java - call subclass methods by superclass reference

Question

I have code like this:

String stringRef = new String("Java");                    // (1)

System.out.println("(2): " + stringRef.getClass());       // (2)
System.out.println("(3): " + stringRef.length());         // (3)

Object objRef = stringRef;                                // (4)

// System.out.println("(5): " + objRef.length());            // (5) Not OK.
System.out.println("(6): " + objRef.equals("Java")); 

Why can I not call length() in line (5); and for which class will equals() be called in line (6)?

Answer 1

equals is declared on Object so you can call it; length isn't. The compiler will not try to determine what exactly object type is referred to by objRef; it works only with the knowledge that it is an Object.

The Java compiler statically resolves the signature of the method to call, based on the static type of the expression to the left of the dot operator. The concept of polymorphism and dynamic dispatch applies only to the resolution of which overriding method to call.