Set Duplicate Prevention

Direct Answer

A Set rejects an element when its implementation's equality rule says an equivalent element is already present. HashSet and LinkedHashSet use hashCode() to find candidates and equals() to match them. TreeSet uses natural ordering or a Comparator, where a comparison result of zero means the elements are the same for that set. add() returns false when no change was made.

Code Example

// Without equals/hashCode - allows duplicates!
class OrderBroken {
    Long id;
    String item;
    OrderBroken(Long id, String item) { this.id = id; this.item = item; }
}

// Proper implementation prevents duplicates
record Order(Long id, String item) {} // records auto-generate equals/hashCode

var brokenSet = new HashSet<OrderBroken>();
brokenSet.add(new OrderBroken(1L, "Laptop"));
brokenSet.add(new OrderBroken(1L, "Laptop")); // both added!
System.out.println(brokenSet.size()); // prints 2 - duplicates allowed

var properSet = new HashSet<Order>();
properSet.add(new Order(1L, "Laptop"));
boolean added = properSet.add(new Order(1L, "Laptop")); // returns false
System.out.println(properSet.size()); // prints 1 - duplicate rejected
System.out.println(added); // false - element already exists

Common Mistakes & Pitfalls

• Overriding equals() without hashCode(): Freshers define logical equality but forget hashCode, breaking the contract — equal objects return different hash codes, so the HashSet cannot find duplicates and stores them in different buckets.
• Using mutable objects in Sets: If you add an object then modify a field used in hashCode, its hash changes but it stays in the old bucket — now contains and remove can't find it, and duplicates creep in.
• Comparing objects with ==: Freshers write if (set.contains(order)) expecting reference equality, not realizing the Set calls equals for value-based comparison.