EvaluatePostfix.java

Below is the syntax highlighted version of EvaluatePostfix.java from §4.3 Stacks and Queues.

/******************************************************************************
 *  Compilation:  javac EvaluatePostfix.java
 *  Execution:    java EvaluatePostfix < file.txt
 *  Dependencies: Stack.java StdIn.java
 *
 *  Evaluates postfix expresions using a stack.
 *
 *  Windows users: replace <Ctrl-d> with <Ctrl-z> so signify end of file.
 *
 *  % java EvaluatePostfix
 *  3 4 5 + *
 *  <Ctrl-d>
 *  27
 *
 *  % java EvaluatePostfix
 *  1 2 3 4 5 * + 6 * * +
 *  <Ctrl-d>
 *  277
 *
 *  % java EvaluatePostfix
 *  7 16 16 16 * * * 5 16 16 * * 3 16 * 1 + + +
 *  <Ctrl-d>
 *  30001
 *
 *  % java EvaluatePostfix
 *  7 16 * 5 + 16 * 3 + 16 * 1 +
 *  <Ctrl-d>
 *  30001
 *
 *  Known bugs
 *  ----------
 *  - No error checking - assumes input is legal postfix expression.
 *  - All token must be separated by whitespace, e.g., 1 5+ is illegal.
 *
 ******************************************************************************/

public class EvaluatePostfix {
    public static void main(String[] args) {
        Stack<Integer> stack = new Stack<Integer>();

        while (!StdIn.isEmpty()) {
            String s = StdIn.readString();
            if      (s.equals("+")) stack.push(stack.pop() + stack.pop());
            else if (s.equals("*")) stack.push(stack.pop() * stack.pop());
            else stack.push(Integer.parseInt(s));
        }
        StdOut.println(stack.pop());
    }
}