Step 2 of 5

Records as keys

In module 32, a class needed its own equals and hashCode before a HashMap could find its objects as keys. A record has both from the start, written by Java from the components.

Equal when the components are equal

Main.java

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        Pin first = new Pin(3, 5);
        Pin second = new Pin(3, 5);
        System.out.println(first == second);
        System.out.println(first.equals(second));

        HashMap<Pin, String> places = new HashMap<>();
        places.put(new Pin(3, 5), "bakery");
        places.put(new Pin(8, 1), "library");
        System.out.println(places.get(new Pin(8, 1)));
        System.out.println(places.containsKey(new Pin(1, 8)));
    }
}

Pin.java

public record Pin(int x, int y) {
}

Output

false
true
library
false

first == second is still false: new makes two objects, and == asks whether two references point to the very same object (module 32).
first.equals(second) is true. A record's equals is true when the other object is a record of the same type and every component is equal: components of a primitive type such as int are compared by their value (doubles with Double.compare), and objects such as Strings with their own equals.
places.get(new Pin(8, 1)) finds the library, although the key in the map is another object: the map uses the record's hashCode and equals, and both look only at the components. new Pin(1, 8) has the same numbers the other way round, so it's a different key.

What hashCode gives

A record's hashCode is computed from its components, so equal records always have the same hash code, which is the rule from module 32 that a HashMap needs. Java doesn't promise what the number itself is, though: it may differ from one Java version to another. So don't print it or write code that depends on its value. The same goes for printing a whole HashMap or HashSet of records: it shows them in an order of its own (module 31). Print what you look up, or go through a list.

Records in a HashSet

A HashSet keeps each value once, by equals and hashCode, so records work in sets too. A set of pins dropped by friends who meet up, with one pin dropped twice:

Main.java

import java.util.HashSet;

public class Main {
    public static void main(String[] args) {
        HashSet<Pin> dropped = new HashSet<>();
        System.out.println(dropped.add(new Pin(3, 5)));
        System.out.println(dropped.add(new Pin(6, 2)));
        System.out.println(dropped.add(new Pin(3, 5)));
        System.out.println(dropped.size() + " different places");
    }
}

Pin.java

public record Pin(int x, int y) {
}

Output

true
true
false
2 different places

add returns false when an equal value is already in the set, and leaves the set as it was.

ArrayList<Cell> path = new ArrayList<>(); path.add(new Cell(0, 0)); path.add(new Cell(0, 1)); path.add(new Cell(0, 0)); path.add(new Cell(2, 1)); path.add(new Cell(0, 0)); HashMap<Cell, Integer> visits = countVisits(path); System.out.println(visits.size() + " different cells"); System.out.println(visits.get(new Cell(0, 0))); System.out.println(visits.get(new Cell(2, 1))); System.out.println(visits.getOrDefault(new Cell(1, 0), 0));

ArrayList<Sighting> notes = new ArrayList<>(); notes.add(new Sighting("heron", 4)); notes.add(new Sighting("crane", 4)); notes.add(new Sighting("heron", 4)); notes.add(new Sighting("heron", 7)); notes.add(new Sighting("crane", 4)); ArrayList<Sighting> unique = withoutRepeats(notes); System.out.println(unique); System.out.println(notes.size() + " notes, " + unique.size() + " sightings");

Records as keys

Equal when the components are equal

What hashCode gives

Records in a HashSet

Your task

Challenge 2

Challenge 3