Java OO Tutorial - HashMap

A HashMap is a data structure that is used to store key-value pairs. It is part of the java.util package and is implemented as a hash table. HashMaps are useful when you need to store and retrieve data based on keys rather than indexes, as in the case of arrays and ArrayLists.

In a HashMap, the keys must be unique, but the values can be duplicated. 

import java.util.HashMap;

public class Main {	
	
	public static void main(String[] args) {
		
		HashMap<String, String> people = new HashMap<String, String>();
		
		people.put("Alabama", "Monica");
		people.put("Arizona", "John");
		people.put("Idaho", "Samantha");
		
		//Get by Key - left side
		System.out.println(people.get("Alabama"));
		
		//Number of elements
		System.out.println("Hash Size: " + people.size());
		
		//Get all pairs
		
		for (String x : people.keySet()) {
			System.out.println("Pair: " + x + "->" + people.get(x));
		}
		
		//Remove pair
		people.remove("Idaho");
		System.out.println(people);
		
		//Clear all pairs
		people.clear();
		System.out.println(people);
	}		
}

Explanation, block by block: 

import java.util.HashMap;

public class Main {	
	
	public static void main(String[] args) {
		
		HashMap<String, String> people = new HashMap<String, String>();
		
		people.put("Alabama", "Monica");
		people.put("Arizona", "John");
		people.put("Idaho", "Samantha");

• We start by importing the HashMap class from the java.util package and creating a new Main class.

• In the main method, we create a new HashMap object called people that will hold String keys and String values.

• We use the put method to add three key-value pairs to the people map. The keys are U.S. state names and the values are people's names. 

		//Get by Key - left side
		System.out.println(people.get("Alabama"));

• We use the get method to retrieve the value associated with the key "Alabama" and print it to the console. 

		//Number of elements
		System.out.println("Hash Size: " + people.size());

• We use the size method to print the number of key-value pairs in the people map. 

		//Get all pairs
		for (String x : people.keySet()) {
			System.out.println("Pair: " + x + "->" + people.get(x));
		}

• We use a for loop to iterate over all the keys in the people map using the keySet method. For each key, we use the get method to retrieve its associated value and print both to the console. 

		//Remove pair
			people.remove("Idaho");
			System.out.println(people);

• We use the remove method to remove the key-value pair with the key "Idaho" from the people map, and then print the updated map to the console. 

		//Clear all pairs
		people.clear();
		System.out.println(people);
	}		
}

• Finally, we use the clear method to remove all the key-value pairs from the people map and then print the empty map to the console.