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Design Patterns in Java - Composite Pattern

Composite Pattern

  • Compose object into tree structures
  • Ex. HashMap:  You can add only one element or  a subtree without any difference 

  • Component is an abstract class

Ex. Create a Menu that can accept accept a MenuItem or another Menu without any difference

--------------------- Main  ----------------------------

public class CompositePattron {
    
    public static void main(String args[]){
    
        Menu mainMenu = new Menu("Main", "/main");
        
        MenuComponent mainMenu2 = new MenuItem("Main", "/main");
        
        mainMenu.add(mainMenu2);
        
        System.out.println(mainMenu.toString());
        
        
        
    }
    
}

  • First create an abstract class to describe the behavior 
    • The method toString will be used to print all type of objects (Menu or MenuItem)
--------------- Abstract class --------------------------
public abstract class MenuComponent {
    
     String head;
     String path;
    
    List<MenuComponent> list = new ArrayList<>();
   
    /**
     * @return the head
     */
    public String getHead() {
        return head;
    }

    /**
     * @return the path
     */
    public String getPath() {
        return path;
    }
    
    
    public abstract String toString();

    String print(MenuComponent mc){
        StringBuilder sb = new StringBuilder(this.head);
        sb.append(": ");
        sb.append(this.path);
        sb.append("\n");
        
        return sb.toString();
    }
    
    public abstract MenuComponent add(MenuComponent mc);
    public abstract MenuComponent remove(MenuComponent mc);
}

  • Second create the Leaf object
    • Because this is a leaf the methods add & remove doesn't have any functionality defined
---------------------- MenuItem ---------------------------------------

public class MenuItem extends MenuComponent {

    public MenuItem(String head, String path) {
        this.head = head;
        this.path = path;

    }

    @Override
    public String toString() {
        return print(this);
    }

    @Override
    public MenuComponent add(MenuComponent mc) {
        throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
    }

    @Override
    public MenuComponent remove(MenuComponent mc) {
        throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
    }

}

  • Create the Composite object, this object contains Leaf objects
    • Due to the leaf objects the methods add & remove now have functionality.
------------------------ Menu ----------------------------
public class Menu extends MenuComponent{
    
    public Menu(String head, String path){
        this.head = head;
        this.path = path;
    }
    
    @Override
    public MenuComponent add(MenuComponent mc){
        this.list.add(mc);
        return mc;
    }
    
    @Override
    public MenuComponent remove(MenuComponent mc){
        this.list.remove(mc);
        return mc;
    }

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder(this.head);
        sb.append(print(this));
        
        Iterator<MenuComponent> itr = this.list.iterator();
        while(itr.hasNext()){
            MenuComponent mc = itr.next();
            sb.append(mc.toString());
        }
        
        return sb.toString();
    }
}








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