HashSet

Learn unique-element sets with HashSet.

Unique set: HashSet

When you want to drop duplicates automatically, you use a HashSet. If an element is already present when you add it again, the second add is a no-op; the set stays the same.

HashSet<String> tags = new HashSet<>();
tags.add("java");
tags.add("java");
tags.add("code");
// [java, code]

Checking existence with contains is very fast; much faster than indexOf on a list. A set does not guarantee order; if order matters, prefer LinkedHashSet.

Looping over a set

You can walk a HashSet with a for-each loop. The output order of the elements may not match the insertion order, because HashSet makes no ordering promise.

for (String tag : tags) {
  System.out.println(tag);
}

To compute the intersection or union of two sets, you use retainAll and addAll. They let you do set operations in a single line.

Exercise Collect unique tags

Create a HashSet<String>, add "java", "code", "java" and "maven". Then print the size of the set.

Show example solution

HashSet<String> tags = new HashSet<>();
tags.add("java");
tags.add("code");
tags.add("java");
tags.add("maven");
System.out.println(tags.size());

Test yourself

Match scenario to collection

Ordered, indexed list

ArrayList
Key-value table

HashMap
Unique element set

HashSet
Ordered unique set

LinkedHashSet

Fill in the blank: is it in the set?

HashSet<String> tags = new HashSet<>();
tags.add("java");
boolean has = tags.____("java");

Show answer

HashSet<String> tags = new HashSet<>();
tags.add("java");
boolean has = tags.contains("java");

What happens if you add the same element twice to a HashSet?
- Two copies are stored
- The second add is ignored; the set stays unchanged
- It throws an error
- The old element is removed and the new one is added
Show answer

The second add is ignored; the set stays unchanged — Set elements are unique; the second add is ignored.

Next lesson: Summary - Collections

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