Java Hashset Treeset — Free Java Tutorial

TL;DR: Learn Java Hashset Treeset in Java with a free, beginner-friendly tutorial, examples and practice for Indian students on Syllab.in.

Java Hashset Treeset in Java

A Set stores unique elements — no duplicates allowed. Adding a duplicate has no effect and does not throw an error.

HashSet uses a hash table internally. O(1) average for add, remove, contains. Does not maintain any order.

TreeSet uses a Red-Black tree internally. O(log n) operations. Always maintains elements in sorted natural order (or custom Comparator order).

LinkedHashSet maintains insertion order while still enforcing uniqueness — a middle ground between HashSet (no order) and TreeSet (sorted order).

Set<String> hashSet = new HashSet<>();
hashSet.add("apple");
hashSet.add("apple");      // ignored - duplicate
hashSet.contains("apple"); // true

TreeSet<Integer> treeSet = new TreeSet<>();
treeSet.first();           // smallest
treeSet.last();            // largest
treeSet.headSet(5);        // el

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