Question

我在堆栈溢出中浏览了一些问题，但它没有解决我的问题。

这是我的代码。

   public class MenuListActivity extends ListActivity {
String MenuNames[] = {"Consultation", "Medicine", "Diseases", "Doctor"};
protected void onCreateView(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_menu_list);
    ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, MenuNames);
    final ListView lv = (ListView) findViewById(R.id.listView1);

    lv.setAdapter(adapter);

    lv.setOnItemClickListener(new OnItemClickListener() {
        public void onItemClick(AdapterView<?> parent, View v,
                int position, long id) {

            if(position == 1) {
                //code specific to first list item    
                Intent myIntent = new Intent(v.getContext(), ConsultationActivity.class);
                startActivity(myIntent);
            }


        }

    }
     );

}

logcat的。

    10-12 10:53:37.572: E/AndroidRuntime(10605): FATAL EXCEPTION: main
    10-12 10:53:37.572: E/AndroidRuntime(10605): Process: com.capstone.medicinehealthguide, PID:     10605
    10-12 10:53:37.572: E/AndroidRuntime(10605): java.lang.RuntimeException: Unable to start activity      ComponentInfo{com.capstone.medicinehealthguide/com.capstone.medicinehealthguide.MenuListActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference
    10-12 10:53:37.572: E/AndroidRuntime(10605):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2255)
    10-12 10:53:37.572: E/AndroidRuntime(10605):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2317)
    10-12 10:53:37.572: E/AndroidRuntime(10605):    at android.app.ActivityThread.access$800(ActivityThread.java:143)
    10-12 10:53:37.572: E/AndroidRuntime(10605):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1258)
    10-12 10:53:37.572: E/AndroidRuntime(10605):    at android.os.Handler.dispatchMessage(Handler.java:102)
    10-12 10:53:37.572: E/AndroidRuntime(10605):    at android.os.Looper.loop(Looper.java:135)
    10-12 10:53:37.572: E/AndroidRuntime(10605):    at android.app.ActivityThread.main(ActivityThread.java:5070)
    10-12 10:53:37.572: E/AndroidRuntime(10605):    at java.lang.reflect.Method.invoke(Native Method)
    10-12 10:53:37.572: E/AndroidRuntime(10605):    at java.lang.reflect.Method.invoke(Method.java:372)
    10-12 10:53:37.572: E/AndroidRuntime(10605):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:836)
    10-12 10:53:37.572: E/AndroidRuntime(10605):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:631)
    10-12 10:53:37.572: E/AndroidRuntime(10605): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference
    10-12 10:53:37.572: E/AndroidRuntime(10605):    at com.capstone.medicinehealthguide.MenuListActivity.onCreate(MenuListActivity.java:24)
    10-12 10:53:37.572: E/AndroidRuntime(10605):    at android.app.Activity.performCreate(Activity.java:5720)
    10-12 10:53:37.572: E/AndroidRuntime(10605):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1102)
    10-12 10:53:37.572: E/AndroidRuntime(10605):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2208)
    10-12 10:53:37.572: E/AndroidRuntime(10605):    ... 10 more

XML文件

    <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.capstone.medicinehealthguide.MenuListActivity" >

<ListView
    android:id="@+id/listView1"
    android:layout_width="match_parent"
    android:layout_height="wrap_content" >

</ListView>

我正在尝试制作一个包含列表视图的应用，当您点击它时，其他活动将会打开

Answer 1

你实际上是在使用这个错误：

final ListView lv = (ListView) getListView().findViewById(R.id.listView1);

下面：

final ListView lv = (ListView) findViewById(R.id.listView1);

问题在于，您在获取列表视图，然后尝试从列表视图中获取列表视图。

尝试上面的选项，只需从内容视图中获取列表视图。

Answer 2

在调用findByViewId之前，视图必须膨胀。你要做的是..将此代码从onCreate方法移动到'onCreateView'方法。在onCreateView方法中，将其放在setContentView()行以下

ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, MenuNames);

final ListView lv = (ListView) getListView().findViewById(R.id.listView1);

lv.setAdapter(adapter);

lv.setOnItemClickListener(new OnItemClickListener() {
    public void onItemClick(AdapterView<?> parent, View v,
            int position, long id) {

        if(position == 1) {
            //code specific to first list item    
            Intent myIntent = new Intent(v.getContext(), ConsultationActivity.class);
            startActivity(myIntent);
        }
}

由以下原因引起：java.lang.NullPointerException：尝试在空对象引用上调用虚方法'void ...

2 个答案: