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     hm put 0   zero    hm put 1   one     Iterator iter    Iterator  hm keySet   iterator     while iter hasNext           Map Entry entry    Map Entry  iter next        System out println entry getKey       -     entry getValue          First  I needed to cast Iterator on hm keySet   iterator    because otherwise it said  Type mismatch  cannot convert from java util Iterator to Iterator   But then I get  The method hasNext   is undefined for the type Iterator   and  The method hasNext   is undefined for the type Iterator

User · Answer

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<Integer,String> entry = (Entry<Integer,String>) iter.next();  
         System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());  
     }  

     System.out.println();


    //Iterating using for-each construct on Entry Set
    Set<Entry<Integer, String>> entrySet = hm.entrySet();
    for (Entry<Integer, String> 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

User · Answer

With Java 8   hm forEach  k  v  - gt        System out println  Key       k     -     v

User · Answer

Map lt String  Car gt  carMap   new HashMap lt String  Car gt  16   float  0 75        there is no iterator for Maps  but there are methods to do this           Set lt String gt  keys   carMap keySet       returns a set containing all the keys         for String c   keys                         System out println c                      Collection lt Car gt  values   carMap values       returns a Collection with all the objects         for Car c   values                        System out println c getDiscription                         keySet and the values methods serve as    views    into the Map            The elements in the set and collection are merely references to the entries in the map             so any changes made to the elements in the set or collection are reflected in the map  and vice versa                                                                                  The entrySet method returns a Set of Map Entry objects             Entry is an inner interface in the Map interface            Two of the methods specified by Map Entry are getKey and getValue            The getKey method returns the key and getValue returns the value             Set lt Map Entry lt String  Car gt  gt  cars   carMap entrySet             for Map Entry lt String  Car gt  e   cars                        System out println  Keys       e getKey                 System out println  Values       e getValue   getDiscription       n

User · Answer

Iterator through keySet will give you keys  You should use entrySet if you want to iterate entries   HashMap hm   new HashMap     hm put 0   zero    hm put 1   one     Iterator iter    Iterator  hm entrySet   iterator     while iter hasNext           Map Entry entry    Map Entry  iter next        System out println entry getKey       -     entry getValue

User · Answer

The cleanest way is to not  directly  use an iterator at all    type your map with generics use a foreach loop to iterate over the entries    Like this   Map lt Integer  String gt  hm   new HashMap lt Integer  String gt      hm put 0   zero    hm put 1   one     for  Map Entry lt Integer  String gt  entry   hm entrySet             do something with the entry     System out println entry getKey       -     entry getValue            the getters are typed      Integer key   entry getKey        String value   entry getValue        This is way more efficient than iterating over keys  because you avoid n calls to get key

User · Answer

Using EntrySet   and for each loop    for Map Entry lt String  String gt  entry  hashMap entrySet           System out println  quot Key Of map    quot   entry getKey                             quot    value of map    quot    entry getValue           Using keyset   and for each loop          for String key   hashMap keySet                  System out println  quot Key Of map    quot   key    quot                           value of map    quot    hashMap get key                    Using EntrySet   and java Iterator       for String key   hashMap keySet              System out println  quot Key Of map    quot   key    quot                           value of map    quot    hashMap get key                 Using keyset   and java Iterator      Iterator lt String gt  keysIterator   keySet iterator        while  keysIterator hasNext              String key   keysIterator next            System out println  quot Key Of map    quot   key    quot    value of map    quot    hashMap get key             Reference    How to iterate over Map or HashMap in java

User · Answer

You should really use generics and the enhanced for loop for this   Map lt Integer  String gt  hm   new HashMap lt  gt     hm put 0   zero    hm put 1   one     for  Integer key   hm keySet          System out println key       System out println hm get key        http   ideone com sx3F0K  Or the entrySet   version   Map lt Integer  String gt  hm   new HashMap lt  gt     hm put 0   zero    hm put 1   one     for  Map Entry lt Integer  String gt  e   hm entrySet          System out println e getKey         System out println e getValue

User · Answer

Can we see your import block  because it seems that you have imported the wrong Iterator class   The one you should use is java util Iterator  To make sure  try   java util Iterator iter   hm keySet   iterator        I personally suggest the following   Map Declaration using Generics and declaration using the Interface Map lt K V gt  and instance creation using the desired implementation HashMap lt K V gt   Map lt Integer  String gt  hm   new HashMap lt  gt       and for the loop   for  Integer key   hm keySet          System out println  Key       key     -     hm get key        UPDATE 3 5 2015  Found out that iterating over the Entry set will be better performance wise   for  Map Entry lt Integer  String gt  entry   hm entrySet          Integer key   entry getKey        String value   entry getValue         UPDATE 10 3 2017  For Java8 and streams  your solution will be  Thanks  Shihe Zhang    hm forEach  key  value  - gt  System out println key          value

User · Answer

Several problems here    You probably don t use the correct iterator class  As others said  use import java util Iterator If you want to use Map Entry entry    Map Entry  iter next    then you need to use hm entrySet   iterator    not hm keySet   iterator    Either you iterate on the keys  or on the entries

[java] Iterator over HashMap in Java

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