Java HashMap

A HashMap keeps data as key and value pairs, like a dictionary. You find a value very fast by its key. Keys are unique.

Basic use

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

public class Main {
    public static void main(String[] args) {
        Map<String, Integer> marks = new HashMap<>();

        marks.put("Riya", 88);
        marks.put("Aman", 72);
        marks.put("Meera", 95);
        marks.put("Aman", 75);          // same key: the value is replaced

        System.out.println(marks.get("Riya"));
        System.out.println(marks.get("Unknown"));              // null when missing
        System.out.println(marks.getOrDefault("Unknown", 0));
        System.out.println(marks.containsKey("Meera"));
        System.out.println(marks.containsValue(72));
        System.out.println(marks.size());

        marks.remove("Meera");
        System.out.println(marks);
    }
}

Loop over a map

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

public class Loop {
    public static void main(String[] args) {
        Map<String, Integer> stock = new TreeMap<>();     // TreeMap keeps the keys sorted
        stock.put("pen", 40);
        stock.put("book", 15);
        stock.put("bag", 5);

        for (Map.Entry<String, Integer> e : stock.entrySet()) {
            System.out.println(e.getKey() + " = " + e.getValue());
        }
        for (String key : stock.keySet()) {
            System.out.print(key + " ");
        }
        System.out.println();
        for (int value : stock.values()) {
            System.out.print(value + " ");
        }
        System.out.println();
        stock.forEach((k, v) -> System.out.println(k + " -> " + v));
    }
}

Count the words

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

public class Words {
    public static void main(String[] args) {
        String text = "to be or not to be that is the question to be";
        Map<String, Integer> count = new TreeMap<>();

        for (String word : text.split(" ")) {
            count.merge(word, 1, Integer::sum);          // add 1, or start with 1
        }
        System.out.println(count);
    }
}

Useful methods

Method	Use
put(k, v)	Add or replace
get(k)	Get the value (null when missing)
getOrDefault(k, d)	Get the value or a default
putIfAbsent(k, v)	Add only if the key is not there
computeIfAbsent(k, f)	Make a value when the key is missing
merge(k, v, f)	Combine values, great for counting
remove(k)	Delete
containsKey(k)	Check a key
keySet(), values(), entrySet()	Views for looping

Which map?

Class	Order	Speed
HashMap	No order	The fastest. The default choice
LinkedHashMap	Order of insert	Fast
TreeMap	Sorted by key	Slower, but sorted

Keys need equals and hashCode

The key class must have good equals() and hashCode() methods. String, Integer and records already have them.

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

record Point(int x, int y) { }

public class Keys {
    public static void main(String[] args) {
        Map<Point, String> places = new HashMap<>();
        places.put(new Point(1, 2), "School");
        System.out.println(places.get(new Point(1, 2)));
    }
}

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