You need to push the left parenthesis onto the stack, and process the stack like so when you encounter a right parenthesis:
// opening (
if (in_fix.peek().type == 4) {
post_fix.push(in_fix.pop());
}
//closing )
if(in_fix.peek().type == 5){
while(!(post_fix.isEmpty() || post_fix.peek().type == 4)){
postfixstr.append(post_fix.pop());
}
if (post_fix.isEmpty())
; // ERROR - unmatched )
else
post_fix.pop(); // pop the (
in_fix.pop(); // pop the )
}