复选框无法正常工作

Question

我有一个复选框和一个textView,当选中复选框时,textview应显示该复选框被按下.但是当我运行我的代码时,我得到一个nullpointerexception.这是我的代码:

public class MyActivity extends Activity {

public void onCreate(Bundle savedInstanceState) {
    CheckBox android=(CheckBox)findViewById(R.id.android);
    CheckBox ios=(CheckBox)findViewById(R.id.ios);
    CheckBox webos=(CheckBox)findViewById(R.id.webos);
    CheckBox windows=(CheckBox)findViewById(R.id.windows);
    Button generate=(Button)findViewById(R.id.generate);
    final TextView result=(TextView)findViewById(R.id.result);
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    android.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            if(((CheckBox)view).isChecked())
                result.setText("Android Checked");
        }
    });

   }
  }

这是我的问题所在的logcat的一部分:

12-21 17:25:00.324: ERROR/AndroidRuntime(960): FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.GridAndroid/com.example.GridAndroid.MyActivity}: java.lang.NullPointerException
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
    at android.app.ActivityThread.access$600(ActivityThread.java:130)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:137)
    at android.app.ActivityThread.main(ActivityThread.java:4745)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:511)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
    at dalvik.system.NativeStart.main(Native Method)
    Caused by: java.lang.NullPointerException
    at com.example.GridAndroid.MyActivity.onCreate(MyActivity.java:26)
    at android.app.Activity.performCreate(Activity.java:5008)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)

所以我的问题是我的代码有什么问题？

Answer 1

在setcontentview()之后获取id

public class MyActivity extends Activity {

public void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

CheckBox android=(CheckBox)findViewById(R.id.android);
    CheckBox ios=(CheckBox)findViewById(R.id.ios);
    CheckBox webos=(CheckBox)findViewById(R.id.webos);
    CheckBox windows=(CheckBox)findViewById(R.id.windows);
    Button generate=(Button)findViewById(R.id.generate);
    final TextView result=(TextView)findViewById(R.id.result);

    android.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            if(((CheckBox)view).isChecked())
                result.setText("Android Checked");
        }
    });

   }
  }