Printing a stack

Question

ok i've been working on a program for a couple of weeks and all I have left to do is print the stack. I'm pushing Grids as 2D arrays onto the stack and that part is working correctly (used the peek method) but when I try to print the entire stack it prints the last Grid the length of the stack. My code is:
while(!solution.empty())
temp= (char [][])solution.pop();
System.out.println();
for(int r=0; r<7; r++)
System.out.println();
for(int c=0; c<7; c++)
System.out.print(temp[r][c]+" ");
Can anyone see a problem with this to make it just print the same grid?

Answer 1

Or looked at differently
when you take your char[][] foo and push it onto the stack then you modify foo, you are simultaneously modifying the one that is out on the stack because all the stack has on it, is a pointer back to the foo object.
Eventually you have a dozen pointers in the stack to foo. They all point to exactly the same single one copy that you ever had. So no surprise, when you change one you change them all. They are all the same.
If you want a copy, you must create a new char[][] object, and use a couple nested for loops to copy each char out of the original into the new object. Now you can push the copy and modify the original without screwing up the one on the stack.
BTW if you ever hear anyone use the phrase "pointer semantics" as in "objects in Java (and every other OOL) have pointer semantics" what they mean is that unless you are careful it is very easy to make exactly the mistake that you just made.
I would estimate that there are exactly zero good programmers that did not learn about this paritcular blunder by making this same mistake and then wondering for hours if they hadn't uncovered some bug in the library or the compiler because, "There is NOTHING wrong with my code!"
Enjoy!