Parse simple math expression

Parse simple math expression

This task use java to parse simple math expression.

Parse simple math expression

1. Create Formula class as following
2. Call Formula.calculate() method as following

Call Formula.calculate() method

---

Map<String, Object> data = new HashMap<String, Object>();
data.put("a", 2);
data.put("b", 3);
String formula = "a - b * a";
Object result = Formula.calculate(data, formula);
if (result != null) {
    logger.info(result.toString());
} else {
    logger.info("NULL");
}
    

Formula class

---

public static class Formula {

    private Map<String, Object> data;
    private String formula;

    public Formula(Map<String, Object> data, String formula) {
        this.data = data;
        this.formula = formula;
    }
     
    public static Object calculate(Map<String, Object> data, String formula) {
        return new Formula(data, formula).calculate();
    }
     
    public Object calculate() {
        int pos = lastOperator(formula, new String[] { "+", "-" });
        if (pos < 0) {
            pos = lastOperator(formula, new String[] { "*", "/" });
            if (pos < 0) {
                return calculate(formula);
            } else {
                return calculate(formula.substring(0, pos).trim(), formula.charAt(pos) + "", formula.substring(pos + 1).trim());
            }
        } else {
            return calculate(formula.substring(0, pos).trim(), formula.charAt(pos) + "", formula.substring(pos + 1).trim());
        }
    }
     
    private Object calculate(String leftSide, String operator, String rightSide) {
        Object valLeft = null;
        Object valRight = null;
        int pos = lastOperator(leftSide, new String[] { "+", "-" });
        if (pos < 0) {
            pos = lastOperator(leftSide, new String[] { "*", "/" });
            if (pos < 0) {
                valLeft = calculate(leftSide);
            } else {
                valLeft = calculate(leftSide.substring(0, pos).trim(), leftSide.charAt(pos) + "", leftSide.substring(pos + 1).trim());
            }
        } else {
            valLeft = calculate(leftSide.substring(0, pos).trim(), leftSide.charAt(pos) + "", leftSide.substring(pos + 1).trim());
        }
        pos = lastOperator(rightSide, new String[] { "*", "/" });
        if (pos < 0) {
            valRight = calculate(rightSide);
        } else {
            valRight = calculate(rightSide.substring(0, pos).trim(), rightSide.charAt(pos) + "", rightSide.substring(pos + 1).trim());
        }
        return calculate(valLeft, operator, valRight);
    }
     
    private Object calculate(Object leftSide, String operator, Object rightSide) {
        Object tag = null;
        if (leftSide == null || rightSide == null) return null;
        if (isNumber(leftSide) && isNumber(rightSide)) {
            double valLeft = parseNumber(leftSide);
            double valRight = parseNumber(rightSide);
            tag = calculate(valLeft, operator, valRight);
        } else if (isString(leftSide) || isString(rightSide)) {
            if ("+".equals(operator)) {
                tag = leftSide.toString() + rightSide.toString();
            }
        }
        return tag;
    }
     
    private Object calculate(double leftSide, String operator, double rightSide) {
        if ("+".equals(operator)) {
            return leftSide + rightSide;
        } else if ("-".equals(operator)) {
            return leftSide - rightSide;
        } else if ("*".equals(operator)) {
            return leftSide * rightSide;
        } else if ("/".equals(operator) && rightSide != 0) {
            return leftSide / rightSide;
        } else {
            return null;
        }
    }
     
    private double parseNumber(Object src) {
        double tag = 0;
        try {
            tag = Double.parseDouble(src.toString());
        } catch (Exception e) {
            tag = 0;
        }
        return tag;
    }
     
    private boolean isString(Object src) {
        String kind = src.getClass().getName();
        if ("java.lang.String".equals(kind)) return true;
        return false;
    }
     
    private boolean isNumber(Object src) {
        if (src == null) return false;
        String[] types = new String[] { "java.lang.Character", "java.lang.Byte", "java.lang.Short", "java.lang.Integer", "java.lang.Long", "java.lang.Float", "java.lang.Double" };
        String kind = src.getClass().getName();
        for (int i = 0; i < types.length; i++) {
            if (types[i].equalsIgnoreCase(kind)) return true;
        }
        return false;
    }
     
    private Object calculate(String src) {
        if (data.containsKey(src)) {
            return data.get(src);
        } else {
            return null;
        }
    }
     
    private int lastOperator(String src, String[] operators) {
        int tag = -1;
        for (int i = 0; i < operators.length; i++) {
            int pos = src.lastIndexOf(operators[i]);
            if (pos < 0) continue;
            if (tag < 0) {
                tag = pos;
            } else {
                if (pos > tag) {
                    tag = pos;
                }
            }
        }
        return tag;
    }
     
}
    

Java