What is the essence of the Decorator pattern?

Design PatternsMiddle~1 min read

Decorator is a structural pattern that lets you dynamically add new behavior to an object without changing its class and without breaking the open/closed principle (OCP).

1. The Core Idea

Instead of creating many subclasses to extend functionality, a decorator wraps an object in another object that implements the same interface, but adds extra behavior before or after calling the original methods.

2. When to Use It

When you need to extend an object's behavior at runtime, rather than at inheritance time.
When it is impossible or undesirable to change the source code of the class (for example, it comes from a third-party library).
When you need to combine several features without causing an explosion of subclasses.

3. Example (Java)

java

interface DataSource {
    void write(String data);
}

class FileDataSource implements DataSource {
    public void write(String data) {
        System.out.println("Writing to file: " + data);
    }
}

class EncryptionDecorator implements DataSource {
    private DataSource wrappee;

    public EncryptionDecorator(DataSource source) {
        this.wrappee = source;
    }

    public void write(String data) {
        data = encrypt(data);
        wrappee.write(data);
    }

    private String encrypt(String data) {
        return "Encrypted(" + data + ")";
    }
}

Usage:

java

DataSource source = new EncryptionDecorator(new FileDataSource());
source.write("Hello");

Result: FileDataSource works as before, but now the data is encrypted before being written.

4. Advantages

Behavior can be added dynamically, without changing the source class.
You can combine several decorators (logging + compression + encryption).
Supports the principle of composition over inheritance.

5. Summary

Decorator is a way to "put extra features on" an object, while leaving its interface unchanged. It turns rigid inheritance into a flexible, dynamic extension of behavior.

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What is the essence of the Decorator pattern?

#1. The Core Idea

#2. When to Use It

#3. Example (Java)

#4. Advantages

#5. Summary