meet it

A keeps each value once and ignores duplicates.

A HashSet is a bag that refuses a . .add("red") puts red in. Add red again and nothing changes: .add gives back false to say it was already there.

Sets are fast at one question: is this in here? .contains answers it, and .size() counts the different values. A HashSet has no index and no order, so reach for a when you want the values sorted.

To clean the repeats out of a list, pour it into a set: new TreeSet<>(stamps).

import java.util.*;
public class Main {
    public static void main(String[] args) {
        HashSet<String> colours = new HashSet<>();
        System.out.println(colours.add("red"));
        System.out.println(colours.add("blue"));
        System.out.println(colours.add("red"));
        System.out.println(colours.size() + " " + colours.contains("blue"));
        List<String> stamps = List.of("UK", "France", "UK", "Spain", "France");
        System.out.println(new TreeSet<>(stamps));
    }
}

Use a list when order or repeats matter, and a set when they do not.

guess first

Visitors sign the guest book. 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) {
        String[] visits = {"Sam", "Ali", "Sam", "Jo", "Ali", "Sam"};
        HashSet<String> visitors = new HashSet<>();
        for (String name : visits) {
            visitors.add(name);
        }
        System.out.println(visits.length + " visits, " + visitors.size() + " visitors");
    }
}

your turn

Your turn. Play the word game.

Type a word between the quotes on line 5. Try a new word, then try apple. Press run each time to see the sorted set.

Your code

output

press run to see what happens

fix it

The badge count has repeats. Fix it!

Broken code to fix

output

press run to see what happens

It counts 4 badges, but swimming is there twice. Which kind of collection keeps repeats?
A list keeps every item, repeats included. A set keeps each value once.
Change line 5 to HashSet<String> badges = new HashSet<>();, then press run.

nailed it

you banished the duplicates. :D

A HashSet keeps each value only once.
.add gives false when the value was already there.
Pour a list into a TreeSet for sorted values with no repeats.

new words in your collection

HashSet
duplicate
TreeSet

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A HashSet keeps each value once and ignores duplicates.

Visitors sign the guest book. What will this show?

Your turn. Play the word game.

The badge count has repeats. Fix it!

you banished the duplicates. :D

A keeps each value once and ignores duplicates.