我正在制作音乐播放器应用程序。
它在类中给出NullPointerException。这是player.java的代码
package com.example.mudit.take6;
import android.content.Intent;
import android.os.Bundle;
import android.os.Environment;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;
import java.io.File;
import java.util.ArrayList;
public class MainActivity extends AppCompatActivity {
ListView lv;
String[] items;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.content_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
lv = (ListView) findViewById(R.id.lvplaylist);
final ArrayList<File> mySongs = findsongs(Environment.getExternalStorageDirectory());
items = new String[mySongs.size()];
for (int i = 0; i < mySongs.size(); i++) {
//toast(mySongs.get(i).getName().toString());
items[i] = mySongs.get(i).getName().toString().replace(".mp3", "").replace("", "");
}
ArrayAdapter<String> adp = new ArrayAdapter<String>(getApplicationContext(), R.layout.song_layout, R.id.textView, items);
lv.setAdapter(adp);
lv.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
startActivity(new Intent(getApplicationContext(), Player.class).putExtra("pos", position).putExtra("SongList", mySongs));
}
}
);
}
public ArrayList<File> findsongs(File root) {
ArrayList<File> al = new ArrayList<File>();
File[] files = root.listFiles();
for (File SingleFiles : files) {
if (SingleFiles.isDirectory() && !SingleFiles.isHidden()) {
al.addAll(findsongs(SingleFiles));
} else {
if (SingleFiles.getName().endsWith(".mp3")) {
al.add(SingleFiles);
}
}
}
return al;
}
public void toast(String text) {
Toast.makeText(getApplicationContext(), text, Toast.LENGTH_SHORT).show();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
}
错误： -
04-16 01:00:56.575 17684-17684/? I/art: Late-enabling -Xcheck:jni
04-16 01:00:56.640 17684-17684/com.example.mudit.take6 W/System: ClassLoader referenced unknown path: /data/app/com.example.mudit.take6- 1/lib/arm
04-16 01:00:56.822 17684-17684/com.example.mudit.take6 D/AndroidRuntime: Shutting down VM
04-16 01:00:56.825 17684-17684/com.example.mudit.take6 E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.mudit.take6, PID: 17684
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.mudit.take6/com.example.mudit.take6.MainActivity}: java.lang.NullPointerException: Attempt to get length of null array
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2426)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2490)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1354)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5443)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
Caused by: java.lang.NullPointerException: Attempt to get length of null array
at com.example.mudit.take6.MainActivity.findsongs(MainActivity.java:59)
at com.example.mudit.take6.MainActivity.onCreate(MainActivity.java:33)
at android.app.Activity.performCreate(Activity.java:6259)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1130)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2379)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2490)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1354)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5443)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
如何成功运行它。请告诉我，我做错了。如果你能以正确的方式讲述。 任何帮助将不胜感激。
答案 0 :(得分：0)
在尝试访问外部存储空间时，您需要在Manifest文件中包含权限。
由于您只是阅读SD卡，因此您只能包含读取权限 -
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
如果你试图修改SD卡内容，你应该包括 -
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
答案 1 :(得分：0)
在第59行上抛出异常，这是
for (File SingleFiles : files)，这里的files数组为null，导致NullPointerException。
更改代码以验证文件数组是否为空。
public ArrayList<File> findsongs(File root) {
ArrayList<File> al = new ArrayList<File>();
File[] files = root.listFiles();
if(files != null){
for (File SingleFiles : files) {
if (SingleFiles.isDirectory() && !SingleFiles.isHidden()) {
al.addAll(findsongs(SingleFiles));
} else {
if (SingleFiles.getName().endsWith(".mp3")) {
al.add(SingleFiles);
}
}
}
}
return al;
}
答案 2 :(得分：0)
它看起来像你的数组列表＆＃34; mySongs＆＃34;一片空白。通过在代码中添加类似内容来检查它：
if(mySongs != null && mySongs.length > 0) {
...
}