meet it

A files each value under a .

A list finds things by index. A HashMap finds them by name, like a coat check: hand over your coat, get a ticket, and the ticket gets your coat back. The ticket is the key, and the coat is its value.

HashMap<String, Integer> stock = new HashMap<>(); makes a map with String keys and Integer values. stock.put("apples", 12) files 12 under apples. stock.get("apples") hands it back, and .size() counts the pairs.

Ask for a key that is not there and you get , Java's word for nothing at all.

import java.util.*;
public class Main {
    public static void main(String[] args) {
        HashMap<String, Integer> stock = new HashMap<>();
        stock.put("apples", 12);
        stock.put("pears", 7);
        System.out.println(stock.get("apples"));
        System.out.println(stock.size() + " kinds of fruit");
        System.out.println(stock.get("mangoes"));
    }
}

Each key appears only once in a map, but two keys can share the same value.

guess first

Look up a phone number. 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, String> phones = new HashMap<>();
        phones.put("Mum", "07700 900123");
        phones.put("Gran", "07700 900456");
        System.out.println(phones.get("Gran"));
        System.out.println(phones.get("gran"));
    }
}

your turn

Your turn. Price up a snack.

On line 6, add a snack of your own and its price in pence. On line 7, type the same snack's name, then press run to look it up.

Your code

output

press run to see what happens

fix it

The apples went missing. Can you fix it?

Broken code to fix

output

press run to see what happens

Java stopped with a NullPointerException on line 7. What does get give for a key that is not there?
Keys are case-sensitive. The map has apples with a small a, so "Apples" gives null, and null cannot go in an int box.
Change "Apples" to "apples" on line 7, then press run.

nailed it

you filed values under keys. :D

A HashMap stores pairs: each key leads to one value.
.put files a value, and .get hands it back.
A missing key gives null, which means nothing at all.

new words in your collection

HashMap
key
null

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

A HashMap files each value under a key.

Look up a phone number. What will this show?

Your turn. Price up a snack.

The apples went missing. Can you fix it?

you filed values under keys. :D

A files each value under a .