Q. Can you explain the result of the following example? Oh, my! 

class Base { 
  public boolean foo(Base b) { 
    return true;     
  } 
} 
class Sub extends Base { 
  public boolean foo(Sub s) { 
    return false; 
  } 
} 

public class Test { 
  public static void main(String argv[]) { 
    Base bb = new Base(); 
    Base bs = new Sub(); 
    Sub  ss = new Sub(); 

    System.out.println(bb.foo(bb)); //true 

    System.out.println(bs.foo(bs)); //true ??? 
    System.out.println(bs.foo(ss)); //true ???
    System.out.println(bs.foo(bb)); //true ???
    System.out.println(ss.foo(bs)); //true ???
    System.out.println(ss.foo(bb)); //true ???

    System.out.println(ss.foo(ss)); //false 
  } 
}
A: The foo methods are overloaded in the Sub. In Base, there is only one foo method, and it is not overridden by the Sub!
Overloading is fundamentally different then overriding. There is no polymorphism or dynamic binding here!!! 
All decisions are made at compile time!!! See detailed explanation in the same code below, documented! 

class Base { 
  // There is only one foo method in Base!!!
  public boolean foo(Base b) { 
    return true;     
  } 
} 
class Sub extends Base { 
  // differnt signature, method overloading
  // there are 2 foo methods in the Sub
  public boolean foo(Sub s) { 
    return false; 
  } 
} 

public class Test { 
  public static void main(String argv[]) { 
    // bb is a Base ref to the compiler, Base obj at runtime
    Base bb = new Base(); 
    // bs is a Base ref to the compiler, Sub obj at runtime
    Base bs = new Sub();  
    // ss is a Sub ref to the compiler, Sub obj at runtime
    Sub  ss = new Sub();  
          
    // All these 4 lines are Base ref 
    // call Base.foo(Base) method, and only one foo available
    // bs is a Base ref, and ss ISA Base ref too
    // Everything is fine!!!
    System.out.println(bb.foo(bb)); //true 
    System.out.println(bs.foo(bs)); //true 
    System.out.println(bs.foo(ss)); //true 
    System.out.println(bs.foo(bb)); //true 
          
    // bb, bs are both Base refs to Compiler
    // ss is a Sub ref
    // call Sub.foo(Base) which is inherited from Base
    System.out.println(ss.foo(bs)); //true 
    System.out.println(ss.foo(bb)); //true 
          
    // no doubt about this one, am I right?
    System.out.println(ss.foo(ss)); //false 
  } 
}