Question

我目前正在开发一个包含自定义ListView的应用程序。我开发了一个定制阵列适配器。我想我的应用程序在这里崩溃了：

ListView DirectoryView = (ListView) findViewById(R.id.fileListView);

所以我认为错误发生在activity_main.xml中：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="fill_parent"
android:layout_height="fill_parent" >

<ListView
    android:id="@+id/fileListView"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:layout_weight="1" >
</ListView>

这是我的LogCat：

09-09 11:19:21.254: E/Trace(1152): error opening trace file: No such file or directory (2)
09-09 11:19:21.484: D/AndroidRuntime(1152): Shutting down VM
09-09 11:19:21.484: W/dalvikvm(1152): threadid=1: thread exiting with uncaught exception (group=0x40a13300)
09-09 11:19:21.504: E/AndroidRuntime(1152): FATAL EXCEPTION: main
09-09 11:19:21.504: E/AndroidRuntime(1152): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.teamdroid.explorer/com.teamdroid.explorer.MainActivity}: java.lang.NullPointerException
09-09 11:19:21.504: E/AndroidRuntime(1152):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
09-09 11:19:21.504: E/AndroidRuntime(1152):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
09-09 11:19:21.504: E/AndroidRuntime(1152):     at android.app.ActivityThread.access$600(ActivityThread.java:130)
09-09 11:19:21.504: E/AndroidRuntime(1152):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
09-09 11:19:21.504: E/AndroidRuntime(1152):     at android.os.Handler.dispatchMessage(Handler.java:99)
09-09 11:19:21.504: E/AndroidRuntime(1152):     at android.os.Looper.loop(Looper.java:137)
09-09 11:19:21.504: E/AndroidRuntime(1152):     at android.app.ActivityThread.main(ActivityThread.java:4745)
09-09 11:19:21.504: E/AndroidRuntime(1152):     at java.lang.reflect.Method.invokeNative(Native Method)
09-09 11:19:21.504: E/AndroidRuntime(1152):     at java.lang.reflect.Method.invoke(Method.java:511)
09-09 11:19:21.504: E/AndroidRuntime(1152):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
09-09 11:19:21.504: E/AndroidRuntime(1152):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
09-09 11:19:21.504: E/AndroidRuntime(1152):     at dalvik.system.NativeStart.main(Native Method)
09-09 11:19:21.504: E/AndroidRuntime(1152): Caused by: java.lang.NullPointerException
09-09 11:19:21.504: E/AndroidRuntime(1152):     at android.app.Activity.findViewById(Activity.java:1825)
09-09 11:19:21.504: E/AndroidRuntime(1152):     at com.teamdroid.explorer.listDirectory.getDirectory(listDirectory.java:20)
09-09 11:19:21.504: E/AndroidRuntime(1152):     at com.teamdroid.explorer.MainActivity.onCreate(MainActivity.java:33)
09-09 11:19:21.504: E/AndroidRuntime(1152):     at android.app.Activity.performCreate(Activity.java:5008)
09-09 11:19:21.504: E/AndroidRuntime(1152):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
09-09 11:19:21.504: E/AndroidRuntime(1152):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
09-09 11:19:21.504: E/AndroidRuntime(1152):     ... 11 more

请你帮我。我正在搜索此错误2天。谢谢！

修改我有第三个文件，其中列出了此listview：

中的目录

    public class listDirectory extends ListActivity {

    String[] DirList;

    public void onCreate(Bundle icicle) {
        setContentView(R.layout.activity_main);
        super.onCreate(icicle);
    }   

    public void getDirectory (){
        MainActivity main = new MainActivity();
        String path = main.getPath();

        ListView DirectoryView = (ListView) findViewById(R.id.fileListView);
        CustomArrayAdapter adapter = new CustomArrayAdapter(getApplicationContext(), DirList);

        File file = new File(path);
        File[] FileList = file.listFiles();

        java.util.Arrays.sort(FileList);

        for(int i = 0; i < FileList.length; i++){
            if(FileList[i].isDirectory()){
                DirList[i] = (FileList[i].getName() + " [folder]");
            } else{
                DirList[i] = (FileList[i].getName() + " [file]");
            }
         }

         DirectoryView.setAdapter(adapter);
    }
}

我认为在这个文件中是错误的。

Answer 1

我认为您没有在onCreate（）方法中设置内容视图，因此findViewById无法找到视图对象，因此返回null。

假设您的XML布局文件是my_layout.xml。在onCreate（...）里面添加以下行：

setContentView(R.layout.my_layout);

Answer 2

非常令人困惑的代码......

我认为在您的MainActivity新建listDirectory的实例时，这不会被listDirectory.onCreate()调用，因此内容视图为空。因此findViewById() listDirectory.getDirectory()在listDirectory的内容视图中查找视图而不是MainActivity，这是null并导致NPE

Android开发应用程序在启动时崩溃

2 个答案: