Polymorphism

Overloading means same name with different parameters, resolved at compile time. Overriding means same name with the same parameters in a child class, resolved at runtime. Overloading changes the form; overriding changes the behavior.

Override requires same name and params in the child. Return: same or covariant. Access: same or wider. Checked exceptions: same or narrower. Cannot override final, static, or private. Always use @Override.

Widen OK (protected to public). Narrow NOT OK (public to private). Private is not inherited. Static is hidden. Final is sealed. The Liskov Substitution Principle requires the child to be at least as accessible as the parent.

Polymorphism means one call, many behaviors. Compile-time: overloading (compiler picks by args). Runtime: overriding (JVM picks by actual type). It eliminates if-else chains, enables the Open/Closed Principle, and powers the Strategy pattern.

Covariant return means the overriding method returns a subtype of the parent's return type. Parent returns Account, child can return SavingsAccount. Avoids casting. Cleaner API. Added in Java 5.

Overriding (instance): runtime dispatch based on actual object type. Hiding (static): compile-time binding based on reference type. Overriding is polymorphism. Hiding is not polymorphism.

Static binding resolves at compile time using the reference type, for private, static, final, and overloaded methods. Dynamic binding resolves at runtime using the actual object type via the vtable, for overridden instance methods.