Iterator over HashMap in Java

Question

I tried to iterate over hashmap in Java, which should be a fairly easy thing to do. However, the following code gives me some problems:

HashMap hm = new HashMap(         


        

Answer 1

You are getting a keySet iterator on the HashMap and expecting to iterate over entries.

Correct code:

    HashMap hm = new HashMap();

    hm.put(0, "zero");
    hm.put(1, "one");

    //Here we get the keyset iterator not the Entry iterator
    Iterator iter = (Iterator) hm.keySet().iterator();

    while(iter.hasNext()) {

        //iterator's next() return an Integer that is the key
        Integer key = (Integer) iter.next();
        //already have the key, now get the value using get() method
        System.out.println(key + " - " + hm.get(key));

    }

Iterating over a HashMap using EntrySet:

     HashMap hm = new HashMap();
     hm.put(0, "zero");
     hm.put(1, "one");
     //Here we get the iterator on the entrySet
     Iterator iter = (Iterator) hm.entrySet().iterator();


     //Traversing using iterator on entry set  
     while (iter.hasNext()) {  
         Entry entry = (Entry) iter.next();  
         System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());  
     }  

     System.out.println();


    //Iterating using for-each construct on Entry Set
    Set> entrySet = hm.entrySet();
    for (Entry entry : entrySet) {  
        System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());  
    }           

Look at the section -Traversing Through a HashMap in the below link. java-collection-internal-hashmap and Traversing through HashMap