HashMap In Java With Example

Here we will learn about the Java HashMap class with an example along with the functions provided by the Java HashMap class which resides in the Java util package.

The Java HashMap class implements the Map interface, which is used for storing key-value pairs. HashMap is represented as HashMap<Key, Value> or HashMap<K, V>, the key and value can be of different data types.

HashMap works on the principle of Hashing(is a way to assign unique code to variable/Value)

Properties of HashMap

• HashMap cannot contain duplicate keys.
• HashMap is an unordered collection.
• HashMap allows null values and null keys.
• HashMap is not thread-safe.

Creating HashMap

HashMap can be created from a Map interface or from another HashMap. below are two ways HashMap can be created.

import java.util.HashMap;
import java.util.Map;

public class HashMapDemo {
 
 public static void main(String [] args) {
  HashMap<Integer, String> mapOne = new HashMap<>();
  mapOne.put(1, "One");
  mapOne.put(2, "Two");
  mapOne.put(3, "Three");
  mapOne.put(4, "Four");
  System.out.println(mapOne);
  
  Map<Integer, String> mapTwo = new HashMap<Integer, String>();
  mapTwo.put(5, "Five");
  mapTwo.put(6, "Six");
  mapTwo.put(7, "Seven");
  mapTwo.put(8, "Eight");
  System.out.println(mapTwo);
 }

OutPut:

{1=One, 2=Two, 3=Three, 4=Four}
{5=Five, 6=Six, 7=Seven, 8=Eight}

HashMap Methods

• put(k, v) : add values to HashMap.
• remove(k): remove value with key “k“.
• get(k): gets the value of key “k“.
• clear(): clears the HashMap, makes it an empty HashMap.
• size(): returns the size of HashMap.
• containsKey(k): returns a boolean value true or false, depends on if key presents in the HashMap.
• conatinsValue(Object value): similar to containsKey it checks if the specified value is present or not.
• isEmpty(): checks if the HashMap is empty or not.

import java.util.HashMap;
import java.util.Map;

public class HashMapDemo {
 
 public static void main(String [] args) {
  HashMap<Integer, String> mapOne = new HashMap<>();
  mapOne.put(1, "One");
  mapOne.put(2, "Two");
  mapOne.put(3, "Three");
  mapOne.put(4, "Four");
  System.out.println(mapOne);
  
  Map<Integer, String> mapTwo = new HashMap<Integer, String>();
  mapTwo.put(5, "Five");
  mapTwo.put(6, "Six");
  mapTwo.put(7, "Seven");
  mapTwo.put(8, "Eight");
  System.out.println(mapTwo);
  
  //remove method
  
  mapOne.remove(4);
  System.out.println("after removal : "+mapOne);
  
  //get method
  
  System.out.println("using get method to get value of 2 : "+mapOne.get(2));
  
  //size method
  
  System.out.println("size of map 2 : "+mapTwo.size());
  
  //containsKey(k)
  
  System.out.println("checking contains key "+mapTwo.containsKey(7));
  
  //containsValue
  
  System.out.println("checking contains value "+mapTwo.containsValue("Six"));
  
  //clear
  mapOne.clear();
  System.out.println("printing map after clear function "+mapOne);
 }

}

OutPut:

{1=One, 2=Two, 3=Three, 4=Four}
{5=Five, 6=Six, 7=Seven, 8=Eight}
after removal : {1=One, 2=Two, 3=Three}
using get method to get value of 2 : Two
size of map 2 : 4
checking contains key true
checking contains value true
printing map after clear function {}

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