Java HashMap
Java HashMap<K, V> implements Map interface, it represents Map data structure, each element in the map is a key, value pair.
- HashMap keys are always unique.
- HashMap is not thread safe.
- A key will always correspond to a single value.
- Key should be immutable.
- HashMap uses hashing algorithms to uniquely store and find data.
- HashMap is similar to HashTable but it is not synchronized safe on other had HashTable is synchronized(thread safe).
- Default initalCapacity of hashMap is 16 and load factor is 0.75.
HashMap Constructors
HashMap has below 4 types of constructors:
1. HashMap() – This constructor has no parameters.
Example:
HashMap myMap = new HashMap();
2.HashMap(int initialCapacity) – HashMap with initial capacity parameter.
Example:
HashMap myMapWithCap = new HashMap(12);
3. HashMap(int initialCapacity, float loadFactor) – HashMap with initial capacity and load factor.
Example:
HashMap myMapWithCapAndLoadFactor = new HashMap(12, .75F);
4. HashMap(Map map) – HashMap which takes another map as parameter.
Example:
HashMap myMap = new HashMap();
HashMap mapWhichTakesAnotherMap = new HashMap(myMap);
HashMap Example Non Generic
In a non-generic or old way, we don’t specify the data type of key and value.
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
public class MapExamples {
public static void main(String[] args) {
   HashMap myMap = new HashMap();
   myMap.put("101", "Tom");
   myMap.put("102", "Rom");
   myMap.put("103", "Jonny");
   myMap.get("101");
   Set eSet= myMap.entrySet();
   Iterator itr = eSet.iterator();
   while(itr.hasNext()) {
    Map.Entry entry = (Map.Entry)itr.next();
       System.out.println("Key:"+entry.getKey()+" value:"+entry.getValue());
   }
}
}
Output
Key:101 value:Tom
Key:102 value:Rom
Key:103 value:Jonny
HashMap Example Generic
In the generic map we specify the data type of key and value pair.
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
public class MapExamples {
public static void main(String[] args) {
   HashMap <String, String> myMap = new HashMap<String, String>();
   myMap.put("101", "Tom");
   myMap.put("102", "Rom");
   myMap.put("103", "Jonny");
   myMap.get("101");
    Set eSet= myMap.entrySet();
   Iterator itr = eSet.iterator();
   while(itr.hasNext()) {
       Map.Entry entry = (Map.Entry)itr.next();
       System.out.println("Key:"+entry.getKey()+" value:"+entry.getValue());
   }
}
}
Output
Key:101 value:Tom
Key:102 value:Rom
Key:103 value:Jonny
HashMap Add, Remove and Iterate Operations
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
public class HashMapExamples {
public static void main(String[] args) {
   HashMap<String, String> myMap = new HashMap<String, String>();
   myMap.put("101", "Tom"); //Adds element in map
   myMap.put("102", "Rom");
    myMap.put("103", "Jonny");
   //Get a value for given key
   String getValue = myMap.get("101");
   //Remove
   myMap.remove("103");
   Set eSet = myMap.entrySet();
    Iterator itr = eSet.iterator();
   while (itr.hasNext()) {
       Map.Entry entry = (Map.Entry) itr.next();
       System.out.println("Key:" + entry.getKey() + " value:" + entry.getValue());
   }
}
}
Output
Key:101 value:Tom
Key:102 value:Rom