java - 为什么 setOnClick 会使这个应用程序崩溃？

Question

我是 android 新手(对于 java 来说并不是完全陌生)。尝试遵循有点过时的教程。我不明白为什么 setOnClickListener 会使这个应用程序崩溃。我知道某处存在空指针异常，但我不知道如何修复它。另外，旁注，为什么我的类扩展 ActionBarActivity 而不是像我到目前为止看到的所有示例中那样扩展 Activity？

public class StartingPoint extends ActionBarActivity {

    int counter;
    Button add, sub;
    TextView display;

    @Override
    protected void onCreate(Bundle savedInstanceState) {

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

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment()).commit();
        } // end of if

        counter = 0;

        add = (Button) findViewById(R.id.bAdd);
        sub = (Button) findViewById(R.id.bSub);
        display = (TextView) findViewById(R.id.tvDisplay);

        add.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                counter++;
                display.setText("Counter equals" + counter);

            }
        });


    } // 



    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.starting_point, 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_starting_point,
                    container, false);
            return rootView;
        }
    }
}

Answer 1

您的按钮和 TextView 位于 fragment 布局中，而不是 Activity 布局中，因此您必须将代码移至 onCreateView:

public static class PlaceholderFragment extends Fragment {

    public PlaceholderFragment() {
    }

    int counter=0;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container,
            false);

        Button add = (Button) rootView.findViewById(R.id.bAdd);
        Button sub = (Button) rootView.findViewById(R.id.bSub);
        TextView display = (TextView) rootView.findViewById(R.id.tvDisplay);

        add.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                counter++;
                display.setText("Counter equals" + counter);
            }
        });

        return rootView;
    }
}

另外，你必须清除你的onCreate，就离开:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_starting_point);

    if (savedInstanceState == null) {
         getSupportFragmentManager().beginTransaction()
          .add(R.id.container, new PlaceholderFragment()).commit();
    }
}