Composite Design Pattern in Java

Composite design pattern

  1. Create a “lowest common denominator” that makes classes interchangeable
  2. All concrete classes declare an “isa” relationship to the interface
  3. All “container” classes couple themselves to the interface
  4. “Container” classes use polymorphism as they delegate to their children
// 1. "lowest common denominator"
interface Component {
    void traverse();
}

// 2. "Isa" relationship
class Primitive implements Component {
    private int value;

    public Primitive(int val) {
        value = val;
    }

    public void traverse() {
        System.out.print( value + "  " );
    }
}

// 2. "Isa" relationship
abstract class Composite implements Component {
    // 3. Couple to interface
    private Component[] children = new Component[9];
    private int total = 0;
    private int value;
    public Composite(int val) {
        value = val;
    }

    // 3. Couple to interface
    public void add(Component c) {
        children[total++] = c;
    }

    public void traverse() {
        System.out.print(value + "  ");
        for (int i=0; i < total; i++) {
            // 4. Delegation and polymorphism
            children[i].traverse();
        }
    }
}

// Two different kinds of "container" classes.  Most of the
// "meat" is in the Composite base class.
class Row extends Composite {
    public Row(int val) {
        super(val);
    }

    public void traverse() {
        System.out.print("Row");
        super.traverse();
    }
}

class Column extends Composite {
    public Column(int val) {
        super(val);
    }

    public void traverse() {
        System.out.print("Col");
        super.traverse();
    }
}

public class CompositeDemo {
    public static void main( String[] args ) {
        Composite first  = new Row( 1 );
        Composite second = new Column( 2 );
        Composite third  = new Column( 3 );
        Composite fourth = new Row( 4 );
        Composite fifth  = new Row( 5 );
        first.add(second);
        first.add(third);
        third.add(fourth);
        third.add(fifth);
        first.add(new Primitive(6));
        second.add(new Primitive(7));
        third.add(new Primitive(8));
        fourth.add(new Primitive(9));
        fifth.add(new Primitive(10));
        first.traverse();
    }
}

Output

Row1  Col2  7  Col3  Row4  9  Row5  10  8  6

Code examples