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📝 Day Six: Adapter Design Pattern in Java

🔍 What is the Adapter Pattern?

The Adapter Pattern allows incompatible classes to work together by converting the interface of one class into an interface expected by the client.

✅ When Should You Use It?

  • When you want to reuse an existing class but its interface doesn’t match what your code expects.
  • When you want to create a bridge between legacy code and modern systems.
  • When you want to avoid modifying source code of existing (3rd-party or legacy) classes.

🧠 Real-World Analogy

Imagine you’re traveling from India to the USA, and you want to plug in your charger 🪫. But the sockets don’t match — so you use a socket adapter. It converts the plug type so your charger can still work!

🧱 Structure 

+--------------------+ +----------------------+ | Client |----->| Target | +--------------------+ +----------------------+ ^ | +-------------------+ | Adapter | +-------------------+ | v +---------------+ | Adaptee | +---------------+
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💡 Example: Media Player Adapter

Let’s say you have a MediaPlayer interface that supports .mp3 files, but now you want to add support for .vlc, .mp4 using an AdvancedMediaPlayer. The interfaces don’t match — so we’ll use an Adapter.

✅ 1. Target Interface 

public interface MediaPlayer { void play(String audioType, String fileName); }
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✅ 2. Adaptee Class (Incompatible Interface) 

public interface AdvancedMediaPlayer { void playVlc(String fileName); void playMp4(String fileName); } public class VlcPlayer implements AdvancedMediaPlayer { public void playVlc(String fileName) { System.out.println("Playing vlc file: " + fileName); } public void playMp4(String fileName) { // Do nothing } } public class Mp4Player implements AdvancedMediaPlayer { public void playMp4(String fileName) { System.out.println("Playing mp4 file: " + fileName); } public void playVlc(String fileName) { // Do nothing } }
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✅ 3. Adapter Class 

public class MediaAdapter implements MediaPlayer { AdvancedMediaPlayer advancedMediaPlayer; public MediaAdapter(String audioType) { if (audioType.equalsIgnoreCase("vlc")) { advancedMediaPlayer = new VlcPlayer(); } else if (audioType.equalsIgnoreCase("mp4")) { advancedMediaPlayer = new Mp4Player(); } } public void play(String audioType, String fileName) { if (audioType.equalsIgnoreCase("vlc")) { advancedMediaPlayer.playVlc(fileName); } else if (audioType.equalsIgnoreCase("mp4")) { advancedMediaPlayer.playMp4(fileName); } } }
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✅ 4. Concrete Target Class (Using the Adapter) 

public class AudioPlayer implements MediaPlayer { MediaAdapter mediaAdapter; public void play(String audioType, String fileName) { if (audioType.equalsIgnoreCase("mp3")) { System.out.println("Playing mp3 file: " + fileName); } else if (audioType.equalsIgnoreCase("vlc") || audioType.equalsIgnoreCase("mp4")) { mediaAdapter = new MediaAdapter(audioType); mediaAdapter.play(audioType, fileName); } else { System.out.println("Invalid media format: " + audioType + " not supported."); } } }
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💻 Client Code 

public class AdapterDemo { public static void main(String[] args) { AudioPlayer audioPlayer = new AudioPlayer(); audioPlayer.play("mp3", "song.mp3"); audioPlayer.play("mp4", "movie.mp4"); audioPlayer.play("vlc", "concert.vlc"); audioPlayer.play("avi", "series.avi"); } }
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🧪 Output 

Playing mp3 file: song.mp3 Playing mp4 file: movie.mp4 Playing vlc file: concert.vlc Invalid media format: avi not supported.
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🎯 Benefits

✅ Promotes code reusability

✅ Acts as a bridge between legacy and new systems

✅ Adds flexibility without modifying existing classes

✅ Helps follow the Single Responsibility Principle

🔌 Java Libraries Using Adapter

  • java.util.Arrays#asList() — wraps an array into a List
  • javax.xml.bind.annotation.adapters.XmlAdapter — to map between XML and Java objects

🧠 Summary Table

Aspect Description
Pattern Type Structural
Problem Solved Interface incompatibility
Key Benefit Reusability & flexibility
Common Use Cases Integrating with 3rd party libraries, supporting legacy systems

🗺️ UML Diagram (Text Format) 

+--------------------+ | MediaPlayer | <---------+ +--------------------+ | | +play(type, name) | | +--------------------+ | ^ | | | +----------------+ +---------------------+ | AudioPlayer | | MediaAdapter | +----------------+ +---------------------+ | -AdvancedMediaPlayer | | +play(...) | +---------------------+ | +--------------------------+ | VlcPlayer Mp4Player | +--------------------------+
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💡 Pro Tip: The Adapter pattern is often used when integrating with third-party APIs whose interface you cannot change.

🚀 Up Next for Day 7: Let’s keep the streak going! Want to explore:

  • Facade
  • Chain of Responsibility
  • State

Let me know what you’re feeling next, and I’ll brew the next blog for you!

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