Interfaces

[Vamshi Vrukodar]

Hi,
I am new to this group. I am preparing for the programmers certification exam. I had a doubt regarding Interfaces. If u run the following program,,,
interface A {
String s1 = "A";
String m1();
}
interface B extends A {
String s1 = "B";
String m1();
}
class InterfaceExample implements B {
public String m1() {return s1;}
public static void main(String[] args) {
A a = new InterfaceExample();
System.out.print(a.m1());
}
}
The output is B. But I thought, since a is of type Interface A, the string returned will be that declared in Interface A. Can anyone please give an explanation for the output.
Thank you
Vamshi

[Yi Meng]

a.m1() will invoke the m1() method defined in InterfaceExample and return the s1 that class InterfaceExample sees.

[Yi Meng]

try out the code below and you will see the difference:

  interface A {
String s1 = "A";
String m1(); 
}
interface B extends A {
String s1 = "B";
String m1(); 
}
class InterfaceExample implements B {
public String m1() {return s1;}
public static void main(String[] args) {
A a = new InterfaceExample();
System.out.print(a.m1());
System.out.print(a.s1);
}
}

It will print BA

[Vamshi Vrukodar]

hi Yi Meng,
Thank you...I understood the concept.
Regards

[Saket Barve]

Hi Yi,

a.m1() will invoke the m1() method defined in InterfaceExample and return the s1 that class InterfaceExample sees.

I'm not clear yet. If the InterfaceExample sees "s1" of interface B, why would it print the value of s1 in interface A for the
System.out.println( a.s1 )?
Saket

[Kaz Yosh]

because when you look up the variables, object is looking for the variable with the same type. In this case, object type is A and actual object is interfaceexample. So the variable you are looking for is type A's variable which is "A" in interface A.

[Yi Meng]

hi Saket Barve,
Actually at compile time, the compiler will only look for methods/variables of the reference type class/interface. e.g. if the method m1 is not in interface A, it will simply report a compiler error despite that the method m1 is in the class InterfaceExample.
a.m1() and a.s1 all make references to those in interface A, but for the method, dynamic binding is applied as the method m1 is properly overriden. The actual method binded will depend on the actual object rather than the reference type.
[ May 26, 2003: Message edited by: Yi Meng ]