Re: when using Class type as method parameters, I have a question...
www wrote:
Hi,
I thought I have understood this issue without any problems already.
When using class type parameters for a method, the behavior is kind of
like pass-by-reference. The object can be modified by the code inside
the method, right? But today, I ran into an interesting problem: the
object cannot get the change it wants.
Suppose I have a class called Species. I have a method
//oldSpecies is an object of Species, its content has been set already.
//newSpecies is an object of Species, but its content has not been set
yet. i.e. its content all has been set by the default values. The
purpose of this method is to set newSpecies content based on different
condition.
public static void convertOldSpeciesToNewSpecies(Species oldSpecies,
Species newSpecies)
{
if(..) //BLOCK A
{
newSpecies = oldOne; //just assign it. Here is the problem! The
caller, the second parameter, does not get the value
}
else //BLOCK B
{
//in this case, newSpecies get assigned values. No problem, the
caller gets the value.
newSpecies.setXXX
newSpecies.setYYY
}
//print out newSpecies content
System.out.println(newSpecies.toString());
}
Helper.convertOldSpeciesToNewSpecies(speciesOne, speciesTwo);
Above calling, if the condition fits BLOCK B, result is correct
(speciesTwo gets the values it should get from inside the method).
But if the condition fits BLOCK A, speciesTwo does NOT get the content
of speciesOne. The printing inside the method shows that newSpecies has
got the correct content (same as speciesOne).
What is the problem? Thank you very much.
For my practical reason, I do not want to change the method to
public Species getNewSpecies(Species oldSpecies){..}
I think I see what you are asking although your example is not very
clear, you are using multiple names (newSpecies, oldSpecies, speciesOne,
speciesTwo, oldOne and what have you) which confuses things a bit.
Arguments to methods are passed call by value in Java, it just so
happens that when you pass a reference in, the value is a memory
address. In the method body, copies of the values are used so you will
have a copy of (say) the memory address of newSpecies and a copy of the
memory address of oldSpecies.
Now to begin with, these copies still point to the original Objects so
you can modify those objects directly by calling their methods, however
if you assign one reference to another all you are doing is modifying
the copies not the values (memory addresses) of the arguments.
Write some more code to assign a value to a modified local copy and see
if you can figure out what's going on, stick with it, you will get there
in the end.