Iterator Pattern

Definition

Provide a way to move through a list of collection or aggregated objects without knowing its internal representations.

Where to use & benefits

• Use a standard interface to represent data objects.
• Use s standard iterator built in each standard collection, like List, Sort, or Map.
• Need to distinguish variations in the traversal of an aggregate.
• Similar to Enumeration class, but more effective.
• Need to filter out some info from an aggregated collection.

Example

Employee is an interface, Manager, PieceWorker, HourlyWorker and CommissionWorker are implementation classes of interface Employee. EmployeeTest class will create a list and use a built-in iterator of ArrayList class to traverse the members of the list.

import java.util.*;
interface Employee {   
    public abstract double earnings();
}
class Manager implements Employee {
    private double weeklySalary;
    private String name;
    public Manager(String name, double s) {
        this.name = name;
        setWeeklySalary(s);
    }
    
    void setWeeklySalary(double s) {
        if (s > 0) {
          weeklySalary = s;
        } else
          weeklySalary = 0;
    }
    
    public double earnings() {
        return weeklySalary;
    }
    public String getName() {
     return name;
 }
    public String toString() {
        return "Manager: " + getName();
    }
}

class PieceWorker implements Employee {
    private double wagePerPiece;
    private int quantity;
    private String name;
    public PieceWorker(String name, double w, int q) {
        this.name = name;
        setWagePerPiece(w);
        setQuantity(q);
    }
    
    void setWagePerPiece(double w) {
        if (w > 0) 
          wagePerPiece = w;
        else
          wagePerPiece = 0;
    }
    
    void setQuantity(int q) {
        if ( q > 0)
           quantity = q;
        else
           quantity = 0;
    }
    public String getName() {
     return name;
 }    
    public double earnings() {
        return quantity * wagePerPiece;
    }
    
    public String toString() {
        return "Piece worker: " + getName();
    }
}

class HourlyWorker implements Employee {
    private double hourlyWage;
    private double hours;
    private String name;
    public HourlyWorker(String name, double w, double h) {
        this.name = name;
        setHourlyWage(w);
        setHours(h);
    }
    
    void setHourlyWage(double w) {
        if (w > 0)
            hourlyWage = w;
        else
            hourlyWage = 0;
    }
    
    void setHours(double h) {
        if ( 0 <= h && h < 168)
            hours = h;
        else
            hours = 0;
    }
    public String getName() {
     return name;
 }    
    public double earnings() {
        return hourlyWage * hours;
    }
    public String toString() {
        return "Hourly worker: " + getName();
    }
}

class CommissionWorker implements Employee {
    private double salary;
    private double commission;
    private double totalSales;
    private String name;
    public CommissionWorker(String name, 
            double salary, double commission, double totalSales) {  
        this.name = name;
        setSalary(salary);
        setCommission(commission);
        setTotalSales(totalSales);
    }
    void setSalary(double s) {
        if( s > 0)
            salary = s;
        else
            salary = 0;
    }  
    void setCommission(double c) {
        if ( c > 0)
            commission = c;
        else
            commission = 0;
    }
    void setTotalSales(double ts) {
        if (ts > 0 )
            totalSales = ts;
        else
            totalSales = 0;
    }
    public String getName() {
     return name;
 }
    public double earnings() {
        return salary + commission/100*totalSales;
    }  
    public String toString() {
        return "Commission worker:"
            + getName();
    }
}

class EmployeeTest {
    public static void main(String[] args) {
        java.util.List list = new ArrayList();
        list.add(new Manager("Bill", 800.00));
        list.add(new CommissionWorker("Newt", 400.0, 3.75, 159.99));
        list.add(new PieceWorker("Al", 2.5, 200));
        list.add(new HourlyWorker("Babara", 13.75, 40));
        list.add(new Manager("Peter", 1200.00));
        list.add(new CommissionWorker("Margret", 600.0,5.5, 200.25));
        list.add(new PieceWorker("Mark", 4.5, 333));
        list.add(new HourlyWorker("William", 31.25, 50));
    
  System.out.println("Use built-in iterator:");
  Iterator iterator = list.iterator();
  while(iterator.hasNext()) {
      Employee em = (Employee)iterator.next();
      System.out.print(em + " earns $");
   System.out.println(em.earnings());
  }
 }
}
%java EmployeeTest
Use built-in iterator:
Manager: Bill earns $800.0
Commission worker:Newt earns $405.999625
Piece worker: Al earns $500.0
Hourly worker: Babara earns $550.0
Manager: Peter earns $1200.0
Commission worker:Margret earns $611.01375
Piece worker: Mark earns $1498.5
Hourly worker: William earns $1562.5

The above example also shows a dynamic binding feature which is popular in Object-Oriented realm.
If you want to pick up a specific object from the aggregated list, you may use the following code.

while(iterator.hasNext()) {
 Employee em = (Employee)iterator.next();
 if (em instanceof Manager) {
    System.out.print(em + " earns $");
    System.out.println(em.earnings());
 }
}

The above list can also be replaced by an array and achieve the same result.