Object Class, equals & hashCode

java.lang.Object is the implicit superclass of every Java class. It provides toString(), equals(), hashCode(), getClass(), clone(), wait(), notify(), and finalize().

Object is the root class of all Java classes. Objects (java.util.Objects) is a utility class with null-safe static helpers: equals, hash, requireNonNull, toString.

If a.equals(b) is true, then a.hashCode() must equal b.hashCode(). Breaking this contract makes objects unfindable in HashMap and HashSet.

HashMap and HashSet break silently. Equal objects land in different buckets because their hash codes differ, so lookups return null and contains returns false.

It is legal but makes HashMap O(n) instead of O(1) because every object lands in the same bucket. A good hashCode distributes objects evenly.

Override equals() and hashCode() using the same fields, and make the key immutable. If the hash-relevant fields change after insertion, the object becomes unfindable.

Use Objects.equals(a, b) for null-safe comparison. It handles null on both sides without throwing NullPointerException.

Senior developers prevent nulls at the boundary instead of checking them everywhere. Use Optional for return types, requireNonNull for parameters, and empty collections instead of null.

Arrays do not override equals() or toString(). Use Arrays.equals() for content comparison and Arrays.toString() for readable output.