public static void main(String[] args) {
        String infix = "3 > 4 && 4 <= 8";
        System.out.printf("infix:   %s%n", infix);
        System.out.printf("postfix: %s%n", transformToRPN(infix));

        test("3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3", "3 4 2 * 1 5 - 2 3 ^ ^ / + ");
    }

    private static void test(String expected, String infix) {
        if (expected.equals(transformToRPN(infix)))
            System.out.println("test PASSED: " + infix);
        else
            System.out.println("test FAILED: " + infix);
    }