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

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());

}

Answer 2

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

Answer 3

With Java 8:

hm.forEach((k, v) -> {
    System.out.println("Key = " + k + " - " + v);
});

Answer 4

Map<String, Car> carMap = new HashMap<String, Car>(16, (float) 0.75);

// there is no iterator for Maps, but there are methods to do this.

        Set<String> keys = carMap.keySet(); // returns a set containing all the keys
        for(String c : keys)
        {

            System.out.println(c);
        }

        Collection<Car> 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<Map.Entry<String, Car>> cars = carMap.entrySet(); 
        for(Map.Entry<String, Car> e : cars)
        {
            System.out.println("Keys = " + e.getKey());
            System.out.println("Values = " + e.getValue().getDiscription() + "\n");

        }

Answer 5

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

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

for (Map.Entry<Integer, String> 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).

Answer 6

1. Using EntrySet() and for each loop

      for(Map.Entry<String, String> entry: hashMap.entrySet()) {
        System.out.println("Key Of map = "+ entry.getKey() + 
                         " , value of map = " + entry.getValue() );
    }
   

2. Using keyset() and for each loop

            for(String key : hashMap.keySet()) {
               System.out.println("Key Of map = "+ key + " , 
                         value of map = " + hashMap.get(key) );
              }
   

3. Using EntrySet() and java Iterator

         for(String key : hashMap.keySet()) {
           System.out.println("Key Of map = "+ key + " , 
                         value of map = " + hashMap.get(key) );
           }
   

4. Using keyset() and java Iterator

        Iterator<String> keysIterator = keySet.iterator();
       while (keysIterator.hasNext()) {
           String key = keysIterator.next();
           System.out.println("Key Of map = "+ key + " , value of map = " + hashMap.get(key) );
      }
   

Reference : How to iterate over Map or HashMap in java