从TextView设置文本时，Android应用程序崩溃

Question

我已经在stackoverflow中彻底搜索了我的问题的解决方案，但没有成功，所以我必须自己问这个问题。我的问题是，当我

Songname.setText(Fullname);

我的应用程序崩溃。如果我注释掉代码，那么应用程序不会崩溃。我的全名变量是

Fullname = (artist + ":" + album + ":" + track);

来自BroadcastReceiver:

    private BroadcastReceiver mReceiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        String action = intent.getAction();
        String cmd = intent.getStringExtra("command");
        Log.v("tag ", action + " / " + cmd);
        String artist = intent.getStringExtra("artist");
        String album = intent.getStringExtra("album");
        String track = intent.getStringExtra("track");
        Log.v("tag", artist + ":" + album + ":" + track);
        Fullname = (artist + ":" + album + ":" + track);
        Toast.makeText(MusicPlayer.this, track, Toast.LENGTH_SHORT).show();
        update();
    }
};

(我想(我的TextView设置正确，所以我怀疑这是问题所在，但我会发布它以防万一：

TextView Songname;    
TextView Songname = (TextView) findViewById(R.id.musictitle);

以下是整个xml，以防出现问题：

<?xml version="1.0" encoding="utf-8"?>

<TextView
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:textAppearance="?android:attr/textAppearanceLarge"
    android:text="Large Text"
    android:id="@+id/musictitle"
    android:layout_marginTop="46dp"
    android:ellipsize="marquee"
    android:scrollHorizontally="true"
    android:focusable="true"
    android:focusableInTouchMode="true"
    android:layout_alignParentTop="true"
    android:layout_alignParentLeft="true" />
<Button
    android:layout_width="50dp"
    android:layout_height="50dp"
    android:text="New Button"
    android:id="@+id/button"
    android:layout_alignBottom="@+id/imageView"
    android:layout_toLeftOf="@+id/imageView"
    android:layout_marginRight="25dp" />
<Button
    android:layout_width="50dp"
    android:layout_height="50dp"
    android:text="New Button"
    android:id="@+id/button2"
    android:layout_alignTop="@+id/button"
    android:layout_alignParentLeft="true"
    android:layout_marginLeft="35dp" />
<Button
    android:layout_width="50dp"
    android:layout_height="50dp"
    android:text="New Button"
    android:id="@+id/button3"
    android:layout_alignTop="@+id/textView7"
    android:layout_toLeftOf="@+id/textView7" />
<Button
    android:layout_width="50dp"
    android:layout_height="50dp"
    android:text="New Button"
    android:id="@+id/button4"
    android:layout_alignTop="@+id/textView7"
    android:layout_toRightOf="@+id/seekBar" />
<TextView
    android:layout_width="50dp"
    android:layout_height="50dp"
    android:textAppearance="?android:attr/textAppearanceLarge"
    android:text="0"
    android:id="@+id/textView7"
    android:gravity="center"
    android:layout_gravity="center"
    android:layout_alignBottom="@+id/imageView"
    android:layout_toLeftOf="@+id/button4" />
<SeekBar
    android:layout_width="500dp"
    android:layout_height="wrap_content"
    android:id="@+id/seekBar"
    android:indeterminate="false"
    android:layout_below="@+id/musictitle"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="30dp" />
<ImageView
    android:layout_width="180dp"
    android:layout_height="180dp"
    android:id="@+id/imageView"
    android:layout_below="@+id/seekBar"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="24dp" />

以及类文件，如果这是问题所在：

package com.androidddev101.ep8;

import android.app.Activity;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.text.TextUtils;
import android.util.Log;
import android.view.View;
import android.content.IntentFilter;
import android.widget.TextView;
import android.widget.Toast;

public class MusicPlayer extends Activity implements View.OnClickListener {
    public static final String SERVICECMD = "com.android.music.musicservicecommand";
    String Fullname;
String artist, album, track;
TextView Songname;
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.musicplayer);

    TextView Songname = (TextView) findViewById(R.id.musictitle);
    IntentFilter iF = new IntentFilter();
    iF.addAction("com.android.music.metachanged");
    iF.addAction("com.android.music.playstatechanged");
    iF.addAction("com.android.music.playbackcomplete");
    iF.addAction("com.android.music.queuechanged");
    iF.addAction("com.htc.music.metachanged");
    iF.addAction("fm.last.android.metachanged");
    iF.addAction("com.sec.android.app.music.metachanged");
    iF.addAction("com.nullsoft.winamp.metachanged");
    iF.addAction("com.amazon.mp3.metachanged");
    iF.addAction("com.miui.player.metachanged");
    iF.addAction("com.real.IMP.metachanged");
    iF.addAction("com.sonyericsson.music.metachanged");
    iF.addAction("com.rdio.android.metachanged");
    iF.addAction("com.samsung.sec.android.MusicPlayer.metachanged");
    iF.addAction("com.andrew.apollo.metachanged");
    iF.addAction("com.jrtstudio.AnotherMusicPlayer.metachanged");

    registerReceiver(mReceiver, iF);
}


private BroadcastReceiver mReceiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        String action = intent.getAction();
        String cmd = intent.getStringExtra("command");
        Log.v("tag ", action + " / " + cmd);
        String artist = intent.getStringExtra("artist");
        String album = intent.getStringExtra("album");
        String track = intent.getStringExtra("track");
        Log.v("tag", artist + ":" + album + ":" + track);
        Fullname = (artist + ":" + album + ":" + track);
        Toast.makeText(MusicPlayer.this, track, Toast.LENGTH_SHORT).show();
        update();
    }
};
public void update() {
    {
       // Fullname = (artist + ":" + album + ":" + track);
        Log.v("tag", Fullname + "123");
        Songname.setText(Fullname);
        Songname.setSelected(true);
        Songname.setEllipsize(TextUtils.TruncateAt.MARQUEE);
        Songname.setSingleLine(true);
    }
}
@Override
public void onClick(View v) {
}

}

注意：toast消息有效，并为我提供曲目名称

这是Logcat 的错误

导致原因：java.lang.NullPointerException网址：com.androiddev101.ep8.MusicPlayer.update(MusicPlayr.java:81(网址：com.androiddev101.ep8.MusicPlayer$1.onReceive(MusicPlayer.java:72(

感谢您的帮助。

Answer 1

TextView Songname; // not instantiated for global use
TextView Songname = (TextView) findViewById(R.id.musictitle); // instantiated in only oncreate

把它改成这个

TextView Songname;    
Songname = (TextView) findViewById(R.id.musictitle);

Answer 2

Songname的引用是onCreate的本地引用，您不需要声明另一个名为Songname。

package com.androidddev101.ep8;

import android.app.Activity;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.text.TextUtils;
import android.util.Log;
import android.view.View;
import android.content.IntentFilter;
import android.widget.TextView;
import android.widget.Toast;

    public class MusicPlayer extends Activity implements View.OnClickListener {
      public static final String SERVICECMD = "com.android.music.musicservicecommand";
      String Fullname;
      String artist, album, track;
      TextView Songname;
      @Override
      public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.musicplayer);

        Songname = (TextView) findViewById(R.id.musictitle);
        IntentFilter iF = new IntentFilter();
        iF.addAction("com.android.music.metachanged");
        iF.addAction("com.android.music.playstatechanged");
        iF.addAction("com.android.music.playbackcomplete");
        iF.addAction("com.android.music.queuechanged");
        iF.addAction("com.htc.music.metachanged");
        iF.addAction("fm.last.android.metachanged");
        iF.addAction("com.sec.android.app.music.metachanged");
        iF.addAction("com.nullsoft.winamp.metachanged");
        iF.addAction("com.amazon.mp3.metachanged");
        iF.addAction("com.miui.player.metachanged");
        iF.addAction("com.real.IMP.metachanged");
        iF.addAction("com.sonyericsson.music.metachanged");
        iF.addAction("com.rdio.android.metachanged");
        iF.addAction("com.samsung.sec.android.MusicPlayer.metachanged");
        iF.addAction("com.andrew.apollo.metachanged");
        iF.addAction("com.jrtstudio.AnotherMusicPlayer.metachanged");

        registerReceiver(mReceiver, iF);
    }

    private BroadcastReceiver mReceiver = new BroadcastReceiver() {
      @Override
      public void onReceive(Context context, Intent intent) {
        String action = intent.getAction();
        String cmd = intent.getStringExtra("command");
        Log.v("tag ", action + " / " + cmd);
        String artist = intent.getStringExtra("artist");
        String album = intent.getStringExtra("album");
        String track = intent.getStringExtra("track");
        Log.v("tag", artist + ":" + album + ":" + track);
        Fullname = (artist + ":" + album + ":" + track);
        Toast.makeText(MusicPlayer.this, track, Toast.LENGTH_SHORT).show();
        update();
       }
      };
      public void update() {
        {
          // Fullname = (artist + ":" + album + ":" + track);
          Log.v("tag", Fullname + "123");
          Songname.setText(Fullname);
          Songname.setSelected(true);
          Songname.setEllipsize(TextUtils.TruncateAt.MARQUEE);
          Songname.setSingleLine(true);
        }
      }
      @Override
      public void onClick(View v) {
      }
    }