Java OCA OCP Practice Question 732


What will the following code print when compiled and run?

class Base{ //from  ww  w.  j a v a 2s. co  m
   void methodA(){ 
      System.out.println("base - MethodA"); 

class Main extends Base{ 
   public void methodA(){ 
      System.out.println("sub - MethodA"); 
   public void methodB(){ 
      System.out.println("sub - MethodB"); 
   public static void main (String args []){ 
      Base b=new Main(); //1 
      b.methodA (); //2 
      b.methodB (); //3 

Select 1 option

A. sub - MethodA and sub - MethodB 
B. base - MethodA and sub - MethodB 
C. Compile time error at  //1 
D. Compile time error at  //2 
E. Compile time error at  //3 

Correct Option is  : E


The point to understand here is, b is declared to be a reference of class Base and methodB() is not defined in Base.

So the compiler cannot accept the statement b.methodB() because it only verifies the validity of a call by looking at the declared class of the reference.

For example, the compiler is able to verify that b.methodA() is a valid call because class Base has method methodA. But it does not "bind" the call.

Call binding is done at runtime by the JVM and the JVM looks for the actual class of object referenced by the variable before invoking the method.