HashMap: Keys and Values

Look things up by name

A map stores pairs: a key and its value. You look up a value by its key.

Think of it like this: A dictionary: you look up a word (the key) to find its meaning (the value). Or a phone book: name → number.

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        HashMap<String, Integer> stock = new HashMap<>();
        stock.put("apple", 50);
        stock.put("mango", 20);
        stock.put("banana", 35);

        System.out.println(stock.get("mango"));
        System.out.println(stock.get("grape"));
        System.out.println(stock.getOrDefault("grape", 0));
        System.out.println(stock.containsKey("apple"));

        stock.put("apple", 45);
        stock.remove("banana");
        System.out.println(stock.size() + " items, apple = " + stock.get("apple"));
    }
}

Output

20
null
0
true
2 items, apple = 45

• HashMap<String, Integer>: keys are Strings, values are Integers.
• put adds a pair – or replaces the value if the key already exists. Keys are unique.
• get gives null when the key isn't there, so getOrDefault is often safer.

Counting things

A classic use: count how often each word appears:

import java.util.TreeMap;

public class Main {
    public static void main(String[] args) {
        String[] words = "the cat and the dog and the bird".split(" ");
        TreeMap<String, Integer> counts = new TreeMap<>();
        for (String w : words) {
            counts.put(w, counts.getOrDefault(w, 0) + 1);
        }
        for (String key : counts.keySet()) {
            System.out.println(key + ": " + counts.get(key));
        }
    }
}

Output

and: 2
bird: 1
cat: 1
dog: 1
the: 3

A HashMap doesn't keep any particular order. A TreeMap works the same way but keeps its keys sorted, and a LinkedHashMap keeps them in the order you added them.

Looping over keys and values together

import java.util.LinkedHashMap;
import java.util.Map;

public class Main {
    public static void main(String[] args) {
        LinkedHashMap<String, String> capitals = new LinkedHashMap<>();
        capitals.put("India", "New Delhi");
        capitals.put("Japan", "Tokyo");
        capitals.put("France", "Paris");
        for (Map.Entry<String, String> e : capitals.entrySet()) {
            System.out.println("The capital of " + e.getKey() + " is " + e.getValue());
        }
    }
}

Output

The capital of India is New Delhi
The capital of Japan is Tokyo
The capital of France is Paris