Navigation 正确的代码，但发生了错误

谁能解释这些代码？特别是黑体代码

import android.widget.ListView;

public class MainActivity extends Activity {

private String[] mPlanetTitles;
private DrawerLayout mDrawerLayout;
private ListView mDrawerList;

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

    **mPlanetTitles = getResources().getStringArray(R.array.planets_array);
    mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
    mDrawerList = (ListView) findViewById(R.id.left_drawer);**

    // Set the adapter for the list view
    **mDrawerList.setAdapter(new ArrayAdapter<String>(this,
            R.layout.drawer_list_item, mPlanetTitles));
    // Set the list's click listener
    mDrawerList.setOnItemClickListener(new DrawerItemClickListener());**

}

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没有什么是粗体的…还有什么错误？你能解释一下这些代码吗？特别是抽屉部分。我不明白。