understand Hash map keys- java

Question

I know that HashMap doesn’t allow duplicate keys (It does allow duplicate values). However in this example that works perfectly,all the values have the same key(that means the key is not unique)Maybe I misunderstood the code.Could someone help me understand it right.

this is the code

public class PlayListManager {
//**********************************hashmap keys************************
public static final String ALL_VIDEOS = "AllVideos";
public static final String ALL_Songs = "AllSongs";
//***************************************************************************
 ..
 ...
 ....
       while (cursor.moveToNext()) {
        Songs song = new Songs();
        song.songId = cursor.getString(cursor
                .getColumnIndexOrThrow(MediaStore.Audio.Media._ID));
        song.artist = cursor.getString(cursor
                .getColumnIndexOrThrow(MediaStore.Audio.Media.ARTIST));
        song.title = cursor.getString(cursor
                .getColumnIndexOrThrow(MediaStore.Audio.Media.TITLE));
        song.songName = cursor
                .getString(cursor
                        .getColumnIndexOrThrow(MediaStore.Audio.Media.DISPLAY_NAME));
        song.duration = Integer.parseInt(cursor.getString(cursor
                .getColumnIndexOrThrow(MediaStore.Audio.Media.DURATION)));
        song.albumId = Integer.parseInt(cursor.getString(cursor
                .getColumnIndexOrThrow(MediaStore.Audio.Media.ALBUM_ID)));
        song.songData = cursor.getString(cursor
                .getColumnIndexOrThrow(MediaStore.Audio.Media.DATA));


        //*******here it uses the same ky for all the videos?!!*************
        HashMap<String, Object> songMap = new HashMap<String, Object>();
        songMap.put(ALL_Songs, song);
        songsList.add(songMap);

    }

}

What's the output when you try to print out the keys in your map? http://docs.oracle.com/javase/7/docs/api/java/util/HashMap.html#put(K,%20V) see this link. You are replacing this key at all times. — arin, Jul 24 '14 at 19:21
[Here](http://stackoverflow.com/questions/1669885/what-happens-when-a-duplicate-key-is-put-into-a-hashmap) is an explanation. — betteroutthanin, Jul 24 '14 at 19:21

score 3 · Accepted Answer · edited Jun 20 '20 at 09:12

3

In each iteration of the while loop, the code is creating a new HashMap instance and filling it with only one entry that maps a String key ("AllSongs") to the Song object.

This map (which contains only one entry) will then be added to the list songsList.

After the while loop is finished, you would have a list of HashMaps where each map basically maps a hardcoded keyword to one song.

songsList:

<"AllSongs", song1>, <"AllSongs", song2>, <"AllSongs", song3>, ...

In this case, using a HashMap seems redundant. You could just fill the list with the Song instances without saving each one in a map.

The key concept here is that there are many HashMap (each iteration creates one), as opposed to having a "global" HashMap. If it were global, each song would overwrite the other since they are always mapped using the same hardcoded key.

edited Jun 20 '20 at 09:12

Community

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answered Jul 24 '14 at 19:25

M A

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I understand now.But why not use hashmap object to be added to the list Songlist? why not use object Song to be added to the list Songlist instead?---Im talking about this specific example – user3392377 Jul 24 '14 at 19:32
You're right: In this case, using a HashMap seems redundant. You could just fill the list with the Song instances without saving each one in a map. But I don't know who wrote this code and why he or she decided on this approach. – M A Jul 24 '14 at 19:36

understand Hash map keys- java

1 Answers1