Composite

• Want to represent a part-whole relationship like tree folder system
• Group components to form larger components, which in turn can be grouped to form still larger components.
• Related patterns include
  • Decorator, which is often used with composite pattern and with the same parent class.
  • Flyweight, which is used with composite pattern to share components.
  • Iterator, which is used to traverse the composites.
  • Visitor, which localizes operations across composite and leaf classes.

Example

A component has many elements and itself which has many elements and itself, etc. A file system is a typical example. Directory is a composite pattern. When you deal with Directory object, if isFile() returns true, work on file, if isDirectory() returns true, work on Directory object.

class Directory {
   Directory dir;
   File[] f;
   ...
   
   boolean isDirectory() {
       return f == null;
   }
   boolean isFile() {
       return f != null;
   }
   File getFile(int i) {
      if (isFile())
         return f[i];
      return null'
   }
   Directory getDirectory() {
      if (isDirectory())
          return dir;
      return null;
   }
   ....
}

For example, General Manager may have several employees and some of employees are Managers which have several employees. To illustrate such issue, we design a simple Manager class.

class Employee {
   String name;
   double salary;
   Employee(String n, double s){
       name = n;
       salary = s;
   }
   String getName() {
      return name;
   }
   double getSalary() {
      return salary;
   }
   public String toString() {
       return "Employee " + name;
   }
}
class Manager {
   Manager mgr;
   Employee[] ely;
   String dept;
   Manager(Manager mgr,Employee[] e, String d ) {
       this(e, d);
       this.mgr = mgr;
   }
   
   Manager(Employee[] e, String d) {
       ely = e;
       dept =d;
   }
   String getDept() {
       return dept;
   }
   Manager getManager() {
       return mgr;
   }
   Employee[] getEmployee() {
       return ely;
   }
   public String toString() {
       return dept + " manager";
   }
}

class Test {
   public static void main(String[] args) {
       Employee[] e1 = {new Employee("Aaron", 50),
                        new Employee("Betty", 60)};
       Manager m1 = new Manager(e1, "Accounting");
       
       Employee[] e2 = {new Employee("Cathy", 70),
                        new Employee("Dan", 80),
                        new Employee("Eliz", 90)};
       Manager m2 = new Manager(m1, e2, "Production");
       
       System.out.println(m2);
       Employee[] emp = m2.getEmployee();
       if (emp != null)
             for (int k = 0; k < emp.length; k++)
                  System.out.println(" "+emp[k]+" Salary: $"+ emp[k].getSalary());       
       Manager m = m2.getManager();
       System.out.println(" " + m);
       if (m!= null) {
          Employee[] emps = m.getEmployee();
          if (emps != null)
             for (int k = 0; k < emps.length; k++)
                   System.out.println("    " + emps[k]+" Salary: $"+ emps[k].getSalary());
        
       }   
   }
}

 C:\ Command Prompt

C:\> java Test Production manager Employee Cathy Salary: $70.0 Employee Dan Salary: $80.0 Employee Eliz Salary: $90.0 Accounting manager Employee Aaron Salary: $50.0 Employee Betty Salary: $60.0

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