Java Choose What Uniqueness and Order Mean

Understand the idea

HashSet uses equals and hashCode and does not promise a useful iteration order. LinkedHashSet retains encounter order for its normal insertion-based use. TreeSet uses an ordering comparison; values comparing as zero are treated as the same set position. Select this policy deliberately, especially when a comparator ignores part of an object’s state.

Run it locally

Use a local JDK 21 or newer. Save the complete example as Main.java in a separate lab folder, then compile and run it with the commands below. Java examples do not run in Bhylu’s browser playground. The -ea option enables assertion checks used by some lessons.

JDK and terminal setup

javac --release 21 -encoding UTF-8 -d out Main.java
java -ea -cp out Main

A worked example

import java.util.Set;
import java.util.LinkedHashSet;
import java.util.TreeSet;

public class Main {
    public static void main(String[] args) {
        Set<String> encounter = new LinkedHashSet<>();
        encounter.add("Wood");
        encounter.add("Clay");
        encounter.add("Wood");
        Set<String> sorted = new TreeSet<>(encounter);
        System.out.println(encounter);
        System.out.println(sorted);
    }
}

Expected output

[Wood, Clay]
[Clay, Wood]

The duplicate Wood does not add another entry. The first result retains arrival order; the second follows the strings’ natural order. If the task needs locale-sensitive human text order, natural String ordering is not automatically the right comparison. Set membership can be useful without printing or depending on the full traversal order.