Question

我是Android App Development的新手。我正在创建一个简单的音乐应用程序，从用户的外部存储中获取歌曲并播放它。我正在使用Fragments。

这是我的SongList.class代码============================

package com.android.jarvis.easysplay;

import android.content.Intent;
import android.os.Bundle;
import android.os.Environment;
import android.support.annotation.Nullable;
import android.support.annotation.StringDef;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;

import java.io.File;
import java.util.ArrayList;
import java.util.List;

/**
 * Created by Devil on 5/17/2017.
 */

public class SongsList extends android.support.v4.app.Fragment {
    ListView lv;
    String items;

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {

        lv =(ListView) getView().findViewById(R.id.lvPlaylist);
        ArrayList<File> mySongs = findSongs(Environment.getExternalStorageDirectory());
        for(int i=0; i<mySongs.size(); i++){
            toast(mySongs.get(i).getName().toString());
        }

        return inflater.inflate(R.layout.songs_list, container, false);

    }
    public ArrayList<File> findSongs(File root){
        ArrayList<File> al = new ArrayList<>();
        File[] files = root.listFiles();
        for(File singleFile:files){
            if(singleFile.isDirectory() && !singleFile.isHidden()){
                al.addAll(findSongs(singleFile));
            }else {
                if(singleFile.getName().endsWith(".mp3") || singleFile.getName().endsWith(".wav")){
                    al.add(singleFile);
                }
            }
        }
        return al;
    }
    public void toast(String text){
        Toast.makeText(getActivity(), text, Toast.LENGTH_SHORT).show();
    }

}

当我开始此活动时，它显示以下错误。我不知道该怎么做

05-18 21:58:56.747 18725-18725/com.android.jarvis.easysplay E/AndroidRuntime: FATAL EXCEPTION: main
                                                                              Process: com.android.jarvis.easysplay, PID: 18725
                                                                              java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference
                                                                                  at com.android.jarvis.easysplay.SongsList.onCreateView(SongsList.java:32)
                                                                                  at android.support.v4.app.Fragment.performCreateView(Fragment.java:2192)
                                                                                  at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1299)
                                                                                  at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1528)
                                                                                  at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1595)
                                                                                  at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:758)
                                                                                  at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2363)
                                                                                  at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2149)
                                                                                  at android.support.v4.app.FragmentManagerImpl.optimizeAndExecuteOps(FragmentManager.java:2103)
                                                                                  at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2013)
                                                                                  at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:710)
                                                                                  at android.os.Handler.handleCallback(Handler.java:815)
                                                                                  at android.os.Handler.dispatchMessage(Handler.java:104)
                                                                                  at android.os.Looper.loop(Looper.java:194)
                                                                                  at android.app.ActivityThread.main(ActivityThread.java:5643)
                                                                                  at java.lang.reflect.Method.invoke(Native Method)
                                                                                  at java.lang.reflect.Method.invoke(Method.java:372)
                                                                                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:960)
                                                                                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)

Answer 1

您尝试在充分展开之前访问您的观点，因此它尚不存在（因此NullPointerException）。将您的onCreateView()方法更改为以下内容：

@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.songs_list, container, false);
    lv =(ListView) view.findViewById(R.id.lvPlaylist);

    ArrayList<File> mySongs = findSongs(Environment.getExternalStorageDirectory());
    for(int i=0; i<mySongs.size(); i++){
        toast(mySongs.get(i).getName().toString());
    }

    return view
}

创建简单音乐应用程序

1 个答案: