GateShala
Playground

HashMap

Look values up by key with HashMap and TreeMap.

A map stores key-value pairs: give it a key and it returns the matching value. HashMap is the most used implementation. Like ArrayList, it lives in java.util and uses generics: HashMap<String, Integer> maps strings to integers.

Java
Open in playground
import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        HashMap<String, Integer> stock = new HashMap<>();
        stock.put("pens", 40);
        stock.put("notebooks", 25);
        stock.put("pens", 35);        // same key: replaces the value

        System.out.println(stock.get("pens"));
        System.out.println(stock.get("erasers"));
        System.out.println(stock.getOrDefault("erasers", 0));
        System.out.println(stock.containsKey("notebooks"));
        System.out.println(stock.size());

        stock.remove("notebooks");
        System.out.println(stock.size());
    }
}
Output
35
null
0
true
2
1

Counting things

merge adds to an existing value, or inserts the starting value if the key is new. It is perfect for counting.

Java
Open in playground
import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        String text = "to be or not to be";
        HashMap<String, Integer> counts = new HashMap<>();
        for (String word : text.split(" ")) {
            counts.merge(word, 1, Integer::sum);
        }
        System.out.println("to: " + counts.get("to"));
        System.out.println("be: " + counts.get("be"));
        System.out.println("not: " + counts.get("not"));
    }
}
Output
to: 2
be: 2
not: 1

Looping in a predictable order

A HashMap does not keep its keys in any particular order. When order matters, use TreeMap (sorted by key) or LinkedHashMap (in insertion order). They have the same methods.

Java
Open in playground
import java.util.Map;
import java.util.TreeMap;

public class Main {
    public static void main(String[] args) {
        Map<String, Double> cgpa = new TreeMap<>();
        cgpa.put("Ravi", 7.9);
        cgpa.put("Asha", 9.1);
        cgpa.put("Meera", 8.4);

        for (Map.Entry<String, Double> e : cgpa.entrySet()) {
            System.out.println(e.getKey() + " -> " + e.getValue());
        }
        System.out.println(cgpa);
    }
}
Output
Asha -> 9.1
Meera -> 8.4
Ravi -> 7.9
{Asha=9.1, Meera=8.4, Ravi=7.9}