Composite

處理樹狀、有階層的資料結構(part-whole hierarchies)，讓使用者用相同的介面操作單獨(Leaf)和合成(Composite)的物件。

from: Composite Pattern Tutorial with Java Examples

Example

Component

public interface Employee {

     public void add(Employee employee);
     public void remove(Employee employee);
     public Employee getChild(int i);
     public String getName();
     public double getSalary();
     public void print();
}

Composite

public class Manager implements Employee {

    private String name;
    private double salary;

    public Manager(String name, double salary) {
        this.name = name;
        this.salary = salary;
    }

    // sub-tree
    List<Employee> employees = new ArrayList<Employee>();

    public void add(Employee employee) {
        employees.add(employee);
    }

    public Employee getChild(int i) {
        return employees.get(i);
    }

    public String getName() {
        return name;
    }

    public double getSalary() {
        return salary;
    }

    public void print() {
        System.out.println("-------------");
        System.out.println("Name ="+getName());
        System.out.println("Salary ="+getSalary());
        System.out.println("-------------");

        Iterator<Employee> employeeIterator = employees.iterator();
        while(employeeIterator.hasNext()) {
            Employee employee = employeeIterator.next();
            employee.print();
        }
    }

    public void remove(Employee employee) {
        employees.remove(employee);
    }
}

Leaf

public class Developer implements Employee {

    private String name;
    private double salary;

    public Developer(String name,double salary){
        this.name = name;
        this.salary = salary;
    }

    public void add(Employee employee) {
        //this is leaf node so this method is not applicable to this class.
    }

    public Employee getChild(int i) {
        //this is leaf node so this method is not applicable to this class.
        return null;
    }

    public String getName() {
        return name;
    }

    public double getSalary() {
        return salary;
    }

    public void print() {
        System.out.println("-------------");
        System.out.println("Name ="+getName());
        System.out.println("Salary ="+getSalary());
        System.out.println("-------------");
    }

    public void remove(Employee employee) {
        //this is leaf node so this method is not applicable to this class.
    }
}

Usage

//
//    generalManager
//      /      \
//    emp3    manager1
//             /    \
//            emp1  emp2
//
public class CompositeDesignPatternMain {

 public static void main(String[] args) {
     Employee emp1 = new Developer("John", 10000);
     Employee emp2 = new Developer("David", 15000);
     Employee manager1 = new Manager("Daniel",25000);
     manager1.add(emp1);
     manager1.add(emp2);

     Employee emp3 = new Developer("Michael", 20000);
     Manager generalManager=new Manager("Mark", 50000);
     generalManager.add(emp3);
     generalManager.add(manager1);

     generalManager.print();
 }
}

from: Composite Design Pattern in Java