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The main purpose of the **Adapter** pattern is to **make incompatible interfaces compatible**, that is, to **let objects with different interfaces work together** without changing their code.

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### 1. **The Core Idea**

Sometimes you need to use a third-party or legacy class whose interface **does not match** what the rest of the system expects.
The adapter acts as a **layer (wrapper)** that "translates" calls from one interface into another, ensuring compatibility.

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### 2. **When It Is Used**

- When you need to integrate **legacy code** with a new system.
- When there is a **library or external API** whose interface cannot be changed.
- When different components of a system must work **through a single interface** but are implemented differently.

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### 3. **Example (Java)**

Suppose we have a modern interface:

```java
interface MediaPlayer {
    void play(String file);
}
```

and an old class:

```java
class OldPlayer {
    void startPlayback(String filePath) {
        System.out.println("Playing " + filePath);
    }
}
```

The adapter "translates" the interface:

```java
class PlayerAdapter implements MediaPlayer {
    private OldPlayer oldPlayer = new OldPlayer();

public void play(String file) {
        oldPlayer.startPlayback(file);
    }
}
```

Now the old `OldPlayer` can be used in the new system through the `MediaPlayer` interface.

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### 4. **Advantages**

- Allows you to **reuse existing code** without changing it.
- Provides **flexibility and compatibility** between different systems.
- Supports the **open/closed principle (OCP)**: the adapter is added, it does not modify existing code.

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### 5. **Conclusion**

Adapter solves the problem of **integrating incompatible interfaces**, turning them into a single interaction format without breaking encapsulation or requiring changes to the original classes.

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