Question

我不熟悉Android编程，但我对Java和C ++非常熟悉。以下是我的应用程序的Java代码，它适用于吐司，但当我尝试更改文本时，它会导致＆＃34;您的应用程序已停止工作＆＃34;。

代码

public class MainActivity extends ActionBarActivity {

int counter;
Button sum, sub;
TextView display;

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    counter = 0;
    display = (TextView) findViewById(R.id.show);
    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment()).commit();
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {

    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

/**
 * A placeholder fragment containing a simple view.
 */
public static class PlaceholderFragment extends Fragment {

    public PlaceholderFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container,
                false);
        return rootView;
    }
}
public void addButtonClicked(View v) {
     counter++;
     tToast("Your Total Is : " + counter);
        display.setText("Your Total Is : " + counter);

    }
public void subButtonClicked(View v) {
     counter--;
     tToast("Your Total Is : " + counter);
        display.setText("Your Total Is : " + counter);

    }
 private void tToast(String s) {
        Context context = getApplicationContext();
        int duration = Toast.LENGTH_LONG;
        Toast toast = Toast.makeText(context, s, duration);
        toast.show();
    }
}

StackTrace：

07-05 05:22:42.888: E/AndroidRuntime(1650): FATAL EXCEPTION: main
07-05 05:22:42.888: E/AndroidRuntime(1650): Process: com.example.counter, PID: 1650
07-05 05:22:42.888: E/AndroidRuntime(1650): java.lang.IllegalStateException: Could not execute method of the activity
07-05 05:22:42.888: E/AndroidRuntime(1650):     at android.view.View$1.onClick(View.java:3823)
07-05 05:22:42.888: E/AndroidRuntime(1650):     at android.view.View.performClick(View.java:4438)
07-05 05:22:42.888: E/AndroidRuntime(1650):     at android.view.View$PerformClick.run(View.java:18422)
07-05 05:22:42.888: E/AndroidRuntime(1650):     at android.os.Handler.handleCallback(Handler.java:733)
07-05 05:22:42.888: E/AndroidRuntime(1650):     at android.os.Handler.dispatchMessage(Handler.java:95)
07-05 05:22:42.888: E/AndroidRuntime(1650):     at android.os.Looper.loop(Looper.java:136)
07-05 05:22:42.888: E/AndroidRuntime(1650):     at android.app.ActivityThread.main(ActivityThread.java:5017)
07-05 05:22:42.888: E/AndroidRuntime(1650):     at java.lang.reflect.Method.invokeNative(Native Method)
07-05 05:22:42.888: E/AndroidRuntime(1650):     at java.lang.reflect.Method.invoke(Method.java:515)
07-05 05:22:42.888: E/AndroidRuntime(1650):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
07-05 05:22:42.888: E/AndroidRuntime(1650):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
07-05 05:22:42.888: E/AndroidRuntime(1650):     at dalvik.system.NativeStart.main(Native Method)
07-05 05:22:42.888: E/AndroidRuntime(1650): Caused by: java.lang.reflect.InvocationTargetException
07-05 05:22:42.888: E/AndroidRuntime(1650):     at java.lang.reflect.Method.invokeNative(Native Method)
07-05 05:22:42.888: E/AndroidRuntime(1650):     at java.lang.reflect.Method.invoke(Method.java:515)
07-05 05:22:42.888: E/AndroidRuntime(1650):     at android.view.View$1.onClick(View.java:3818)
07-05 05:22:42.888: E/AndroidRuntime(1650):     ... 11 more
07-05 05:22:42.888: E/AndroidRuntime(1650): Caused by: java.lang.NullPointerException
07-05 05:22:42.888: E/AndroidRuntime(1650):     at com.example.counter.MainActivity.addButtonClicked(MainActivity.java:104)
07-05 05:22:42.888: E/AndroidRuntime(1650):     ... 14 more

Answer 1

这是因为findViewById()在activity_main布局中搜索，而TextView位于片段的布局fragment_main中。

在片段的onCreateView()方法中移动该段代码：

//...
View rootView = inflater.inflate(R.layout.fragment_main, container, false);
display = (TextView)rootView.findViewById(R.id.show);

请注意，现在您可以通过rootView view访问它：

display = (TextView)rootView.findViewById(R.id.show);

否则你会再次NullPointerException。

应用已停止使用display.setText（）

1 个答案: