CompositeGeometricShapes.java

// Composite design pattern - geometric shapes

package structural.composite;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

// Notice this is not an abstract class
class GraphicObject
{
    protected String name = "Group"; // hints at what this class does

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public GraphicObject()
    {

    }
    public String color;
    public List<GraphicObject> children = new ArrayList<>();

    private void print(StringBuilder stringBuilder,  int depth)
    {
        stringBuilder.append(String.join("", Collections.nCopies(depth, "*")))
            .append(depth > 0 ? " " : "")
            .append((color == null || color.isEmpty()) ? "" : color + " ")
            .append(getName())
            .append(System.lineSeparator());
        // This for loop is key of the composite design pattern
        // Recursively traversing the list allows us to treat a Collection like a singular object
        for (GraphicObject child : children)
            child.print(stringBuilder,  depth+1);
    }

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        print(sb, 0);
        return sb.toString();
    }
}

class Circle extends GraphicObject
{
    public Circle(String color) {
        name = "Circle";
        this.color = color;
    }
}

class Square extends GraphicObject
{
    public Square(String color) {
        name = "Square";
        this.color = color;
    }
}

class CompositeShapesDemo
{
    public static void main(String[] args) {
        GraphicObject drawing = new GraphicObject();
        drawing.setName("My Drawing");
        drawing.children.add(new Square("Red"));
        drawing.children.add(new Circle("Yellow"));

        GraphicObject group = new GraphicObject();
        group.children.add(new Circle("Blue"));
        group.children.add(new Square("Blue"));

        drawing.children.add(group); // adding a GraphicObject to a graphic object
        System.out.println(drawing); // This is KEY. Regardless whether it is an object (circle or square) or a list of objects, it always recursively prints!
    }
}