编写android代码时此处理程序的用途是什么？

Question

Here in the below code a songsManager object is created and then why is this.songsList used to store the song files and why not only songsList is used. Main question is what is the use of this and what exactly is it and when it is used? My main doubt is that here since no other songsList is declared so there is no chance of songsList clashing so why to specifically refer to it as the songsList declared in the present class. Mainly I use it when there are arguments passed to a function whose names are same as that of objects or variables declared within the class so to avoid confusion and to tell the compiler that I want to use the object declared in that class and not the one passed as an argument I used this.. Please correct me if I am wrong and add to my knowledge about this.

The code lines of interest are followed by // please see to it

public class CustomizedListView extends Activity{

private int currentIndex;
private String[] menuItems = {"Play","Share Music Via","Details"};
private LinkedList<File> songsList = new LinkedList<File>();//
private ArrayList<HashMap<String, String>> songsListdata = new ArrayList<HashMap<String, String>>();
private MediaMetadataRetriever mmr = new MediaMetadataRetriever();
private Utilities utils=new Utilities();
ListView list=null;
ModifiedAdapter adapter=null;
SongsManager plm=null;//
Button search;
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.playlist);

    plm = new SongsManager();//
    File extStore = Environment.getExternalStorageDirectory();
    // get all songs from sdcard
    this.songsList = plm.getFilesInFolder(extStore);//
    for (int i = 0; i < songsList.size(); i++) {
        // creating new HashMap
        HashMap<String, String> song = new HashMap<String, String>();
        mmr.setDataSource(songsList.get(i).getAbsolutePath().toString());

        //getting artist
        String artist = mmr.extractMetadata(MediaMetadataRetriever.METADATA_KEY_ALBUMARTIST);
        if(artist==null)
            artist=mmr.extractMetadata(MediaMetadataRetriever.METADATA_KEY_ARTIST);

        //getting Duration
        String len = mmr.extractMetadata(MediaMetadataRetriever.METADATA_KEY_DURATION);

        long Len=0;
        try
        {
            Len=Integer.parseInt(len);
        }
        catch(Exception e)
        {
            Log.i(null, ":conversion error");
        }
        len=utils.milliSecondsToTimer(Len);
        Log.i(null, "length"+len);
        song.put("songTitle", (songsList.get(i)).getName().substring(0, ((songsList.get(i)).getName().length() - 4)));
        song.put("songArtist", artist);
        song.put("duration", len);
        song.put("songPath",songsList.get(i).getAbsolutePath().toString());
        // adding HashList to ArrayList
        songsListdata.add(song);
    }

    list=(ListView)findViewById(R.id.list);
    // Getting adapter by passing xml data ArrayList
    adapter=new ModifiedAdapter(this, songsListdata);
    list.setAdapter(adapter);
    // Click event for single list row
    list.setOnItemClickListener(new OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View view,
                final int position, long id) {

            final String songPath =songsList.get(position).getAbsolutePath().toString();
            AlertDialog.Builder builder = new AlertDialog.Builder(CustomizedListView.this);
            builder.setTitle((songsList.get(position)).getName().substring(0, ((songsList.get(position)).getName().length() - 4)));
            builder.setItems(menuItems, new DialogInterface.OnClickListener()
             {

                    public void onClick(DialogInterface dialog, int item)
                    {  

                        if(item==0)
                        {
                            Intent in = new Intent(getApplicationContext(),MainActivity.class);
                            // Sending songIndex to PlayerActivity
                            in.putExtra("songIndex", position);
                            setResult(100, in);
                            // Closing PlayListView
                            finish();
                        }
                        else if(item==2)
                        {
                            Intent details = new Intent(getApplicationContext(),Details.class);
                            details.putExtra("songPath", songPath);
                            startActivity(details);
                        }
                        else if(item==1)
                        {
                            Intent intent = new Intent();  
                            intent.setAction(Intent.ACTION_SEND);  
                            intent.setType("audio/*");
                            intent.putExtra(Intent.EXTRA_STREAM, Uri.fromFile(new File(songPath)));
                            startActivity(intent);
                        }
                    }
             });
            AlertDialog alert = builder.create();
            alert.show();
        }
    });

    //Search for a song implementations
    search=(Button)findViewById(R.id.searchForSong);
    search.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View arg0) {
            // TODO Auto-generated method stub
            Intent launchBrowser=new Intent(getApplicationContext(), Browser.class);
            startActivity(launchBrowser);
        }
    });
}

}

Answer 1

this keyword is used to refere to the current object

So you can access any member of current object using this.member. As in your example you are accesig songList within the current object so there is no difference between using this and not using this.

More use of this keyword

as you mentioned about the following example

private int a;
void method(int a){
    this.a = a;
}

here this is used to refer to the member of current object as the names are same. if you used

void method(int b){
    a = b;
}

then there would be no difference between using this and not using this

Some More Example

private int a = 5;

public void method() {
    int a = 6;
    System.out.println(a); // will print 6 
    System.out.println(this.a);  // will print 5
}

int the following example the second one is pointing to the member variable of current object so it is printing 5.

Answer 2

Actually this answer should be broken in a few steps

1

THIS operator

it will refer to the current object/scope in which it is used

for eg: say a button listener is made like this

new button(context).setOnClickListener(new View.onClickListener(public void onClick(View v){ //Using this here to refers to this onclicklistener

});

// for a constructor

public classname(int arg1){ //so to initialise the arg1 of yur class with this arg1

//for just thesake of.clarity you write this.arg1=arg1;

}

2

this used here with the songlist is redundant and is of no signficance as ther e is no conflict.

Hope this helps you.