Application
Programming
Hend Alkittawi

Java Iteration
Iterating through objects in Java
Collections

INTRODUCTION
-

Related to the discussion on Java generics and collections is the
discussion on the following interfaces
-

-

Iterator, Iterable, Comparable, Comparator

The Java API has a consistent approach to iterators that are
implemented by nearly all collections in the class Library.

-

Iterators are implemented in the Java API using two primary interfaces:
-

Iterator: used to define an object that can be used as an iterator.

-

Iterable: used to define a collection from which an iterator can be
extracted.

-

The Comparable and Comparator interfaces in Java facilitate comparisons
between objects

THE ITERATOR INTERFACE
-

The Iterator interface is defined in the Java APIs.

-

The interface is used by a class that represents a collection
of objects, providing a means to move through the collection
one object at a time.

-

An Iterator provides a consistent and simple mechanism for
systematically processing a group of objects.

-

An Iterator is an object that has methods that allow you to
process a collection of items one at a time.

-

An Iterator object in Java is defined using the Iterator
interface.

THE ITERATOR INTERFACE
-

Every iterator object has a method called hasNext() that
returns a boolean value indicating whether there is at least
one more item to process.

-

Every Iterator also has a method called next() to retrieve the
next item in the collection to process.

-

The Iterator interface also has a method called remove() which
takes no parameters and has a void return type. A call to the
remove() method removes the object that was most recently
returned by the next method from the underlying collection.

THE ITERABLE INTERFACE
-

The Iterable interface has a single method iterator() that returns an Iterator
object.

-

If an object has implemented the Iterable interface, we can use a variation of the
for loop to process items using a simplified syntax → The enhanced for loop
(for-each loop).

-

For example if bookList is an Iterable object that contains book objects we can use
a for loop to process each book object as follows
for (Book myBook: bookList)
system.out.println( myBook)

-

This version of the for Loop processes each object in the Iterator in turn. It is
equivalent to the following:
Book myBook;
while( bookList.hasNext(){
myBook = bookList.next();
System.out.println( myBook) }

import java.util.Iterator;
public class Range implements
Iterable<Integer>{

import java.util.Iterator;
import java.util.NoSuchElementException;

import java.util.Iterator;
public class RangeTest {

public class RangeIterator implements
Iterator<Integer>{

public static void main(String[] args){
Range range = new Range(1, 7);
System.out.println("Looping with an
iterator");
Iterator<Integer> it =
range.iterator();
while(it.hasNext()){
int cur = it.next();
System.out.print(cur + "\t");
}

private int start, end;
private int cursor;
private int end;

public Range(int start, int end) {
this.start = start;
this.end = end;
}

public RangeIterator(int start, int
end){
this.cursor = start;
this.end = end;
}

public Iterator<Integer> iterator(){
return new RangeIterator(start,
end);
}

public boolean hasNext() {
return this.cursor < end;
}

}

public Integer next() {
if(this.hasNext()) {
int current = cursor;
cursor++;
return current;
}
throw new NoSuchElementException();
}
public void remove() {
throw new
UnsupportedOperationException();
}
}

System.out.println("\nLooping with a
for-each loop");
for(Integer cur : range){
System.out.print(cur + "\t");
}
}
}

Looping with an iterator
1
2
3
4
5
Looping with a for-each loop
1
2
3
4
5

6
6

public class Course {
private String prefix;
private int number;
private String title;
private String grade;

public class ProgramOfStudy implements
Iterable<Course>{
private List<Course> list;
public ProgramOfStudy(){
list = new LinkedList<Course>();
}

public Course(String prefix, int number,
String title, String grade){
this.prefix = prefix;
this.number = number;
this.title = title;
if (grade == null)
this.grade = "";
else
this.grade = grade;
}
public Course(String prefix, int number,
String title){
this(prefix, number, title, "");
}

public class IterableTest {
public static void main(String[] args) throws
Exception{
ProgramOfStudy pos = new ProgramOfStudy();
pos.loadCourses();
System.out.println(pos);

public void addCourse(Course course){
if (course != null)
list.add(course);
}

for(Course course : pos) {
pos.addCourse(new Course("MATH", 1044,
"Caluclus I"));
}

public String toString(){
String result = "";
for (Course course : list)
result += course + "\n";
return result;
}

System.out.println("Removing courses with
no grades.\n");
Iterator<Course> itr = pos.iterator();
while (itr.hasNext()){
Course course = itr.next();
if (!course.taken())
itr.remove();
}
System.out.println(pos);
}

@Override
public Iterator<Course> iterator() {
return list.iterator();
}

public boolean taken(){
return !grade.equals("");
}

public void loadCourses() {
list.add(new Course("CS", 3443,
"Application Programming", "A+"));
list.add(new Course("CS", 3343,
"Algorithms", "B"));
list.add(new Course("CS", 1173, "Data
Analysis and Visualization", "C+"));
list.add(new Course("CS", 2073,
"Introduction to Programming"));
}

public String toString(){
String result = prefix + " " + number
+ ": " + title;
if (!grade.equals(""))
result += " [" + grade + "]";
return result;
}
}
}

}

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QUESTIONS?

THANK
YOU!

@

hend.alkittawi@utsa.edu

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