Doubt in run time polymorphism

[Thennarasu]

Could You explain the program flow and output?

package com;

class Alpha {
public void foo(String... args) {
System.out.println("Alpha:foo");
}

public void bar(String a) {
System.out.println("Alpha:bar");
}
}

public class RegExCheck extends Alpha {
public void foo(String a) {
System.out.println("Beta:foo");
}

public void bar(String a) {
System.out.println("Beta:bar");
}

public static void main(String[] args) {
Alpha a = new RegExCheck();
RegExCheck b = (RegExCheck) a;
a.foo("test"); b.foo("test");
a.bar("test"); b.bar("test");
}
}

[Tolls]

Could You explain the program flow and output?

How about you tell us:

a. what you get.
b. what you expect.
c. why you expected b.

otherwise we might simply be doing your homework for you.

[Thennarasu]

1) I get O/P as Alpha:foo Beta:foo Alpha:bar Beta:bar
2) I expected it to be Beta:foo Beta:foo Alpha:bar Beta:bar
2) I expected (2) because object a is created of the reference type Beta and hence at run time it should call method Beta class.

[Tolls]

1) I get O/P as Alpha:foo Beta:foo Alpha:bar Beta:bar

You do? That's not either what I expect, or what I get. Did you write the code correctly, or was bar() supposed to be static?

2) I expected it to be Beta:foo Beta:foo Alpha:bar Beta:bar

Well, as you suspect, that's not correct either.

2) I expected (2) because object a is created of the reference type Beta and hence at run time it should call method Beta class.

The runtime looks for the foo() function that matches a.foo(), which takes a String. It finds one called foo(String...). It then looks to see if it's overridden by the subclass, and doesn't find a child version of foo(String...), so it sticks with the Alpha method it has found.

At least, that's the gist of its decision making.

[Thennarasu]

Thanks Tolls