Java
Decorator
Learn the Decorator pattern.
The Decorator pattern
The Decorator adds behavior by wrapping an object. The wrapping class implements the same interface as the wrapped object, so the client sees no difference. Several decorators can nest.
Example: a Coffee interface, a SimpleCoffee base object. A MilkDecorator takes a Coffee inside and adds the milk price to its cost. new MilkDecorator(new SimpleCoffee()) gives a wrapped coffee. Wrapping is preferred over inheritance, so behavior can be added at runtime.
Nested decorators
Decorators can nest: new SugarDecorator(new MilkDecorator(new SimpleCoffee())). Each layer adds its own contribution; the result is the sum of all layers' costs.
This prevents the inheritance hierarchy from bloating. Instead of a new subclass for each feature, an independent decorator is written. The client customizes the object by picking the layers it wants. This flexibility is common in IO streams (for example BufferedReader).
Exercise Add a milk decorator to coffee
Define a Coffee interface with int cost(). A SimpleCoffee class implements it and cost returns 10. A MilkDecorator class implements the Coffee interface; its constructor takes a Coffee inner, and cost returns inner.cost() + 3. In main, wrap with new MilkDecorator(new SimpleCoffee()) and print the cost() result with System.out.println.
Show example solution
interface Coffee { int cost(); }
class SimpleCoffee implements Coffee {
public int cost() { return 10; }
}
class MilkDecorator implements Coffee {
private Coffee inner;
public MilkDecorator(Coffee inner) { this.inner = inner; }
public int cost() { return inner.cost() + 3; }
}
public class DecoratorDemo {
public static void main(String[] args) {
Coffee c = new MilkDecorator(new SimpleCoffee());
System.out.println(c.cost());
}
}
Test yourself
-
What does a Decorator share with the object it wraps?
- The same interface
- The fields
- The constructor
- The static state
Show answer
The same interface — A Decorator implements the same interface as the wrapped object.
-
Match the Decorator concepts
- Coffee
- Common interface
- SimpleCoffee
- Base object
- MilkDecorator
- Wrapper
- inner.cost()
- Delegate to inner
-
Complete the Decorator constructor
public MilkDecorator(____ inner) { this.inner = inner; }
Show answer
public MilkDecorator(Coffee inner) { this.inner = inner; }
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