Android程序崩溃空指针异常

Question

嘿，大家在此刻我试图显示所有的文件在一个ListView的android设备sd卡上的某个文件夹，到目前为止我有坏运气，它崩溃与一个空指针异常，所以我想知道如果有人可以在我的问题上发光一些生命我会张贴我的活动和logcat下面，谢谢。

活动

package com.mkyong.android;
import java.io.File;
import java.util.ArrayList;
import android.app.ListActivity;
import android.os.Bundle;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;
import android.view.View;
public class ListMobileActivity extends ListActivity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.res);
    }
        public ArrayList<String> GetFiles(String DirectoryPath) {
            ArrayList<String> MyFiles = new ArrayList<String>();
            File f = new File(DirectoryPath);
            f.mkdirs();
            File[] files = f.listFiles();
            if (files.length == 0)
                return null;
            else {
                for (int i=0; i<files.length; i++) 
                    MyFiles.add(files[i].getName());
            }
            return MyFiles;
        }
        {
        ListView lv;
        ArrayList<String> FilesInFolder = GetFiles("/sdcard/RootBox");
        lv = (ListView)findViewById(R.id.listView1);
        lv.setAdapter(new ArrayAdapter<String>(this,
            android.R.layout.simple_list_item_1, FilesInFolder));
        lv.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        public void onItemClick(AdapterView<?> parent, View v, int position, long id) {
             }
        });
    }


    @Override
    protected void onListItemClick(ListView l, View v, int position, long id) {
        //get selected items
        String selectedValue = (String) getListAdapter().getItem(position);
        Toast.makeText(this, selectedValue, Toast.LENGTH_SHORT).show();
    }
}

和logcat

05-24 06:42:28.992: E/AndroidRuntime(796): FATAL EXCEPTION: main
05-24 06:42:28.992: E/AndroidRuntime(796): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.mkyong.android/com.mkyong.android.ListMobileActivity}: java.lang.RuntimeException: Your content must have a ListView whose id attribute is 'android.R.id.list'
05-24 06:42:28.992: E/AndroidRuntime(796):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
05-24 06:42:28.992: E/AndroidRuntime(796):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
05-24 06:42:28.992: E/AndroidRuntime(796):  at android.app.ActivityThread.access$600(ActivityThread.java:141)
05-24 06:42:28.992: E/AndroidRuntime(796):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
05-24 06:42:28.992: E/AndroidRuntime(796):  at android.os.Handler.dispatchMessage(Handler.java:99)

Answer 1

如果你在android manifest中请求了权限来读取存储，那么这可能是问题所在。

从代码看来，您正在查询实例块中的列表视图的实例。这可能在setContentView被调用之前被调用。你应该在布局膨胀后用XML实例化UI组件，否则你会遇到异常。

试试这个。

    public class ListMobileActivity extends ListActivity {
    ListView lv;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.res);
        ArrayList<String> FilesInFolder = GetFiles("/sdcard/RootBox");
        lv = (ListView) findViewById(R.id.listView1);
        lv.setAdapter(new ArrayAdapter<String>(this,
                android.R.layout.simple_list_item_1, FilesInFolder));
        lv.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            public void onItemClick(AdapterView<?> parent, View v,
                    int position, long id) {
            }
        });
    }
    public ArrayList<String> GetFiles(String DirectoryPath) {
        ArrayList<String> MyFiles = new ArrayList<String>();
        File f = new File(DirectoryPath);
        f.mkdirs();
        File[] files = f.listFiles();
        if (files.length == 0)
            return null;
        else {
            for (int i = 0; i < files.length; i++)
                MyFiles.add(files[i].getName());
        }
        return MyFiles;
    }
}

Answer 2

日志:

ListMobileActivity}: java.lang.RuntimeException:你的内容必须创建一个id属性为'android.R.id.list'的ListView

你需要在xml中设置ListView id为'android.R.id '。如果你扩展ListActivity来创建ListView。做res布局:

<ListView android:id="@id/android:list" <!--set ListView id to android.R.id.list-->
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:drawSelectorOnTop="false"
        android:scrollbars="vertical"/>

并在代码中将ListView实例初始化为:

lv = (ListView)findViewById(android.R.id.list);

或

lv =ListMobileActivity.this.getListView();

Answer 3

Try This:

package com.mkyong.android;
import java.io.File;
import java.util.ArrayList;
import android.app.ListActivity;
import android.os.Bundle;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;
import android.view.View;
public class ListMobileActivity extends ListActivity {
    ListView lv;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.res);
        ArrayList<String> FilesInFolder = GetFiles("/sdcard/RootBox");
        setListAdapter(new ArrayAdapter<String>(this,
            android.R.layout.simple_list_item_1, FilesInFolder));
    }
    public ArrayList<String> GetFiles(String DirectoryPath) {
        ArrayList<String> MyFiles = new ArrayList<String>();
        File f = new File(DirectoryPath);
        f.mkdirs();
        File[] files = f.listFiles();
        if (files.length == 0)
            return null;
        else {
            for (int i=0; i<files.length; i++){
                MyFiles.add(files[i].getName());
            }
        }
        return MyFiles;
    }    
    @Override
    protected void onListItemClick(ListView l, View v, int position, long id) {
        //get selected items
        String selectedValue = (String) getListAdapter().getItem(position);
        Toast.makeText(this, selectedValue, Toast.LENGTH_SHORT).show();
    }
}