IntermediatePractice in the editor

Java HashMap

A list finds items by their position. Often you want to find them by something more meaningful: a phone number by a name, a price by a product, a capital city by a country. A map stores pairs of a key and a value, and finds a value by its key very quickly. The most used map in Java is HashMap.

put and get

You give two types in angle brackets: the key type and the value type.

ExampleTry it in the editor

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        HashMap<String, String> capitals = new HashMap<>();
        capitals.put("India", "New Delhi");
        capitals.put("Japan", "Tokyo");
        capitals.put("Kenya", "Nairobi");
        System.out.println(capitals.get("Japan"));
        System.out.println(capitals.get("Peru"));
        System.out.println(capitals.size());
        capitals.put("Japan", "TOKYO");
        System.out.println(capitals.get("Japan"));
    }
}

Output

Tokyo
null
3
TOKYO

put(key, value) adds a pair. If the key is already there, its value is replaced: each key appears only once.
get(key) returns the value, or null when the key is not in the map.

Checking, defaults and removing

ExampleTry it in the editor

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        HashMap<String, Integer> stock = new HashMap<>();
        stock.put("pen", 40);
        stock.put("eraser", 12);
        System.out.println(stock.containsKey("pen"));
        System.out.println(stock.getOrDefault("ruler", 0));
        stock.remove("eraser");
        System.out.println(stock.containsKey("eraser"));
        stock.put("pen", stock.get("pen") - 5);
        System.out.println(stock);
    }
}

Output

true
0
false
{pen=35}

getOrDefault avoids the null (and the NullPointerException that unboxing it would cause) by giving a default value for missing keys.

Looping over a map

keySet() gives all the keys, values() all the values, and entrySet() all the pairs. A HashMap does not keep the pairs in the order you added them; the order depends on the keys' hash codes:

ExampleTry it in the editor

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

public class Main {
    public static void main(String[] args) {
        Map<String, Integer> scores = new HashMap<>();
        scores.put("Ravi", 82);
        scores.put("Anu", 95);
        scores.put("Zoya", 77);
        for (Map.Entry<String, Integer> e : scores.entrySet()) {
            System.out.println(e.getKey() + " -> " + e.getValue());
        }
        int total = 0;
        for (int s : scores.values()) {
            total += s;
        }
        System.out.println("Total: " + total);
    }
}

Output

Ravi -> 82
Zoya -> 77
Anu -> 95
Total: 254

Counting with a map

Counting how often something appears is the classic use of a map. merge adds 1 to the count, or puts 1 when the key is new. A TreeMap keeps its keys sorted, which is handy for printing:

ExampleTry it in the editor

import java.util.Map;
import java.util.TreeMap;

public class Main {
    public static void main(String[] args) {
        String text = "the cat and the dog and the bird";
        Map<String, Integer> counts = new TreeMap<>();
        for (String word : text.split(" ")) {
            counts.merge(word, 1, Integer::sum);
        }
        System.out.println(counts);
        System.out.println("'the' appears " + counts.get("the") + " times");
    }
}

Output

{and=2, bird=1, cat=1, dog=1, the=3}
'the' appears 3 times

Integer::sum is a method reference, a short form of a lambda that you will meet in Lambda. Keys must have proper equals and hashCode methods; strings, wrapper classes and records do.

Practice exercises

Store the prices of five fruits in a map and print the price of one the user types.
Count how many times each letter appears in the word "programming".
Print the contents of a map sorted by key using a TreeMap.