android adapter 提示空指针异常

Question

 public class ContactInfoParser extends Activity {
    ListView mListView;
    List<String> contactList = new ArrayList<String>();


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mListView = (ListView) findViewById(R.id.list_contact);
        ArrayAdapter<String> adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, contactList);
        mListView.setAdapter(adapter);
        readContacts();
    }

    private void readContacts() {
        Cursor cursor = null;
        try{
            cursor = getContentResolver().query(ContactsContract.CommonDataKinds.Phone.CONTENT_URI, null, null, null, null);
            while (cursor.moveToNext()) {
                String displayname = cursor.getString(cursor.getColumnIndex(ContactsContract.CommonDataKinds.Phone.DISPLAY_NAME));
                String diplaynumber = cursor.getString(cursor.getColumnIndex(ContactsContract.CommonDataKinds.Phone.NUMBER));
                contactList.add(displayname + "\n" + diplaynumber);
            }
        }catch (Exception e){
            e.printStackTrace();
        }finally {
            if (cursor != null) {
                cursor.close();
            }

        }

    }
}

上面这段代码从其他activity跳转过来，执行到mListView.setAdapter(adapter);这句话的时候，提示一直报错空指针异常。
报错代码如下：
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.liujing.shanzhaisafe/com.liujing.shanzhaisage.fangdaomokuai.dialog.ContactInfoParser}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
at com.liujing.shanzhaisage.fangdaomokuai.dialog.ContactInfoParser.onCreate(ContactInfoParser.java:24)

Answer 1

你的创建方法中缺少setContentView(R.layout.?);
没有设置此activity与之关联的显示页面，所以listview查找不到