Using enums to avoid using switch/if

Hi, I want to avoid using if cases or switch in the following piece of
code:
 package calculator;

public class Operations {

    public double calculateResult() {
        double totalVal = Calculator.calculator.prevValue;
        double val2 = Double.parseDouble
(Calculator.calculator.displayField.getText());

        if (Calculator.calculator.operatorUsed.equals("+")) {
            totalVal += val2;
        }
        if (Calculator.calculator.operatorUsed.equals("-")) {
            totalVal -= val2;
        }
        if (Calculator.calculator.operatorUsed.equals("/")) {
            totalVal /= val2;
        }
        if (Calculator.calculator.operatorUsed.equals("*")) {
            totalVal *= val2;
        }
        if (Calculator.calculator.operatorUsed.equals("=")) {
            totalVal = val2;
        }
        return totalVal;
    }
}

I read in a java tutorial that you can use enums to get rid of this :

 public enum Operation {

        PLUS {

            double eval(double x, double y) {
                return x + y;
            }
        },
        MINUS {

            double eval(double x, double y) {
                return x - y;
            }
        },
        TIMES {

            double eval(double x, double y) {
                return x * y;
            }
        },
        DIVIDE {

            double eval(double x, double y) {
                return x / y;
            }
        };

        abstract double eval(double x, double y);
    }

But here's my problem: In my program I've strings ("+" , "-", "/", "="
etc) instead of enums. I need a mechanism to map string to enum.