HashMap is a part of java.util package.HashMap extends an abstract class AbstractMap which also provides an incomplete implementation of Map interface. It stores the data in (Key, Value) pairs.
We can initialize HashMap using the constructor in four different ways :
1.HashMap()
It is the default constructor with initial capacity 16 and load factor 0.75.HashMap hs=new HashMap();
// Java program to demonstrate simple initialization // of HashMapimport java.util.*;public class GFG {    public static void main(String args[])    {        HashMap<Integer, String> hm = new HashMap<Integer, String>();        hm.put(1, "Ram");        hm.put(2, "Shyam");        hm.put(3, "Sita");        System.out.println("Values " + hm);    }} |
Values {1=Ram, 2=Shyam, 3=Sita}
2.HashMap(int initialCapacity)
It is used to create an empty HashMap object with specified initial capacity under default load factor 0.75.HashMap hs=new HashMap(10);
// Java program to demonstrate initialization // of HashMap with given capacity.import java.util.*;public class GFG {    public static void main(String[] args)    {        HashMap<Integer, String> hm = new HashMap<Integer, String>(3);        hm.put(1, "C");        hm.put(2, "C++");        hm.put(3, "Java");        System.out.println("Values of hm" + hm);    }} |
Values of hm{1=C, 2=C++, 3=Java}
3.HashMap(int initial capacity, float loadFactor)
It is used to create an empty HashMap object with specified initial capacity as well as load factor.HashMap hs=new HashMap(10, 0.5);
// Java program to demonstrate initialization // of HashMap with given capacity and load factor.import java.util.*;public class GFG {    public static void main(String[] args)    {        HashMap<Integer, String> hm =               new HashMap<Integer, String>(3, 0.5f);        hm.put(1, "C");        hm.put(2, "C++");        hm.put(3, "Java");        System.out.println("Values of hm" + hm);    }} |
Values of hm{1=C, 2=C++, 3=Java}
4.HashMap(Map map)
It is used to initializes the hash map by using the elements of the given Map object map.HashMap hs=new HashMap(map);
// Java program to demonstrate initialization // of HashMap from another HashMap.import java.util.*;public class GFG {    public static void main(String[] args)    {        HashMap<Integer, String> hm = new HashMap<Integer, String>();        hm.put(1, "C");        hm.put(2, "C++");        hm.put(3, "Java");        System.out.println("Values of hm" + hm);          // Creating a new map from above map        HashMap<Integer, String> language = new HashMap<Integer, String>(hm);          System.out.println("Values of language " + language);    }} |
Values of hm{1=C, 2=C++, 3=Java}
Values of language {1=C, 2=C++, 3=Java}
Initializing Immutable HashMap
Please see Immutable Map in Java
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