The Adapter Design Pattern is a structural design pattern that allows incompatible interfaces to work together. It acts as a bridge between two objects, allowing them to collaborate even if they have incompatible interfaces.
One of the examples is the memory card reader.
We insert a memory card in the card reader and the card reader in the laptop
then we can read the contents of the memory card in a laptop, this is possible because of the card reader (acting as an adaptor)

Let's take the example of MediaPlayer(legacy) that can only play mp3, now we want it to play mp4 and vlc media as well.
So, MediaPlayer can take help of third party application( which is nothing but an adapter) to play mp4 and vlc media as well.
Adapter works as a bridge between two interfaces that are not related to each other allowing them to work together ( as the example of MediaPlayer needs to play AdvanceMediaPlayer files like mp4 , vlc etc);

Steps:
Creating interface MediaPlayer > create concrete implementations
Creating interface AdvanceMediaPlayer > Create a concrete implementation
Creating MediaAdapter > implementing MediaPlayer and using AdvanceMediaPlayer (as a dependency) to act as a bridge between these two

In summary MediaPlayer uses MediaAdapter and MediaAdapter uses AdvanceMediaPlayer to help MediaPlayer get the features of AdvanceMediaPlayer

MediaPlayer

/**
 * Interface is capable of only playing mp3 media files
*/
public interface MediaPlayer {
    public void play(String mediaType,String fileName);
}

AdvanceMediaPlayer

/**
 * Interface capable of playing mp4 and vlc files both
*/
public interface AdvanceMediaPlayer {

    public void playMp4(String fileName);
    public void playVlc(String fileName);

}

Mp3MediaPlayer

public class Mp3MediaPlayer implements MediaPlayer {
    //dependency
    MediaAdapter mediaAdapter;
    @Override
    public void play(String mediaType,String fileName){
        if(mediaType.equals("mp3")){
            System.out.println("playing "+fileName+  " of type "+ mediaType);
        }
        else if(mediaType.equals("mp4") || mediaType.equals("vlc")){
            mediaAdapter = new MediaAdapter(mediaType);
            mediaAdapter.play(mediaType, fileName);
        }
        else System.out.println("Given media type is not supported ");

    }
}

MediaAdapter

public class MediaAdapter  implements MediaPlayer{

    //dependency in MediaAdapter
    AdvanceMediaPlayer advanceMediaPlayer;

    //dependency injection in the same class(not recommended)
    public MediaAdapter(String mediaType){
        this.advanceMediaPlayer = mediaType.equals("mp4") ? new Mp4MediaPlayer(): (mediaType.equals("vlc") ? new VlcMediaPlayer(): null);
    }

    @Override
    public void play(String mediaType, String fileName){
        if(mediaType.equals("vls")) this.advanceMediaPlayer.playVlc(fileName);
        else if (mediaType.equals("mp4")) this.advanceMediaPlayer.playMp4(fileName);
    }

}

Mp4MediaPlayer


public class Mp4MediaPlayer implements AdvanceMediaPlayer {
    @Override
    public void playMp4(String fileName){
        System.out.println("playing "+fileName+  " of type mp4");
    }
    //note: this is a clear violation of ISP (as this client(Mp4MediaPlayer) is being forced to override a method it does use)
    public void playVlc(String fileName){
        // do nothing
    }
}

VlcMediaPlayer


public class VlcMediaPlayer implements AdvanceMediaPlayer {

    //note: this is a clear violation of ISP (as this client is being forced to override a method it does use), for solution, we can segregate these two methods from the parent interface into separate interfaces and only implement the required one
    @Override
    public void playMp4(String fileName){
        //do nothing
    }
     public void playVlc(String fileName){
        System.out.println("playing "+fileName+  " of type vlc");
    }

}

Main


public class MediaPlayerMain{
    public static void main(String args[]){
        // below class Mp3MediaPlayer can play mp3 player (inbuilt) but can also play mp4 and vlc because of 
        //MediaAdapter
         Mp3MediaPlayer mediaPlayer = new Mp3MediaPlayer();
         mediaPlayer.play("mp3", "classics.mp3");
         mediaPlayer.play("mp4", "Krish.mp4");
         mediaPlayer.play("vlc", "MIB.vlc");
         mediaPlayer.play("csv", "random.csv");


    }
}

Output:

playing classics.mp3 of type mp3
playing Krish.mp4 of type mp4
Given media type is not supported

Adapter design pattern