The history of element iteration in Java

In this tutorial we look at the history of element iteration in Java.

On of the most common tasks in programming is to iterate over a collection of data. This tutorial shows how element iteration evolved with Java language over the course of time.

Enumeration

In the early days of Java, Enumeration was used to iterate over elements of data. The Enumeration interface defines the methods by which we can enumerate (obtain one element at a time) the elements in a collection of objects. The two primary collection classes were Vector and Hashtable.

Today, Enumeration, Vector, and Hashtable are considered obsolete. They are not deprecated, however.

EnumerationEx.java
package com.zetcode;

import java.util.Enumeration;
import java.util.Vector;

public class EnumerationEx {

    public static void main(String[] args) {

        Vector items = new Vector();

        items.add("coin");
        items.add("pen");
        items.add("chair");
        items.add("lamp");
        items.add("cup");
        items.add("spoon");
        
        Enumeration itemsEn = items.elements();

        while (itemsEn.hasMoreElements()) {
            System.out.println(itemsEn.nextElement());
        }
    }
}

We have a vector of strings. We use the Enumeration to loop over the elements of the vector.

Enumeration itemsEn = items.elements();

The elements() method returns the the Enumeration of the vector.

while (itemsEn.hasMoreElements()) {

    System.out.println(itemsEn.nextElement());
}

The elements are traversed in a while loop. The hasMoreElements() returns true while there are still more elements to extract, and false when all the elements have been enumerated.

Iterator

Java 1.2 introduced the standard collection classes (List, Set, Map), and the Iterator. Iterator brings the Iterator design pattern, which is a common behavioral pattern used to access the elements of a collection object in sequential manner without any need to know its underlying representation.

IteratorEx.java
package com.zetcode;

import java.util.Arrays;
import java.util.Iterator;
import java.util.List;

public class IteratorEx {

    public static void main(String[] args) {
        
        List<String> items = Arrays.asList("coin", "ball", "lamp", "spoon");
        
        Iterator it = items.iterator();
        
        while (it.hasNext()) {
            
            System.out.println(it.next());
        }
    }
}

In the example, we loop over a list of elements with Iterator.

List<String> items = Arrays.asList("coin", "ball", "lamp", "spoon");

We use the Arrays.asList() method to define a list in one line.

Iterator it = items.iterator();

We get the iterator from the list using the iterator() method.

while (it.hasNext()) {
    
    System.out.println(it.next());
}

We go through the list of elements in a while loop. We use the hasNext() and next() methods of iterator.

Iterable and enhanced for loop

Java 5 introduced generics, Iterable, and the enhanced for loop. The Iterable interface allows an object to be the target of the enhanced for loop statement. An Iterable is an object that contains a series of elements that can be iterated over. It has one method that produces an Iterator.

EnhancedForLoop.java
package com.zetcode;

import java.util.Arrays;
import java.util.List;

public class EnhancedForLoop {

    public static void main(String[] args) {
        
        List<String> items = Arrays.asList("coin", "ball", "lamp", "spoon");
        
        for (String item: items) {
            
            System.out.println(item);
        }
    }
}

The example uses an enhanced for loop to traverse list elements. The creation of the iterator and calls to the hasNext() and next() methods are not explicit, but they still take place behind the scenes.

Java 8 forEach() method

The forEach() method performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception. The forEach() method uses an internal iterator, while previous methods used an external iterator.

ForEachEx.java
package com.zetcode;

import java.util.Arrays;
import java.util.List;

public class ForEachEx {

    public static void main(String[] args) {

        List<String> items = Arrays.asList("coin", "ball", "lamp", "spoon");
        
        items.stream().forEach(System.out::println);
    }
}

In the example, we loop over the elements with the forEach() method.

In this tutorial, we have looked at the brief history of element iteration in Java.

You might also be interested in the following related tutorials: Filtering a list in Java, Java ArrayList tutorial, Opencsv tutorial, Java tutorial, Reading text files in Java, Reading a web page in Java, or Introduction to Google Guava.