STUDYECRAT Java : List vs Set vs Map
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Java List vs Set vs Map
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★ Java Collections: List vs Set vs Map Key Differences
1. Duplicate Element Handling
List<String> list = new ArrayList<>();
list.add("A"); list.add("A"); // Allowed
Set<String> set = new HashSet<>();
set.add("A"); set.add("A"); // Rejected
Map<String, Integer> map = new HashMap<>();
map.put("A", 1); map.put("A", 2); // Overwrites value
// List allows duplicates, Set rejects them, Map overwrites values
list.add("A"); list.add("A"); // Allowed
Set<String> set = new HashSet<>();
set.add("A"); set.add("A"); // Rejected
Map<String, Integer> map = new HashMap<>();
map.put("A", 1); map.put("A", 2); // Overwrites value
// List allows duplicates, Set rejects them, Map overwrites values
- Tip: Use Set when unique elements are required without manual checks
- Real Use: Shopping cart (List) vs Unique coupon codes (Set) vs Product inventory (Map)
2. Null Element Support
List<String> list = new ArrayList<>();
list.add(null); // Allowed
Set<String> set = new TreeSet<>();
set.add(null); // Throws NullPointerException
Map<String, Integer> map = new HashMap<>();
map.put(null, 1); // Allowed (1 null key)
// TreeSet rejects nulls, HashMap allows 1 null key
list.add(null); // Allowed
Set<String> set = new TreeSet<>();
set.add(null); // Throws NullPointerException
Map<String, Integer> map = new HashMap<>();
map.put(null, 1); // Allowed (1 null key)
// TreeSet rejects nulls, HashMap allows 1 null key
- Tip: Use LinkedList if frequent null additions are needed
- Real Use: Configuration values where absence (null) is valid
3. Order Guarantees
List<Integer> arrayList = new ArrayList<>(List.of(3,1,2));
System.out.println(arrayList); // [3,1,2] (Insertion order)
Set<Integer> treeSet = new TreeSet<>(List.of(3,1,2));
System.out.println(treeSet); // [1,2,3] (Natural order)
Map<Integer, String> linkedMap = new LinkedHashMap<>();
linkedMap.put(3,"C"); linkedMap.put(1,"A");
System.out.println(linkedMap); // {3=C, 1=A} (Insertion order)
// Different collection types maintain different orders
System.out.println(arrayList); // [3,1,2] (Insertion order)
Set<Integer> treeSet = new TreeSet<>(List.of(3,1,2));
System.out.println(treeSet); // [1,2,3] (Natural order)
Map<Integer, String> linkedMap = new LinkedHashMap<>();
linkedMap.put(3,"C"); linkedMap.put(1,"A");
System.out.println(linkedMap); // {3=C, 1=A} (Insertion order)
// Different collection types maintain different orders
- Tip: Use LinkedHashSet for insertion-order uniqueness
- Real Use: Audit logs (order-critical) vs Dictionary (sorted)
4. Time Complexity Comparison
List<Integer> list = new ArrayList<>();
list.contains(5); // O(n) linear search
Set<Integer> set = new HashSet<>();
set.contains(5); // O(1) hash-based lookup
Map<Integer, String> map = new HashMap<>();
map.containsKey(5); // O(1) hash-based key search
// Sets and Maps outperform Lists for search operations
list.contains(5); // O(n) linear search
Set<Integer> set = new HashSet<>();
set.contains(5); // O(1) hash-based lookup
Map<Integer, String> map = new HashMap<>();
map.containsKey(5); // O(1) hash-based key search
// Sets and Maps outperform Lists for search operations
- Tip: Prefer HashSet over ArrayList when frequent contains() checks are needed
- Real Use: User session tracking (fast lookup) vs Transaction history (sequential access)
5. Thread-Safe Alternatives
List<String> syncList = Collections.synchronizedList(new ArrayList<>());
Set<String> concurrentSet = ConcurrentHashMap.newKeySet();
Map<String, Integer> concurrentMap = new ConcurrentHashMap<>();
// Safe for multi-threaded environments
// Standard implementations aren't thread-safe by default
Set<String> concurrentSet = ConcurrentHashMap.newKeySet();
Map<String, Integer> concurrentMap = new ConcurrentHashMap<>();
// Safe for multi-threaded environments
// Standard implementations aren't thread-safe by default
- Tip: Use CopyOnWriteArrayList for read-heavy concurrent scenarios
- Real Use: Real-time stock tickers (high concurrency)
💡 Pro Interview Tip
When asked about collections, always mention: 1) Order guarantees 2) Null handling 3) Thread safety 4) Time complexities. Example: "HashMap offers O(1) lookups but isn't thread-safe, whereas Hashtable is synchronized but slower."
