostExecute(Void result) {
super.onPostExecute(result);
String[] from = { TAG_ID, TAG_NAME };
int[] to = { R.id.stud_id, R.id.stud_name };
ListAdapter adapter = new SimpleAdapter(getApplicationContext(),
studentList, R.layout.list_items, from, to);
setListAdapter(adapter);
PD.dismiss();
}
}
}
此处显示
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.pavan.androidjsondemo"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="17" />
<uses-permission android:name="android.permission.INTERNET" />
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android:name="com.pavan.androidjsondemo.MainActivity"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity android:name=".SecondActivity" ></activity>
<activity android:name=".Student_Detail" >
</activity>
</application>
</manifest>
这里的堆栈跟踪
04-11 00:42:29.751 2280-2280/? I/art: Not late-enabling -Xcheck:jni (already on)
04-11 00:42:29.751 2280-2280/? W/art: Unexpected CPU variant for X86 using defaults: x86
04-11 00:42:29.782 2280-2280/? W/System: ClassLoader referenced unknown path: /data/app/com.pavan.androidjsondemo-1/lib/x86
04-11 00:42:29.972 2280-2280/com.pavan.androidjsondemo D/AndroidRuntime: Shutting down VM
--------- beginning of crash
04-11 00:42:29.972 2280-2280/com.pavan.androidjsondemo E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.pavan.androidjsondemo, PID: 2280
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.pavan.androidjsondemo/com.pavan.androidjsondemo.MainAc}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6077)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.pavan.androidjsondemo.MainAc.onCreate(MainAc.java:23)
at android.app.Activity.performCreate(Activity.java:6662)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2599)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6077)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
答案 0 :(得分：0)
您需要在setContentView(R.layout.activity_layout_xml_file)方法之后在您的MainActivity中加入super.onCreate(savedInstanceState);。阅读更多here。
当您没有设置布局时，Activity中没有视图（除非您以编程方式构造布局;但是更容易坚持布局）。然后，findViewBy(R.id.button1)将返回null，因为无法找到具有此ID的视图。这会导致b.setOnClickListener(...)上的NullPointerException。