HashMap in Java

KeshavCollections, Java Core, OCA, OCP
Here is the list of methods available in HashMap class. I have also covered examples using these methods at the end of this post.
  1. void clear(): It removes all the key and value pairs from the specified Map.
  2. Object clone(): It returns a copy of all the mappings of a map and used for cloning them into another map.
  3. boolean containsKey(Object key): It is a boolean function which returns true or false based on whether the specified key is found in the map.
  4. boolean containsValue(Object Value): Similar to containsKey() method, however it looks for the specified value instead of key.
  5. Value get(Object key): It returns the value for the specified key.
  6. boolean isEmpty(): It checks whether the map is empty. If there are no key-value mapping present in the map then this function returns true else false.
  7. Set keySet(): It returns the Set of the keys fetched from the map.
  8. value put(Key k, Value v): Inserts key value mapping into the map. Used in the above example.
  9. int size(): Returns the size of the map – Number of key-value mappings.
  10. Collection values(): It returns a collection of values of map.
  11. Value remove(Object key): It removes the key-value pair for the specified key. Used in the above example.
  12. void putAll(Map m): Copies all the elements of a map to the another specified map.





import java.util.HashMap;
import java.util.Map;
import java.util.Iterator;
import java.util.Set;
public class Details {

   public static void main(String args[]) {

      /* This is how to declare HashMap */
      HashMap<Integer, String> hmap = new HashMap<Integer, String>();

      /*Adding elements to HashMap*/
      hmap.put(12, “Chaitanya”);
      hmap.put(2, “Rahul”);
      hmap.put(7, “Singh”);
      hmap.put(49, “Ajeet”);
      hmap.put(3, “Anuj”);

      /* Display content using Iterator*/
      Set set = hmap.entrySet();
      Iterator iterator = set.iterator();
      while(iterator.hasNext()) {
         Map.Entry mentry = (Map.Entry)iterator.next();
         System.out.print(“key is: “+ mentry.getKey() + ” & Value is: “);
         System.out.println(mentry.getValue());
      }

      /* Get values based on key*/
      String var= hmap.get(2);
      System.out.println(“Value at index 2 is: “+var);

      /* Remove values based on key*/
      hmap.remove(3);
      System.out.println(“Map key and values after removal:”);
      Set set2 = hmap.entrySet();
      Iterator iterator2 = set2.iterator();
      while(iterator2.hasNext()) {
          Map.Entry mentry2 = (Map.Entry)iterator2.next();
          System.out.print(“Key is: “+mentry2.getKey() + ” & Value is: “);
          System.out.println(mentry2.getValue());
       }

   }
}

The Output is :

key is: 49 & Value is: Ajeet
key is: 2 & Value is: Rahul
key is: 3 & Value is: Anuj
key is: 7 & Value is: Singh
key is: 12 & Value is: Chaitanya
Value at index 2 is: Rahul
Map key and values after removal:
Key is: 49 & Value is: Ajeet
Key is: 2 & Value is: Rahul
Key is: 7 & Value is: Singh
Key is: 12 & Value is: Chaitanya