Java Casting

Introduced in 2026-03-12-object-oriented-programming-ii (Lecture, Week 3). See also java-primitive-and-reference-types.

Casting converts a variable from one type to another.

Primitive casting

• Widening — converting a smaller type to a larger one: byte -> short -> char -> int -> long -> float -> double.
• Narrowing — converting a larger type to a smaller one: double -> float -> long -> int -> char -> short -> byte.

Reference casting

• Upcasting (implicit) — casting a subclass reference to a superclass reference.
• Downcasting (explicit) — casting a superclass reference back to a subclass reference.

Given Object -> Animal -> {Mammal -> {Dog, Cat}, Bird -> Chicken}, upcasting moves up this hierarchy, downcasting moves down it.

Upcasting

Always safe, done implicitly:

public class Animal {
    public void makeSound() { System.out.println("Animal makes a sound"); }
}
public class Cat extends Animal {
    @Override public void makeSound() { System.out.println("Cat meows"); }
}

Animal myAnimal = new Cat(); // upcasting: Cat treated as an Animal
myAnimal.makeSound();        // "Cat meows"

Downcasting

Must check the object is actually an instance of the target subclass with instanceof first, since not every Animal is a Cat:

public class Cat extends Animal {
    @Override public void makeSound() { System.out.println("Cat meows"); }
    public void climb() { System.out.println("I can climb trees"); }
}

Animal myAnimal = new Cat(); // upcasting
if (myAnimal instanceof Cat) {
    Cat myCat = (Cat) myAnimal; // downcasting
    myCat.climb();              // access methods specific to Cat
}

Every class implicitly extends Object

A class with no explicit extends clause (e.g. a plain public class Student { ... }) still has a parent: Object, the root of the Java class hierarchy.