Android 使用微调器时无法启动组件信息

我对安卓非常陌生。对不起，我找不到我的缺点。在emulator上打开应用程序时，我面临NullPointerException。日志显示无法启动组件信息。请帮忙

    package com.myworks.converter;

    import android.os.Bundle;
    import android.support.v4.app.Fragment;
    import android.support.v7.app.ActionBarActivity;
    import android.view.LayoutInflater;
    import android.view.Menu;
    import android.view.MenuItem;
    import android.view.View;
    import android.view.ViewGroup;
    import android.widget.AdapterView;
    import android.widget.AdapterView.OnItemSelectedListener;
    import android.widget.ArrayAdapter;
    import android.widget.Spinner;

    public class MainActivity extends ActionBarActivity implements OnItemSelectedListener 
    {

    private Spinner spinner;
    private static final String items[] = {"Length", "Weight", "Temperature"};

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

        spinner = (Spinner)findViewById(R.id.spinner);

        ArrayAdapter<String> adapter = new ArrayAdapter<String>(MainActivity.this,
                android.R.layout.simple_spinner_item,items);

        adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        spinner.setAdapter(adapter);
        spinner.setOnItemSelectedListener(this);

        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;
        }
    }

    @Override
    public void onItemSelected(AdapterView<?> parent, View view, int position,long id)
    {
        switch (position) 
        {
            case 0:
            // Whatever you want to happen when the first item gets selected
                break;
            case 1:
            // Whatever you want to happen when the second item gets selected
                break;
            case 2:
            // Whatever you want to happen when the thrid item gets selected
       }

    }

    @Override
    public void onNothingSelected(AdapterView<?> parent) 
    {
        // TODO Auto-generated method stub

    }

    }

---

您的车祸原因如下：

@Override
protected void onCreate(Bundle savedInstanceState) 
{
    spinner = (Spinner)findViewById(R.id.spinner); // <------------- returns null!
}

为什么?

因为您正在尝试访问片段视图，所以在xml的片段布局中声明了R.id.spinner 在其活动的onCreate中，即在片段完全膨胀之前

要修复此问题，请更改逻辑，以便从片段本身中引用片段视图：

class PlaceHolderFragment {
   .....

   @Override
   public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
         View view = inflater.inflate(R.layout.event_log, container, false);
         spinner = (Spinner)findViewById(R.id.spinner);
         // here spinner != null  <------------------------
         return view;
   }

}

---

您的微调器在片段中，而不是在活动中。@njzk2是正确的。。您正在尝试在片段被附加之前访问微调器，因此它是空的

@Override
protected void onCreate(Bundle savedInstanceState) 
{
    spinner = (Spinner)findViewById(R.id.spinner); // <------------- returns null!
}

class PlaceHolderFragment {
   .....

   @Override
   public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
         View view = inflater.inflate(R.layout.event_log, container, false);
         spinner = (Spinner)findViewById(R.id.spinner);
         // here spinner != null  <------------------------
         return view;
   }

}