Iterator

Story
Image
UML
Implementation
Usage

Story

Book is a set of written, printed sheets bound together into a volume. You can browse through the book page by page, or quickly jump to interesting chapter. Process of browsing is example of Iterator pattern.

Image

alt text

Open Book by Dave Dugdale, on Flickr" (CC BY-SA 2.0) by Dave Dugdale

UML

Implementation

Iterator.java

package com.hundredwordsgof.iterator;

/**
 * 
 * Iterator defines an interface for accessing and traversing elements.
 *
 */
public interface Iterator {

  Object first();

  Object next();

  boolean isDone();

  Object currentItem();
}

ConcreteIterator.java

package com.hundredwordsgof.iterator;

/**
 * 
 * ConcreteIterator implements the Iterator interface. Keeps track of the
 * current position in the traversal of the aggregate.
 *
 */
public class ConcreteIterator implements Iterator {

  private ConcreteAggregate concreteAggregate;
  private int index = -1;

  public ConcreteIterator(ConcreteAggregate concreteAggregate) {
    this.concreteAggregate = concreteAggregate;
  }

  public Object first() {
    index = 0;
    return concreteAggregate.getRecords()[index];
  }

  public Object next() {
    index++;
    return concreteAggregate.getRecords()[index];
  }

  public boolean isDone() {

    if (concreteAggregate.getRecords().length == (index + 1)) {
      return true;
    }
    return false;
  }

  public Object currentItem() {
    return concreteAggregate.getRecords()[index];
  }
}

Aggregate.java

package com.hundredwordsgof.iterator;

/**
 * 
 * Aggregate defines an interface for creating an Iterator object.
 *
 */
public interface Aggregate {

  Iterator createIterator();
}

ConcreteAggregate.java

package com.hundredwordsgof.iterator;

/**
 * 
 * ConcreteAgregate implements the Iterator creation interface to return an
 * instance of the proper ConcreteIterator.
 *
 */
public class ConcreteAggregate implements Aggregate {

  private final String records[] = { "first", "second", "third", "fourth" };

  public Iterator createIterator() {
    return new ConcreteIterator(this);
  }

  protected String[] getRecords() {
    return records;
  }
}

Usage

IteratorTest.java

package com.hundredwordsgof.iterator;

import static org.junit.Assert.assertEquals;
import org.junit.Test;

/**
 * Test implementation of the Iterator pattern.
 */
public class IteratorTest {

  @Test
  public void testIterator() throws Exception {

    Aggregate aggregate = new ConcreteAggregate();

    Iterator iterator = aggregate.createIterator();

    assertEquals("first", iterator.first());
    assertEquals("first", iterator.currentItem());
    assertEquals(false, iterator.isDone());

    assertEquals("second", iterator.next());
    assertEquals("second", iterator.currentItem());
    assertEquals(false, iterator.isDone());

    assertEquals("third", iterator.next());
    assertEquals("third", iterator.currentItem());
    assertEquals(false, iterator.isDone());

    assertEquals("fourth", iterator.next());
    assertEquals("fourth", iterator.currentItem());
    assertEquals(true, iterator.isDone());
  }
}