Java Interview Questions and Answers

• ArrayList: array-backed, O(1) random access, amortised O(1) append, O(n) insertion/removal in the middle. Best default for ordered lists.
• LinkedList: doubly linked, O(1) insertion/removal at the ends or given a node, O(n) random access and poor cache locality. Rarely the best choice in practice; ArrayDeque is preferred for queues.
• HashMap: hash table with average O(1) get/put, no ordering guarantee, one null key allowed. Collisions are handled with buckets that treeify to red-black trees past a threshold. Use LinkedHashMap for insertion order and TreeMap for sorted keys.

Other essentials: HashSet is a HashMap key set; ConcurrentHashMap for thread-safe maps; choose initial capacity and load factor to avoid rehashing.

Rules:

• If a.equals(b) is true then a.hashCode() == b.hashCode() must be true.
• If two objects are unequal, their hash codes may still collide (allowed).
• equals must be reflexive, symmetric, transitive and consistent.
• Override both together, or hash-based collections (HashMap, HashSet) misbehave.

@Override
public boolean equals(Object o) {
  if (this == o) return true;
  if (!(o instanceof User u)) return false;   // pattern matching (Java 16+)
  return id == u.id && Objects.equals(email, u.email);
}

@Override
public int hashCode() { return Objects.hash(id, email); }

Mutable fields used in equals/hashCode are a classic bug: mutating them after insertion corrupts the collection.

Dependency injection means a component receives its collaborators from outside rather than creating them itself. The Spring container builds and wires the object graph from annotations such as @Component, @Service and @Repository, injecting through constructor, setter or field.

@Service
public class OrderService {
  private final PaymentGateway gateway;
  public OrderService(PaymentGateway gateway) { this.gateway = gateway; }
}

Benefits: loose coupling, easier testing with mocks, centralised configuration and lifecycle management. Constructor injection is preferred because dependencies are explicit, objects can be immutable and missing beans fail fast at startup. Spring Boot adds auto-configuration and a starter-based dependency model.