In Java using HashMap we can store data using custom indexes. In HashMap an Index can be any object type. And for each object index there is a value.

Importing in package declaration

import java.util.HashMap;

Variable declaration:

HashMap<String, String> studentIds = new HashMap<String, String>();

Adding Items:

To add items in HashMap simply use the put(java.lang.Object, java.lang.Object). When you declare a HashMap using specific type of class, you can only add or get items from HashMap using that specific type only.

import java.util.HashMap

public class HashMapExample{
    public static void main(String[] args){
        HashMap<String, String> studentIds = new HashMap<String, String>();
        studentIds.put("20191010115", "Jennifer Lopez");
        studentIds.put("20192010114", "Mira Chitra");
        studentIds.put("20202809779", "Jita Forge");
    }
}

Note that putting multiple items in same key, will replace the previous values;

Accessing Items:

To access a values, simple use the get(java.lang.Object) method. If the corresponding doesn’t exist null will be returned

String name = studentIds.get("20191010115");

Removing Items:

The remove(java.lang.Object) method is there to simply remove a data pair in the HashMap.

studentIds.remove("20191010115");

To clear the HashMap, ie delete all data in it, call the clear() method

studentIds.clear();

Getting the Size of HashMap:

The size() method returns the size of the HashMap. Note that, it only counts the number of data pairs in it, not the size in bytes. Example:

studentIds.size();

Iterating through HashMap

Sometimes, we need to read all the keys and respected references in it. To iterate a HashMap use keySet() in for.

for (String id: studentIds.keySet()){
    System.out.println(id);
}

If you also want to print the reference variable for the key

for (String id: studentIds.keySet()){
    System.out.println(id + "->" + studentIds.get(id));
}

To get only the values:

for (String name: studentIds.values()){
    System.out.println(name);
}