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Java - CollectionsJava - List InterfaceJava - Set InterfaceJava - Map InterfaceJava - Queue Interface

Java - Set Interface

📌 What You Will Learn
  • What the Collection Framework is and why we need it
  • List, Set, Map and Queue — the four big families
  • Choosing the right collection for the right job
  • Comparable vs Comparator
  • The internal working of HashMap (interview favourite)

Set Interface is a core concept of the Java language. This lesson explains Set — No Duplicates with complete, runnable code examples, clear step-by-step explanations, and common mistakes to avoid with exam-style MCQs at the end.

Set — No Duplicates

A Set never allows duplicate values. Use it when the group should have only unique items (like a list of registered student emails).

HashSet vs LinkedHashSet vs TreeSet

PointHashSetLinkedHashSetTreeSet
OrderNo guaranteed order (fastest)Insertion orderSorted (ascending)
Null allowed?YesYesNo (throws error)
DuplicatesNoNoNo
SpeedFastestMediumSlowest (sorting cost)
Best forGeneral unique groupUnique + insertion orderUnique + sorted data
Example01
JCode Cell
1import java.util.*;
2 
3class SetDemo {
4 public static void main(String[] args) {
5 Set<String> set = new HashSet<>();
6 set.add("Apple");
7 set.add("Banana");
8 set.add("Apple"); // ignored - duplicate
9 System.out.println(set);
10 }
11}
Output
[Apple, Banana]
📝 Key Takeaways
  • Collection is an interface; Collections is a utility class with static helpers.
  • List = ordered, allows duplicates. Set = no duplicates. Map = key-value pairs. Queue = FIFO.
  • ArrayList is the everyday list; HashMap is the everyday map.
  • HashMap works on hashCode + buckets; collisions chain, then become trees (Java 8+).
  • Comparable (compareTo) is the natural order; Comparator (compare) gives custom order.
  • Always override equals() and hashCode() together for your own objects.
  • Collections.sort() and Collections.reverse() are the handy helpers.

🧠 Test Your Knowledge

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