Parentheses.java

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

/******************************************************************************
 *  Compilation:  javac Parentheses.java
 *  Execution:    java Parentheses < file.txt
 *  Dependencies: StdIn.java Stack.java
 *
 *  Reads in a text file and checks to see if the paretheses, curly
 *  braces, and square brackets are balanced.
 *
 *  % java  java Parentheses
 *  [()]{}{[()()]()}
 *  true
 *
 *  % java Parentheses
 *  [(])
 *  false
 *
 ******************************************************************************/

public class Parentheses {
    private static final char L_PAREN    = '(';
    private static final char R_PAREN    = ')';
    private static final char L_BRACE    = '{';
    private static final char R_BRACE    = '}';
    private static final char L_BRACKET  = '[';
    private static final char R_BRACKET  = ']';

    public static boolean isBalanced(String s) {
        Stack<Character> stack = new Stack<Character>();
        for (int i = 0; i < s.length(); i++) {

            if      (s.charAt(i) == L_PAREN)   stack.push(L_PAREN);

            else if (s.charAt(i) == L_BRACE)   stack.push(L_BRACE);

            else if (s.charAt(i) == L_BRACKET) stack.push(L_BRACKET);

            else if (s.charAt(i) == R_PAREN) {
                if (stack.isEmpty())        return false;
                if (stack.pop() != L_PAREN) return false;
            }

            else if (s.charAt(i) == R_BRACE) {
                if (stack.isEmpty())        return false;
                if (stack.pop() != L_BRACE) return false;
            }

            else if (s.charAt(i) == R_BRACKET) {
                if (stack.isEmpty())        return false;
                if (stack.pop() != L_BRACKET) return false;
            }

            // ignore all other characters

        }
        return stack.isEmpty();
    }


    public static void main(String[] args) {
        String s = StdIn.readAll();
        StdOut.println(isBalanced(s));
    }

}