用JAVA程序写个计算器,实现加,减,乘,除

Calculator.java

public class Calculator{
private String result = "0";
private int op = 0,add = 1,sub = 2,mul = 3,div = 4;

private double stringToDouble(String x){
double y = Double.parseDouble(x);
return y;
}
private void operate(String x){
double x1 = stringToDouble(x);
double y = stringToDouble(result);
switch (op){
case 0:
result = x;
break;
case 1:
result = String.valueOf(y+x1);
break;
case 2:
result = String.valueOf(y-x1);
break;
case 3:
result = String.valueOf(y*x1);
break;
case 4:
if(x1!=0){
result = String.valueOf(y/x1);
}else{
result = "The divisor can't be zero!";
}
break;
}
}

public String opAdd(String x){
operate(x);
op = add;
return result;
}
public String opSubtract(