Iterator

• 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.
• Related patterns include
  • Composite, which supports recursive structures, whereas an iterator is often applied on it.
  • Factory Method, which provides appropriate instances needed by an iterator subclasses.
  • Memento, which is often used with an Iterator. The iterator stores the memento internally.

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.

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