Differences between iterator and for loop in hashmap

Question

Below are the scenario of iterator and for loop:

1) Using iterator:

  ihm.put("Zara", new Double(3434.34));
  ihm.put("Mahnaz", new Double(123.22));
  ihm.put("Ayan", new Double(1378.00));
  ihm.put("Daisy", new Double(99.22));
  ihm.put("Qadir", new Double(-19.08));

  // Get a set of the entries
  Set set = ihm.entrySet();

  // Get an iterator
  Iterator i = set.iterator();

  // Display elements
  while(i.hasNext()) {
     Map.Entry me = (Map.Entry)i.next();
     System.out.print(me.getKey() + ": ");
     System.out.println(me.getValue());
  }
  System.out.println();

=====================================

Using For Loop:

    HashMap<String, Integer> map = new HashMap<String, Integer>();

    //Adding key-value pairs

    map.put("ONE", 1);

    map.put("TWO", 2);

    map.put("THREE", 3);

    map.put("FOUR", 4);

    //Adds key-value pair 'ONE-111' only if it is not present in map

    map.putIfAbsent("ONE", 111);

    //Adds key-value pair 'FIVE-5' only if it is not present in map

    map.putIfAbsent("FIVE", 5);

    //Printing key-value pairs of map

    Set<Entry<String, Integer>> entrySet = map.entrySet();

    for (Entry<String, Integer> entry : entrySet) 
    {
        System.out.println(entry.getKey()+" : "+entry.getValue());
    }        
}

In the above both cases iterator and for loop are doing the same job.So can anyone tell me what is the differences between them and when i can use iterator and for loop.

the only difference is using iterator is like breathing through nose and using for loop is like breathing through mouth ;) — user6601906
– user6601906, Commented Oct 12, 2016 at 6:50
The second one is clearer and type checked. They both use Iterator in reality. — Peter Lawrey
– Peter Lawrey, Commented Oct 12, 2016 at 8:23

Daniil Dubrovsky · Accepted Answer · 2021-02-05 20:19:02Z

3

The only significant difference between loop and iterator (apart from readability) is that while using iterator you can edit content of the collection through that Iterator instance.

If you try to edit map while looping through it you will get Concurrent ModificationException

You should use iterator when you want to update map while iterating over it e.g. remove entries which match some conditions.

Iterator<Entry> it = map.iterator();
while(it.hasNext())
      if(check(it.next()))
           it.remove();

I have to note that for-loop is a syntactic sugar for iterator and in resulting bytecode they will look the same way.

edited Feb 5, 2021 at 20:19

answered Oct 12, 2016 at 7:01

Daniil Dubrovsky

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1 Comment

Fildor Over a year ago

"while using iterator you can edit the content of map" - using the iterator that is. If you e.g. delete an entry not through the iterator, it will still throw an exception.

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