Question

嘿大家此刻我试图在ListView的android设备SD卡上的某个文件夹中显示所有文件，到目前为止我运气不好，它崩溃了一个nullpointer异常，所以我想知道如果这里的任何人都可以在我的活动和下面的logcat上点亮我的问题，并且感谢。

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

如果您在Android清单中要求权限以读取存储空间，那么这可能就是问题所在。

从代码中可以看出，您正在查询实例块中的列表视图实例。这可能在调用setContentView之前调用。在布局膨胀之后，您应该在XML中实例化一个UI组件，否则您将遇到异常。

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;
    }

}

它应该有用。

Answer 2

在日志中：

ListMobileActivity}：java.lang.RuntimeException：您的内容必须有一个ListView，其id属性是'android.R.id.list'

如果要扩展ListActivity以创建ListView，则需要将xml中的ListView id设置为“android.R.id.list”。将其作为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);

OR

lv =ListMobileActivity.this.getListView();

Answer 3

试试这个：

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();

    }

}

Android编程崩溃空指针异常

3 个答案: