a question about inheritance

[kevin]

Hi all:

look this code:

// code begin
public void actionPerformed(ActionEvent e)
{
Object obj = e.getSource();
if (obj == ok); // ok is declared as JButton
// do something
if (obj == cancel); // cancel is declared as JButton
// do something
.......
}

// code ends

My question is why the expression obj == ok and obj == cancel can work
because
they are two different type class, can different class compared with
"==" and why?



thanks!

kevin

 

[Joona I Palaste]

Hi all:

look this code:

// code begin
public void actionPerformed(ActionEvent e)
{
Object obj = e.getSource();
if (obj == ok); // ok is declared as JButton
// do something
if (obj == cancel); // cancel is declared as JButton
// do something
......
}

// code ends

My question is why the expression obj == ok and obj == cancel can work
because
they are two different type class, can different class compared with
"==" and why?

Yes, different classes can be compared, as long as one of them is a
subclass of the other. In your code, one of the classes is Object, and
every other class is a subclass of Object, so it always works.
The reason for this is polymorphism, and the Java type system. Note that
your variables don't actually contain the objects - they contain
references to them. While an object always has the same class, it can
have references of various class types referring to it. The class
itself, any of its superclasses, or any interface it implements.

 

[Ann]

Hi all:

look this code:

// code begin
public void actionPerformed(ActionEvent e)
{
Object obj = e.getSource();
if (obj == ok); // ok is declared as JButton
// do something
if (obj == cancel); // cancel is declared as JButton
// do something
......
}

// code ends

My question is why the expression obj == ok and obj == cancel can work
because
they are two different type class, can different class compared with
"==" and why?

obj is a reference, ok is a reference, cancel is a reference
the jvm determines what a reference looks like, but if two
different references refer to the same object then == yields true