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Composite Design Pattern: Structure and Implementation

Composite pattern helps manage complex object structures by treating components uniformly yet differently when needed. This pattern defines abstract classes for primitives and containers, enabling the creation of composite objects while handling child components effectively. This design pattern is widely utilized in GUI frameworks and diverse application scenarios. Learn how to implement and use the Composite Design Pattern in Java with real-world examples and insights.

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Composite Design Pattern: Structure and Implementation

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  1. Composite Design Pattern Rick Mercer

  2. Composite Pattern • Recurring problem: • Often complex structures are built with container and primitive objects. Container objects can contain other objects • How can code that uses these classes treat all the objects in the structure identically sometimes, yet differently when it matters? • Solution: • Define an abstract class that represents primitives and containers • Composite was used in the View class of Smalltalk MVC as well as most other GUI toolkits

  3. General Form of Composite

  4. Participants • Component • Declares the interface for all objects in the composition • Implements default behavior, as appropriate • Declares an algorithm interface (set of methods) for accessing and managing child components • Leaf: Has no children: it is a primitive • Composite: Defines behavior for components having children • Also implements child-related operations of Component

  5. Participants • Component has operations that apply to all • The component can be a Composite or a Leaf • Composite adds methods indicating a collection: add(), and remove() • In each method, a Component is passed • Can add either a Child or a Component • Component should not add itself • Should not add a Component to a leaf

  6. Usage Example ArrayList<Object> a = newArrayList<Object>(); a.add("abc"); a.add("cde"); ArrayList<Object> b = newArrayList<Object>(); b.add(1.11); b.add(2.22); System.out.println("a: " + a); System.out.println("b: " + b); b.add(a); b.add(b); // a.add(b); Stack Overflow System.out.println("a: " + a); System.out.println("b: " + b); • What types are the Leafs here? • ___________________ • What type is the Composite? • ___________________ • What type is the Component? • ___________________ • Output? • ________________________________ • ________________________________

  7. Use Example: Java Swing • Java Swing has four major pieces: • Events and EventListeners • Layouts • Drawing • Graphical Components • The root of all is also named Component • Component utilizes the Composite pattern in several ways • One you may find useful or need for your final project

  8. JMenus in Java Swing • Java menus use the Composite Design Pattern • JMenuBar is a composite extending JComponent • JMenuBar is a composite extending JComponent • Can add others like JLabel, JTextField • Can also add JMenuItem to JMenuItem • JMenuItem has three subclasses • JMenu • JRadioButtonMenuItem • JCheckboxMenuItem

  9. JMenuItem menu = new JMenu("Composite"); menu.setMnemonic('C');//Open with alt-C // Create two leafs JLabel label = new JLabel("Label"); JTextField textF = new JTextField("text field"); menu.add(label); menu.add(textF); // Add a Composite JMenuItem menuItem = new JMenuItem("menu item"); menu.add(menuItem); // Add two Composites to a Composite JMenuItem jmi1Nest = new JMenu("Nest 1"); menu.add(jmi1Nest); JMenuItem jmiNested1 = new JMenuItem("Nested in 1"); jmi1Nest.add(jmiNested1); JMenuItem jmiNested2 = new JMenuItem("Nested in 1 also"); jmi1Nest.add(jmiNested2);

  10. JMenuItemDemoComposite // Add two more Composites JMenuItem checkBox = new JCheckBoxMenuItem("Human", false); JMenuItem radioButton = new JRadioButtonMenuItem("Computer", true); menu.add(checkBox); menu.add(radioButton); // Add two more Composites JMenuBar menuBar = new JMenuBar(); setJMenuBar(menuBar); menuBar.add(menu); Run JMenuItemDemoComposite.java See code demo page

  11. Portfolios in Portfolios

  12. Portfolio overall = new Portfolio("My Entire Retirement Portfolio"); // Add two leafs to "my IRA" Stock aStock = new Stock("Seven Eleven", 500, 9.15); overall.add(aStock); BankAccount account = new BankAccount("Swiss Account", 300000); overall.add(account); // Create a tech portfolio Portfolio tech = new Portfolio("Tech Stocks"); Stock sun = new Stock("Sun", 20, 30.50); MutualFund highRisk = new MutualFund("Nasdaq", 13, 45.20); tech.add(sun); // add leaf tech.add(highRisk); // add leaf // Add this 2nd portfolio to the overall portfolio overall.add(tech); // add Composite // overall.add(overall); There is an if to avoid adding this to this // Create a overseas portfolio of tech stocks Portfolio global = new Portfolio("Global Equities"); Stock pacificRim = new Stock("Pacific Rim Tech", 10, 12.34); MutualFund lrgGrow = new MutualFund("Large Growth", 100, 95.21); global.add(pacificRim); global.add(lrgGrow); tech.add(global);

  13. Create a print method for this My Entire Retirement Portfolio with value $315417.0 500 shares of Seven Eleven with value $4575.0 BankAccount: 'Swiss Account' with value $300000.0 Tech Stocks with value $10842.0 20 shares of Sun with value $610.0 13.0 shares of Nasdaq with value $587.6 Global Equities with value $9644.4 10 shares of Pacific Rim Tech with value $123.4 100.0 shares of Large Growth China with value $9521.0

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