Java、后缀表示法

package collection;
 
import java.util.*;
 
public class Exercise20_14 {
    public static void main(String[] args) {
        System.out.print("Enter a postfix expression: ");
        String expression = new Scanner(System.in).nextLine();
 
        System.out.println("expression = " + format(evaluateExpression(expression)));
    }
 
    /** 计算后缀表达式 */
    public static double evaluateExpression(String expression) {
        Stack stack = new Stack<>();    //储值栈
        String[] tokens = expression.split(" ");    //返回表达式按空格分隔数组
        Set set = new HashSet<>(Arrays.asList("+", "-", "*", "/", "^", "%"));
 
        for (String op: tokens) {
            try {   //栈添加数字
                stack.push(Double.parseDouble(op));
            } catch (NumberFormatException ex) {
            }
 
            if (set.contains(op)) { //遇到运算符
                double op1 = stack.pop();   //弹出数字
                double op2 = stack.pop();
 
                switch (op) {   //进行相应操作
                    case "+": stack.push(op2 + op1); break;
                    case "-": stack.push(op2 - op1); break;
                    case "*": stack.push(op2 * op1); break;
                    case "/": stack.push(op2 / op1); break;
                    case "^": stack.push(Math.pow(op2, op1)); break;
                    case "%": stack.push(op2 % op1); break;
                    default:
                }
            }
        }
        return stack.pop();
    }
 
    /** 格式化值 */
    public static String format(double value) {
        String v = String.valueOf(value);
        return v.endsWith("0") ? String.valueOf((int)value) : v;
    }
}