Casting, instanceof & pattern matching

Moving up and down the hierarchy

Upcasting — assigning a subclass reference to a superclass type — is implicit and always safe:

Animal a = new Dog("Rex", "Lab");  // no cast needed

Downcasting — assigning a superclass reference to a subclass type — requires an explicit cast and can throw ClassCastException at runtime if the object is not actually of the target type:

Animal a = new Dog("Rex", "Lab");
Dog d = (Dog) a;          // safe: a actually holds a Dog

Animal a2 = new Cat("Whiskers");
Dog d2 = (Dog) a2;        // compiles, but throws ClassCastException at runtime

instanceof — guard before casting:

if (a instanceof Dog) {
    Dog d = (Dog) a;
    d.fetch();
}

Java 16+ pattern matching collapses the guard and cast into one:

if (a instanceof Dog d) {   // d is in scope only inside the if-body
    d.fetch();              // no explicit cast needed
}

The pattern variable d is created only when the check passes. This removes an entire class of ClassCastException bugs.