Chapter 59: Set Interface in Java

List cares about positions and gladly stores duplicates. Set flips the story: each value appears at most once (by the set’s equals rules). There is no get(index)—you either ask membership or walk everything.

HashSet details arrive in Chapter 60; ordered variants follow.

1. Topic title

Set: membership and uniqueness—not “item number five”

2. What it means

java.util.Set<E> extends Collection<E> with extra equals/hashCode contracts documented on the interface. Practical habits:

• add(x) returns false if x was already considered present.
• contains(x) answers yes/no quickly on HashSet when hashCode behaves.
• Iteration order is not part of the general Set promise—HashSet scrambles, LinkedHashSet remembers insertion order, TreeSet sorts (later chapters).

3. Why it is used

De-duplicate a stream of user ids, tag sets, permission names—anywhere “already seen” matters more than position.

4. Mental flip

If List is a shopping receipt line-by-line, Set is a spice rack label check: cumin either exists or does not; you do not ask for “cumin slot 3.”

5. Java code example

import java.util.HashSet;
import java.util.Set;
 
public class SetInterfaceDemo {
    public static void main(String[] args) {
        Set<String> tags = new HashSet<>();
        tags.add("java");
        tags.add("beginner");
        tags.add("java");
        System.out.println("size " + tags.size());
        System.out.println("has sql? " + tags.contains("sql"));
 
        for (String t : tags) {
            System.out.println("tag: " + t);
        }
    }
}

Adding "java" twice still yields size 2. Loop order is not guaranteed—do not write homework that depends on print order from HashSet.

6. Explanation of code

HashSet is the simplest default Set for learning: fast add/contains average case when keys distribute hashes well.

7. Common mistakes

Assuming Set remembers insertion order—only some implementations do.

Putting mutable objects into a HashSet then changing fields that affect hashCode—membership lookups break in confusing ways (advanced but real).

Using List APIs on a Set—there is no index; use contains, remove, or iterate.

8. Best practices

If you need both uniqueness and stable iteration order, plan for LinkedHashSet (next chapter family).

For custom classes in sets, override equals and hashCode together when you mean “same id means same object”—Chapter 33-style objects revisited.

9. Small practice task

Given List<String> with duplicates, build new HashSet<>(list) and print original size vs set size. Explain the difference in one sentence you would tell a teammate.

Beginner tip

When a problem says “unique”, Set is often the first data shape to sketch—even if you later swap HashSet for TreeSet for sorting.