Iterating a hashmap using Iterator is efficient than using foreach loop

[Balaguru Gupta]

Can anyone explain why Iterating a hashmap using Iterator is efficient than using foreach loop?

[Muneeswaran Balasubramanian]

Hi Balaguru,


Foreach loop only used for Iteration.That means readonly.

But using iterator we can modify the collection using the method remove of Iterator.

This point add the value to your post.Waiting for others quotes,,,,,

[Balaguru Gupta]

Thank you so much Muneeswaran,

I worked out with the same.
Example program ,

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

/**
 * @since @author balaguru
 */
public class SampleIteration
{

    /**
     * Method defined to iterate hashmap using Iterator.
     */
    private void iterateUsingIterator()
    {
        Map<String, String> sampleHashMap = new HashMap<String, String>();

        sampleHashMap.put("A", "One");
        sampleHashMap.put("B", "Two");
        sampleHashMap.put("C", "Three");
        sampleHashMap.put("D", "Four");

        System.out.println("sampleHashMap : " + sampleHashMap);

        Iterator iterator = sampleHashMap.entrySet().iterator();
        while (iterator.hasNext())
        {
            Map.Entry pairs = (Map.Entry) iterator.next();
            System.out.println(pairs.getKey() + ":" + pairs.getValue());
            iterator.remove();//concurrent modification exception does not occur in this case.
        }
        System.out.println("sampleHashMap after remove operation : " + sampleHashMap);
    }

    /**
     * Method defined to iterate hashmap using for each loop.
     */
    private void iterateUsingForEach()
    {
        Map<String, String> sampleHashMap = new HashMap<String, String>();

        sampleHashMap.put("A", "One");
        sampleHashMap.put("B", "Two");
        sampleHashMap.put("C", "Three");
        sampleHashMap.put("D", "Four");

        System.out.println("sampleHashMap : " + sampleHashMap);

        for (Map.Entry<String, String> entry : sampleHashMap.entrySet())
        {
            System.out.println(entry.getKey() + ":" + entry.getValue());;
            sampleHashMap.remove(entry.getKey()); //concurrent modification exception occurs in this case.
        }

        System.out.println("sampleHashMap after remove operation : " + sampleHashMap); //will not be executed, because of the exception arised.
    }

    public static void main(String[] args)
    {
        SampleIteration sampleIteration = new SampleIteration();
        sampleIteration.iterateUsingIterator();
        sampleIteration.iterateUsingForEach();
    }
}

Apart from this reason, Is there any other difference for the same to justify that Iterator is efficient than foreach loop?

[Jesper de Jong]

Balaguru Gupta wrote:Can anyone explain why Iterating a hashmap using Iterator is efficient than using foreach loop?

Why do you think that this is true? Because it isn't.

Note that a foreach loop will use an iterator "under the covers" - the compiler will generate code that loops over the map using an iterator. The foreach syntax is just to make it easier for you to write the code.

You should normally use the foreach loop syntax, because it's clearer and shorter to write. But if you need to remove elements from the map while you're iterating, you have to use the iterator syntax, because you need to be able to call remove() on the iterator, as you demonstrated in your code.

[Balaguru Gupta]

Hello Jesper de Jong,

Thank you so much. Now I am pretty much clear with the concept and difference between using Iterator and foreach.

Cheerz mate