Question

根据this教程，我试图为我的应用构建导航抽屉。

我得到的是这一行的错误：

    getActionBar().setDisplayHomeAsUpEnabled(true);

我的错误日志：

  Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
12-11 18:36:50.670 6386-6386/com.example.mika.newspaper E/AndroidRuntime:     at com.example.mika.newspaper.MainActivity.onCreate(MainActivity.java:56)
12-11 18:36:50.670 6386-6386/com.example.mika.newspaper E/AndroidRuntime:     at android.app.Activity.performCreate(Activity.java:6237)
12-11 18:36:50.670 6386-6386/com.example.mika.newspaper E/AndroidRuntime:     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
12-11 18:36:50.670 6386-6386/com.example.mika.newspaper E/AndroidRuntime:     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
12-11 18:36:50.670 6386-6386/com.example.mika.newspaper E/AndroidRuntime:     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
12-11 18:36:50.670 6386-6386/com.example.mika.newspaper E/AndroidRuntime:     at android.app.ActivityThread.-wrap11(ActivityThread.java) 
12-11 18:36:50.670 6386-6386/com.example.mika.newspaper E/AndroidRuntime:     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
12-11 18:36:50.670 6386-6386/com.example.mika.newspaper E/AndroidRuntime:     at android.os.Handler.dispatchMessage(Handler.java:102) 
12-11 18:36:50.670 6386-6386/com.example.mika.newspaper E/AndroidRuntime:     at android.os.Looper.loop(Looper.java:148) 
12

我的活动：

package com.example.mika.newspaper;

import android.app.Activity;
import android.app.FragmentManager;
import android.app.Fragment;
import android.content.res.Configuration;
import android.support.v4.widget.DrawerLayout;
import android.os.Bundle;
import android.support.v7.app.ActionBarDrawerToggle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;

public class MainActivity extends Activity {

    private String[] mNewsTitles;
    private DrawerLayout mDrawerLayout;
    private ListView mDrawerList;
    private ActionBarDrawerToggle mDrawerToggle;
    private CharSequence mDrawerTitle;
    private CharSequence mTitle;

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

        mNewsTitles = getResources().getStringArray(R.array.newsTypes);
        mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
        mDrawerList = (ListView) findViewById(R.id.left_drawer);
        mDrawerList.setAdapter(new ArrayAdapter<String>(this, R.layout.drawer_list_item, mNewsTitles));
        mDrawerList.setOnItemClickListener(new DrawerItemClickListener());

        mTitle = mDrawerTitle = getTitle();
        mDrawerToggle = new ActionBarDrawerToggle(this, mDrawerLayout,
                R.string.drawer_open, R.string.drawer_close) {

            public void onDrawerClosed(View view) {
                super.onDrawerClosed(view);
                getActionBar().setTitle(mTitle);
                invalidateOptionsMenu();
            }


            public void onDrawerOpened(View drawerView) {
                super.onDrawerOpened(drawerView);
                getActionBar().setTitle(mDrawerTitle);
                invalidateOptionsMenu();
            }
        };

        mDrawerLayout.setDrawerListener(mDrawerToggle);
        getActionBar().setDisplayHomeAsUpEnabled(true);
        getActionBar().setHomeButtonEnabled(true);
    }

    @Override
    protected void onPostCreate(Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);
        mDrawerToggle.syncState();
    }

    @Override
    public void onConfigurationChanged(Configuration newConfig) {
        super.onConfigurationChanged(newConfig);
        mDrawerToggle.onConfigurationChanged(newConfig);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        if (mDrawerToggle.onOptionsItemSelected(item)) {
            return true;
        }
        return super.onOptionsItemSelected(item);

    }

    @Override
    public boolean onPrepareOptionsMenu(Menu menu) {
        boolean drawerOpen = mDrawerLayout.isDrawerOpen(mDrawerList);
        menu.findItem(R.id.action_websearch).setVisible(!drawerOpen);
        return super.onPrepareOptionsMenu(menu);
    }

    private class DrawerItemClickListener implements ListView.OnItemClickListener {


        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
            selectItem(position);
        }

    }

    private void selectItem(int position){
        Fragment fragment = new NewsFragment();
        Bundle args = new Bundle();
        args.putInt(NewsFragment.ARG_NEWS_NUMBER, position);
        fragment.setArguments(args);
        FragmentManager fragmentManager = getFragmentManager();
        fragmentManager.beginTransaction().replace(R.id.content_frame, fragment).commit();

        mDrawerList.setItemChecked(position, true);
        setTitle(mNewsTitles[position]);
        mDrawerLayout.closeDrawer(mDrawerList);
    }



    @Override
    public void setTitle(CharSequence title) {
        mTitle = title;
        getActionBar().setTitle(mTitle);
    }


}

Answer 1

您必须将所有getActionBar()替换为getSupportActionBar()

导航抽屉提供nullPointerExeption

1 个答案: