Postfix evaluation using a stack
Conventions

According to coding conventions, Java class names should start with an uppercase letter. myStack should be MyStack.

Variable names

Whenever you have a comment after declaring a variable, rename the variable to comment itself.

int n1;     //result of 1st popping


Can be renamed to make the variable name "comment itself".

int poppingResult1;


If this, then do nothing, else do something

if (ch == ' ') {
} else {


To achieve the same effect, and make the code cleaner, write instead:

if (ch != ' ') {


Integer.parseInt

Now, it's not clear which datatype you get from list.pop(). If it is an int (or Integer), then variable = list.pop() will be sufficient.

If it is an Object, instead use Integer.parseInt(String.valueOf(list.pop()))

Anyway, if possible, change the values you store in your list to always be of the same type (Integer, preferably). This would make your code cleaner as you don't have to use Integer.parseInt every time you pop something of the list.