Set Interface

A collection that doesn’t allow duplicates.
Implements Collection, but ignores order (unless special types).

Common Implementations

Implementation	Features
HashSet	Unordered, allows 1 null, fastest
LinkedHashSet	Maintains insertion order
TreeSet	Sorted set (natural or custom order)

Key Properties

• No duplicates.
• No index-based access (get(index) not available).
• Fastest membership checks: contains(...).
• Uses quals()and hashCode()to detect duplicates.

Example

HashSet

Set<String> set = new HashSet<>();
set.add("Java");
set.add("Java"); // Duplicate is ignored
set.add("Python");
 
System.out.println(set); // ["Java", "Python"] (order not guaranteed)

TreeSet

Set<Integer> numbers = new TreeSet<>();
numbers.add(3);
numbers.add(1);
numbers.add(2);
 
System.out.println(numbers); // [1, 2, 3] - sorted

Comparison

HashSet vs TreeSet vs LinkedHashSet

Feature	HashSet	LinkedHashSet	TreeSet
Ordering	✗	✅ Insertion order	✅ Sorted
Performance	✅ Fastest	Slightly slower	✗ Slower
Nulls	✅ 1 null allowed	✅ 1 null allowed	✗ No nulls allowed
Under the hood	Hash table	Hash table + linked list	Red-black tree

Use Cases

• HashSet: uniqueness, fast lookup (e.g. filter duplicates).
• LinkedHashSet: uniqueness + predictable order (e.g. caching).
• TreeSet: sorted data (e.g. leaderboards, autocomplete).

Tip

Be ready to explain:

• explain how Set avoids duplicates,
• talk about hashCode() + equals(),
• compare all 3 Set types and when to use each.

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Parent: _Collections