What does Java object assignment mean?

Question

I have the following 2 classes :

class Animal {
    public static void staticMethod(int i) {
        System.out.println("Animal : static -- " + i);
    }

    public void instanceMethod(int i) {
        System.out.println("Animal : instance -- " + i);
    }
}

class Cat extends Animal {
    public static void staticMethod(int i) {
        System.out.println("Cat : static -- " + i);
    }

    public void instanceMethod(int i) {
        System.out.println("Cat : instance -- " + i);
    }

    public static void main(String[] args) {
        Cat myCat = new Cat();
        myCat.staticMethod(1);                       // Cat : static -- 1
        myCat.instanceMethod(2);                     // Cat : instance -- 2
        System.out.println("");

        Animal myAnimal = myCat;
        Animal.staticMethod(3);                      // Animal : static -- 3 
        myAnimal.staticMethod(4);                    // Animal : static -- 4 [ ? ]
        System.out.println("");

        myAnimal.instanceMethod(5);                  // Cat : instance -- 5
    }
} 

And when I run Cat, I got the following results :

Cat : static -- 1
Cat : instance -- 2

Animal : static -- 3
Animal : static -- 4

Cat : instance -- 5

I can understand 1,2,3 and 5, but why #4 is not : " Cat : static -- 4 " ? My understanding would be like this :

myAnimal=myCat means "myAnimal" is now exactly the same as "myCat", so anywhere "myAnimal" apears, you can replace it with "myCat" and get the same result, because everything inside myAnimal is the same as everything inside myCat, therefore "myAnimal.staticMethod(4)" should be the same as "myCat.staticMethod(4)" and the output should be : "Cat : static -- 4", similiar to "myCat.staticMethod(1)" above.

But that doesn't seem to be the case, why ?

Answer 1

You declare myAnimal as Animal. Therefore a static method is called from that class too.

You should never call static methods (or access static fields) from an instance to prevent this kind of confusion.

Answer 2

From Oracle docs:

8.4.8.2. Hiding (by Class Methods)

If a class C declares or inherits a static method m, then m is said to hide any method m', where the signature of m is a subsignature (§8.4.2) of the signature of m', in the superclasses and superinterfaces of C that would otherwise be accessible to code in C.

Example 8.4.8.2-1. Invocation of Hidden Class Methods

A class (static) method that is hidden can be invoked by using a reference whose type is the class that actually contains the declaration of the method. In this respect, hiding of static methods is different from overriding of instance methods. The example:

class Super {
            static String greeting() { return "Goodnight"; }
            String name() { return "Richard"; }
        }
        class Sub extends Super {
            static String greeting() { return "Hello"; }
            String name() { return "Dick"; }
        }
        class Test {
            public static void main(String[] args) {
                Super s = new Sub();
                System.out.println(s.greeting() + ", " + s.name());
            }
        }

produces the output:

Goodnight, Dick

because the invocation of greeting uses the type of s, namely Super, to figure out, at compile time, which class method to invoke, whereas the invocation of name uses the class of s, namely Sub, to figure out, at run time, which instance method to invoke.