Question

我想创建一个BaseActivity Navigation Drawer来扩展其他活动。（对所有活动使用一个抽屉）

我搜索并尝试了许多代码，例如this link，但我仍然有：

无法启动活动（NullPointExeption）错误！

我也在MainActivity：

中编写此代码

super.onCreate(savedInstanceState);
setContentView(R.layout.activity);
super.onCreateDrawer();

但它不起作用：（

BaseActivity：

public abstract class BaseActivity extends Activity
{
    public DrawerLayout drawerLayout;
    public String[] layers;
    private ActionBarDrawerToggle drawerToggle;

    protected void onCreateDrawer()
    {
        // R.id.drawer_layout should be in every activity with exactly the same id.
        drawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);

        drawerToggle = new ActionBarDrawerToggle((Activity) this, drawerLayout, R.drawable.ic_drawer, 0, 0) 
        {
            public void onDrawerClosed(View view) 
            {
                getActionBar().setTitle(R.string.app_name);
            }

            public void onDrawerOpened(View drawerView) 
            {
                getActionBar().setTitle(R.string.hello_world);
            }
        };
        drawerLayout.setDrawerListener(drawerToggle);

        getActionBar().setDisplayHomeAsUpEnabled(true);
        getActionBar().setHomeButtonEnabled(true);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {

        if (drawerToggle.onOptionsItemSelected(item)) {
            return true;
        }
        return super.onOptionsItemSelected(item);

    }

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

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

}

MainActivity：

public class MainActivity extends BaseActivity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        super.onCreateDrawer();
    }
}

drawer_activity.xml

<android.support.v4.widget.DrawerLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/drawer_layout"
android:layout_width="match_parent"
android:layout_height="match_parent">
<!-- The main content view -->
<FrameLayout
    android:id="@+id/content_frame"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >
    <!-- Put what you want as your normal screen in here, you can also choose for a linear layout or any other layout, whatever you prefer -->
</FrameLayout>
<!-- The navigation drawer -->
<ListView android:id="@+id/left_drawer"
    android:layout_width="240dp"
    android:layout_height="match_parent"
    android:layout_gravity="start"
    android:choiceMode="singleChoice"
    android:divider="@android:color/transparent"
    android:dividerHeight="0dp"
    android:background="#111"/>

logcat的：

   09-07 07:12:47.308: E/AndroidRuntime(2620): FATAL EXCEPTION: main
09-07 07:12:47.308: E/AndroidRuntime(2620): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.test/com.example.test.MainActivity}: java.lang.NullPointerException
09-07 07:12:47.308: E/AndroidRuntime(2620):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
09-07 07:12:47.308: E/AndroidRuntime(2620):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
09-07 07:12:47.308: E/AndroidRuntime(2620):     at android.app.ActivityThread.access$600(ActivityThread.java:130)
09-07 07:12:47.308: E/AndroidRuntime(2620):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
09-07 07:12:47.308: E/AndroidRuntime(2620):     at android.os.Handler.dispatchMessage(Handler.java:99)
09-07 07:12:47.308: E/AndroidRuntime(2620):     at android.os.Looper.loop(Looper.java:137)
09-07 07:12:47.308: E/AndroidRuntime(2620):     at android.app.ActivityThread.main(ActivityThread.java:4745)
09-07 07:12:47.308: E/AndroidRuntime(2620):     at java.lang.reflect.Method.invokeNative(Native Method)
09-07 07:12:47.308: E/AndroidRuntime(2620):     at java.lang.reflect.Method.invoke(Method.java:511)
09-07 07:12:47.308: E/AndroidRuntime(2620):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
09-07 07:12:47.308: E/AndroidRuntime(2620):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
09-07 07:12:47.308: E/AndroidRuntime(2620):     at dalvik.system.NativeStart.main(Native Method)
09-07 07:12:47.308: E/AndroidRuntime(2620): Caused by: java.lang.NullPointerException
09-07 07:12:47.308: E/AndroidRuntime(2620):     at com.example.test.NavigationActivity.onCreateDrawer(NavigationActivity.java:38)
09-07 07:12:47.308: E/AndroidRuntime(2620):     at com.example.test.MainActivity.onCreate(MainActivity.java:11)
09-07 07:12:47.308: E/AndroidRuntime(2620):     at android.app.Activity.performCreate(Activity.java:5008)
09-07 07:12:47.308: E/AndroidRuntime(2620):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
09-07 07:12:47.308: E/AndroidRuntime(2620):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)

解决方案是什么？！感谢。

Answer 1

NPE可能很容易导致getActionBar()，具体取决于Android版本和您的清单配置。我强烈建议您延长AppCompatActivity而不是Activity，并使用getSupportActionBar()代替getActionBar()。请注意，两者都可以返回null，具体取决于您在清单上选择的主题。

Answer 2

onCreateDrawer（）方法中出现错误。可能是因为：

不使用活动，而是使用 AppCompatActivity
而不是这一行

drawerToggle = new ActionBarDrawerToggle（（Activity）this， drawerLayout，R.drawable.ic_drawer，0,0）

像这样做

drawerToggle = new ActionBarDrawerToggle（this，drawerLayout， R.drawable.ic_drawer，0,0）

检查您的主题，使用可支持ActionBar的主题以下是Android文档链接Styling Action Bar

BaseActivity中的NullPointExeption

2 个答案: