Java HashMap is a map interface that allows us to store data by key-value pair where key should be unique.

HashMap Syntax:

HashMap<datatype1, datatype2> obj= new HashMap<datatype1, datatype2>();

Let’s see a simple example of HashMap to store key and value pair.

A simple program:

public class Main {
    public static void main(String[] args) {
        HashMap<Integer,String> hashmap=new HashMap<Integer,String>();
        hashmap.put(1,"America");
        hashmap.put(2,"Bangladesh");
        hashmap.put(3,"Canada");
        hashmap.put(4,"Denmark");
        hashmap.put(5,"England");

        System.out.println(hashmap.get(2)); //2 is the key
    }


}

Output:
Bangladesh

If you want to print all of the stored data, then the program looks like:

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

public class Main {
    public static void main(String[] args) {
        HashMap<Integer,String> hashmap=new HashMap<Integer,String>();
        hashmap.put(1,"America");
        hashmap.put(2,"Bangladesh");
        hashmap.put(3,"Canada");
        hashmap.put(4,"Denmark");
        hashmap.put(5,"England");
        for(Map.Entry m : hashmap.entrySet()){
            System.out.println(m.getKey()+" "+m.getValue());
        }
    }
    
}

Output:
1 America
2 Bangladesh
3 Canada
4 Denmark
5 England

Duplicate Key:

Method: hashmap.put(duplicate_key, newValue);

If you duplicate the key, then first one will be replaced by second one.

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

public class Main {
    public static void main(String[] args) {
        HashMap<Integer,String> hashmap=new HashMap<Integer,String>();
        hashmap.put(1,"America");
        hashmap.put(2,"Bangladesh");
        hashmap.put(3,"Canada");
        hashmap.put(4,"Denmark");
        hashmap.put(5,"England");
        hashmap.put(1,"Germany");
        for(Map.Entry m : hashmap.entrySet()){
            System.out.println(m.getKey()+" "+m.getValue());
        }
    }

}

Output:
1 Germany
2 Bangladesh
3 Canada
4 Denmark
5 England

Delete Data:

Method: hashmap.remove(key)

If you want to remove a data from HashMap, then the program is:

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

public class Main {
    public static void main(String[] args) {
        HashMap<Integer,String> hashmap=new HashMap<Integer,String>();
        hashmap.put(1,"America");
        hashmap.put(2,"Bangladesh");
        hashmap.put(3,"Canada");
        hashmap.put(4,"Denmark");
        hashmap.put(5,"England");
        hashmap.remove(5);
        for(Map.Entry m : hashmap.entrySet()){
            System.out.println(m.getKey()+" "+m.getValue());
        }
    }

}

Output:
1 America
2 Bangladesh
3 Canada
4 Denmark

Replace Data:

Method: hashmap.replace(key,newValue);

If you want to replace a value with another value, then the program looks like:

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

public class Main {
    public static void main(String[] args) {
        HashMap<Integer,String> hashmap=new HashMap<Integer,String>();
        hashmap.put(1,"America");
        hashmap.put(2,"Bangladesh");
        hashmap.put(3,"Canada");
        hashmap.put(4,"Denmark");
        hashmap.put(5,"England");
        hashmap.replace(5,"France");
        for(Map.Entry m : hashmap.entrySet()){
            System.out.println(m.getKey()+" "+m.getValue());
        }
    }

}

Output:
1 America
2 Bangladesh
3 Canada
4 Denmark
5 France