Object Class Methods — toString, hashCode, equals, clone, finalize

Every Java class inherits from Object. The five methods interviewers focus on are toString(), equals(), hashCode(), clone(), and finalize().

equals/hashCode — the contract that causes production bugs

• Override equals() without hashCode() and HashMap silently loses data: put() stores in one bucket, get() with a logically equal key looks in a different bucket, returns null
• Fix: always override both together using the same fields; Objects.hash(name, age) is the standard pattern
• Use a record when possible — it generates both correctly from all components automatically

toString()

• Override it so logs show Person{name=Alice, age=30} instead of Person@1a2b3c
• Lombok @ToString or records handle this automatically

clone() and finalize() — avoid both

• clone() bypasses constructors, returns Object requiring a cast, and shares mutable internals; use copy constructors
• finalize() was deprecated in Java 9 and removed in Java 18 — unpredictable GC timing, potential object resurrection; use try-with-resources for cleanup

I recommend using Java records for any class used as a map key — they generate correct equals() and hashCode() automatically, eliminating the most common source of silent data loss.