Initialize HashMap in Java

The HashMap class uses a hashtable to implement the Map interface. This allows the execution time of basic operations, such as get( ) and put( ), to remain constant even for large sets.

Following is the list of constructors supported by the HashMap class.

Following is an example displaying how to initialize and work with HashMap in Java −

Example

 Live Demo

import java.util.*;
public class Demo {
   public static void main(String args[]) {
      HashMap<Integer, String> hashMap = new HashMap<Integer, String>(7);
      hashMap.put(1, "Laptop");
      hashMap.put(2, "LCD");
      hashMap.put(3, "LED");
      hashMap.put(4, "Desktop");
      hashMap.put(5, "Laptop");
      hashMap.put(6, "Notebook");
      hashMap.put(7, "Mobile");
      System.out.println("HashMap = " + hashMap);
   }
}

Output

HashMap = {1=Laptop, 2=LCD, 3=LED, 4=Desktop, 5=Laptop, 6=Notebook, 7=Mobile}

Let us see another example −

Example

 Live Demo

import java.util.*;
public class Main {
   public static void main(String args[]) {
      HashMap hashMap = new HashMap();
      hashMap.put("John", new Integer(10000));
      hashMap.put("Tim", new Integer(25000));
      hashMap.put("Adam", new Integer(15000));
      hashMap.put("Katie", new Integer(30000));
      hashMap.put("Jacob", new Integer(45000));
      hashMap.put("Steve", new Integer(23000));
      hashMap.put("Nathan", new Integer(25000));
      hashMap.put("Amy", new Integer(27000));
      Set set = hashMap.entrySet();
      Iterator iterator = set.iterator();
      while(iterator.hasNext()) {
         Map.Entry map = (Map.Entry)iterator.next();
         System.out.print(map.getKey() + ": ");
         System.out.println(map.getValue());
      }
      System.out.println();
      System.out.println("Size of IdentintyHashMap: "+hashMap.size());
      int bonus = ((Integer)hashMap.get("Amy")).intValue();
      hashMap.put("Amy", new Integer(bonus + 5000));
      System.out.println("Amy's salary after bonus = " + hashMap.get("Amy"));
      int deductions = ((Integer)hashMap.get("Steve")).intValue();
      hashMap.put("Steve", new Integer(deductions - 3000));
      System.out.println("Steve's salary after deductions = " + hashMap.get("Steve"));
   }
}

Output

Adam: 15000
Nathan: 25000
Katie: 30000
Steve: 23000
John: 10000
Tim: 25000
Amy: 27000
Jacob: 45000
Size of IdentintyHashMap: 8
Amy's salary after bonus = 32000
Steve's salary after deductions = 20000