Carlo Cruz-Albrecht

Casting: compiler and runtime errors

Both compiler errors and runtime errors can occur in regards to casting.

(A) var

  1. The compiler will allow you to cast var to A as long as A is a superclass or subclass of the static type of var. Even though we don’t get compile errors in this case, we still could get a runtime error.

  2. If A is not a super/sub class of the static type of var, then the compiler knows something is wrong and we get a compiler error.

So, for example:

Fish fishy = new Fish();
Dog d = new Dog();

((Salmon) fishy).swim();  // cast allowed, but gives
                          // runtime error (ClassCastException)
((Dog) fishy).bark();  // compiler error (incompatible types)

More examples:

public class CastingDemo {
  static class Fish { 
    public void swim() {}
  static class Salmon extends Fish {
    public void sally() {}
  static class Cod extends Fish {
    public void coddy() {}
  public static void main(String[] args) {
    Fish fishy = new Fish();
    Salmon sal = new Salmon();
    Fish bob = new Salmon();

    // CASTING: *temporarily* change the static type of a variable. 
    // Long story short: you can only cast to:
    // the dynamic type  OR  a superclass of the dynamic type.
((Fish) sal).swim();

sal has dynamic type Salmon. Since Fish is a supercass of Salmon, this line is fine. You can cast to a superclass of the dynamic type but normally no reason too.

((Fish) sal).sally();

COMPILE ERROR: this errors b/c the cast gives ((Fish) sal) static type of Fish, which doesn’t have the sally method

((Salmon) sal).sally();

no error but does nothing b/c the static type of sal is already Salmon


COMPILE ERROR: this errors b/c the static type of bob (Fish) does not have the sally method

((Salmon) bob).sally();

bob has dynamic type Salmon. Here the cast changes its static type to Salmon so we can run the sally method now

((Salmon) fishy).sally();

RUNTIME ERROR: static type of fishy is Fish. Salmon is a subclass of Fish so the compiler allows the cast w/o error. But, when we try to run the program we get a ClassCastException b/c fishy’s dynamic type is Fish! (not Salmon). We can only cast to the dynamic type or a superclass of the dynamic type.

((Cod) sal).swim();

COMPILE ERROR: incompatible types. sal has static type Salmon. Since Cod is not a super/sub class of Salmon, the compiler knows this is wrong