synopsis · Java for DSA · chapter 3

OOP for DSA & Design Rounds

stagechapter

9 cards, about a minute. Read them the night before an interview, or right after the chapter to lock it in.

what it is
A class is the blueprint, an object one instance on the heap. Instance fields: one copy per object. static fields: one copy per class.
template
private static class Node { int val; Node next; Node(int v) { val = v; } } inside your class: no hidden outer reference.
use it when
"Design / Implement the X class": private fields for state, a constructor, one method per operation that checks, updates, returns.
cost
HashMap lookups are O(1) only if hashCode spreads the keys. A constant hashCode puts every key in one bucket: up to O(n) per lookup.
trap
Override equals without hashCode and HashSet/HashMap stop finding equal keys. Override both, from the same fields.
trap
static counters in a design class are shared by every object the judge creates. Per-object state = instance fields.
pro tip
Pairs, cells, composite keys: record Cell(int r, int c) {} gives equals, hashCode and toString for free (Java 16+).
pro tip
A fixed set of options (directions, states) → an enum. == is safe on enum constants; cache values().
what it is
Interfaces say what (Deque, Map, Comparable), classes say how. Declare by interface: Deque<Integer> st = new ArrayDeque<>();