Composite Pattern

What

Composes objects into tree structures and lets clients treat individual objects and compositions uniformly.

When

You need to treat part-whole hierarchies of objects the same way. You want to treat individual and group objects uniformly (e.g., a tree).

class Circle {
    public void draw() {
        System.out.println("Drawing Circle");
    }
}
 
class Drawing {
    private List<Object> elements = new ArrayList<>(); // no common interface
 
    public void drawAll() {
        for (Object o : elements) {
            if (o instanceof Circle) {
                ((Circle) o).draw();
            } // messy, unscalable logic
        }
    }
}
 

How

• Define a common interface for leaf and composite classes.
• Composite holds child components and delegates operations.

interface Graphic { void draw(); }
 
class Circle implements Graphic {
    public void draw() { System.out.println("Circle"); }
}
 
class Drawing implements Graphic {
    private List<Graphic> elements = new ArrayList<>();
    public void add(Graphic g) { elements.add(g); }
    public void draw() { for (Graphic g : elements) g.draw(); }
}
 

Examples

• GUI components like buttons, panels, and windows.
• File systems with files and directories.

Why

Advantages

• Simplifies client code.
• Makes it easy to add new kinds of components.

Disadvantages

• Can make the design overly general.
• Harder to restrict certain composite-leaf relationships.