java - 简单抽屉导航返回空指针异常

Question

我正在尝试为我的应用程序实现抽屉式导航，但我什至无法启动它，我也不知道为什么。它在 setAdapter 返回一个空指针异常，但我不明白，因为这几乎是从 android 和其他类似教程中复制过来的。谁能告诉我我做错了什么？

主要 Activity

import android.app.Activity;
import android.os.Bundle;
import android.support.v4.widget.DrawerLayout;
import android.util.Log;
import android.view.Menu;
import android.widget.ArrayAdapter;
import android.widget.ListView;

public class MainActivity extends Activity {
    private String[] mContentTitles;
    private DrawerLayout mDrawerLayout;
    private ListView mDrawerList;
    private final String TAG = "MainActivity";


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

        mContentTitles = getResources().getStringArray(R.array.contentTitles);
        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, mContentTitles));

    }

    @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;
    }

}

DrawerLayout.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" />
    <!-- The navigation drawer -->

    <ListView
        android:id="@+id/left_drawer"
        android:layout_width="240dp"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:background="#111"
        android:choiceMode="singleChoice"
        android:divider="@android:color/transparent"
        android:dividerHeight="0dp" />

</android.support.v4.widget.DrawerLayout>

drawer_list_item.xml

<?xml version="1.0" encoding="utf-8"?>

<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@android:id/text1"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:textColor="#fff"
    android:textSize="20sp"
    android:gravity="center_vertical"
    android:paddingStart="14.5sp"
    android:paddingEnd="14.5sp"
    android:minHeight="35sp"
/>

字符串.xml

<?xml version="1.0" encoding="utf-8"?>

<string name="app_name">FrontRowReady</string>
<string name="action_settings">Settings</string>
<string name="home">Home</string>
<string name="settings">Settings</string>
<string name="inbox">Inbox</string>
<string name="contacts">Contacts</string>

<array name="contentTitles">
    <item>Home</item>
    <item>Inbox</item>
    <item>Contacts</item>
    <item>Settings</item>
</array>

错误代码

12-23 14:29:11.315: E/AndroidRuntime(30524): FATAL EXCEPTION: main 12-23 14:29:11.315: E/AndroidRuntime(30524): Process: PID: 30524 12-23 14:29:11.315: E/AndroidRuntime(30524): java.lang.RuntimeException: Unable to start activity java.lang.NullPointerException 12-23 14:29:11.315: E/AndroidRuntime(30524): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195) 12-23 14:29:11.315: E/AndroidRuntime(30524): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245) 12-23 14:29:11.315: E/AndroidRuntime(30524): at android.app.ActivityThread.access$800(ActivityThread.java:135) 12-23 14:29:11.315: E/AndroidRuntime(30524): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196) 12-23 14:29:11.315: E/AndroidRuntime(30524): at android.os.Handler.dispatchMessage(Handler.java:102) 12-23 14:29:11.315: E/AndroidRuntime(30524): at android.os.Looper.loop(Looper.java:136) 12-23 14:29:11.315: E/AndroidRuntime(30524): at android.app.ActivityThread.main(ActivityThread.java:5017) 12-23 14:29:11.315: E/AndroidRuntime(30524): at java.lang.reflect.Method.invokeNative(Native Method) 12-23 14:29:11.315: E/AndroidRuntime(30524): at java.lang.reflect.Method.invoke(Method.java:515) 12-23 14:29:11.315: E/AndroidRuntime(30524): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779) 12-23 14:29:11.315: E/AndroidRuntime(30524): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595) 12-23 14:29:11.315: E/AndroidRuntime(30524): at dalvik.system.NativeStart.main(Native Method) 12-23 14:29:11.315: E/AndroidRuntime(30524): Caused by: java.lang.NullPointerException 12-23 14:29:11.315: E/AndroidRuntime(30524): at com.maxxpotential.frontrowready.MainActivity.onCreate(MainActivity.java:27) 12-23 14:29:11.315: E/AndroidRuntime(30524): at android.app.Activity.performCreate(Activity.java:5231) 12-23 14:29:11.315: E/AndroidRuntime(30524): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087) 12-23 14:29:11.315: E/AndroidRuntime(30524): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159) 12-23 14:29:11.315: E/AndroidRuntime(30524): ... 11 more

Answer 1

修改这部分代码:

setContentView(R.layout.activity_main); 

为此:

setContentView(R.layout.DrawerLayout);