后缀评估计算结果错误

问：

我正在尝试使用堆栈实现后缀评估，但是计算后的结果不正确，我无法弄清楚是哪个部分的计算错误，非常感谢。

import java.util.Stack;

public class PostFixEvaluate {
    public static float calculate(Float operand_1,char operator,Float operand_2 ){
        switch(operator){
            case '+':
                return operand_2 + operand_1;

            case '-':
                return operand_2 - operand_1;

            case '*':
                return operand_2 * operand_1;

            case '/':
                return operand_2 / operand_1;
        }

        return 0;
    }   //end calculate()

    public static float postFixEvaluation(String expr){
        Stack<Float> stack = new Stack<>();
        int index = 0;

        while(index < expr.length()){
            char token = expr.charAt(index);
            boolean x = isOperand(token);

            if(x == true){     //operand
                float operandIn = (float)token;
                stack.push(operandIn);
            }
            else{           //operator
                float a = stack.pop();   //operand_1
                float b = stack.pop();   //operand_2
                char w = token;         //operator
                stack.push(calculate(a, w, b));
            }
            index += 1;
        }   //end while

        float result = stack.pop();
        return result;
     }  //postFixEvaluation()

    /*Operand or operator? operator return 1, operand return 2*/
     public static boolean isOperand(char c){
        switch(c){
            case '+':
            case '-':
            case '*':
            case '/':
                return false;   //an operator
            default:
                return true;   //an operand
        }   //end switch()
    }  //end isOperand()
}

输入后缀“312*+456*+97-/+”后，结果是3958.0，预计是22.0，这比错误多得多，但我仍然无法弄清楚为什么结果是3958.0。