meet it

Put a new value under an old key and the old one is gone.

A key holds one value at a time, so .put on a key that is already there replaces its value. .remove("pears") takes the whole pair out, and tells you whether a key is there.

gives the value under kiwis, or 0 if there is none. No more null surprises.

Together they make a . For each vote, read the old count with getOrDefault, add one and put it back: votes.put(pet, votes.getOrDefault(pet, 0) + 1);.

import java.util.*;
public class Main {
    public static void main(String[] args) {
        String[] ballots = {"cat", "dog", "cat", "rabbit", "cat"};
        HashMap<String, Integer> votes = new HashMap<>();
        for (String pet : ballots) {
            votes.put(pet, votes.getOrDefault(pet, 0) + 1);
        }
        System.out.println("cat: " + votes.get("cat"));
        System.out.println("dog: " + votes.get("dog"));
        System.out.println("hamster: " + votes.getOrDefault("hamster", 0));
        System.out.println(votes.containsKey("rabbit"));
    }
}

.put never complains about a new key, so a typo can quietly start a second count.

guess first

The scores keep changing. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

import java.util.*;
public class Main {
    public static void main(String[] args) {
        HashMap<String, Integer> scores = new HashMap<>();
        scores.put("Ava", 10);
        scores.put("Leo", 8);
        scores.put("Ava", 15);
        scores.remove("Leo");
        System.out.println(scores.size() + " " + scores.get("Ava"));
    }
}

your turn

Your turn. Count the drinks order.

Add two drinks of your choice on line 4. On line 7, fill in the map method that gives a backup value when a key is missing. Then press run.

Your code

output

press run to see what happens

fix it

The bird count is stuck on 1. Can you fix it?

Broken code to fix

output

press run to see what happens

Robins were seen three times, but the count says 1. What does line 7 do on each turn?
.put replaces the old value, so every sighting resets the count to 1. Read the old count first.
Change line 7 to birds.put(bird, birds.getOrDefault(bird, 0) + 1);, then press run.

nailed it

you kept a tally with a map. :D

.put on an old key replaces its value.
.getOrDefault gives a backup value for a missing key.
To tally, read the count, add one and put it back.

new words in your collection

containsKey
getOrDefault
tally

This lesson isn’t saved. A free account remembers every lesson you finish.