how to dynamically calculate formulas in java?

Question

i am developing an application in JAVA jdbc, in which i add new shapes and their formulas than calculate its area, question is something like this,

web page should show a list of shapes (circle, square, rectangle, elipse) and statistics for each shape.

When you click on a shape, you are asked to enter dimensions.

For circle, only diameter is required

For elipse, height and width is required

... etc ...

After entering the dimensions, the area of the shape will be calculated, and the statistics will be updated.

It should be easy to add new types of shapes in the future.

i have developed almost everything, but only problem is how to dynamically calculate formulas, when a shape is entered its formula saved in database, now when i ask for calculate formula, so i gave values from frontend and their formula arrive from dataabse so it calculate and show result, recently i hard code formulas in servlet like this

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        Shapes shape = new Shapes();
        int id = Integer.parseInt(request.getParameter("id"));

        int count = Integer.parseInt(request.getParameter("count")); 
        double values = Double.parseDouble(request.getParameter("name0")); // here values of shapes came from form jsp (form)

        double area = 3.14159 * values * values; // i don't want to hardcode formulas
        shape.setShapeId(id);
        shape.setShapeCount(count);
        shape.setShapeArea(area);
        shapesDao.updateShapeArea(shape);
}

**All i Want to formula values came from jsp (forms) and in servlet formula of that shape came from database by its id, than formula executed and return result **

i searched and figure out it is done by JEXL apache library, but i am not able to use this library kindly provide me help with code, i'll be greatfull to you.

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        String formula = null;
        Shapes shape = new Shapes();
        int id = Integer.parseInt(request.getParameter("id"));

        int count = Integer.parseInt(request.getParameter("count"));
        double values = Double.parseDouble(request.getParameter("name0"));

        Map m  = request.getParameterMap();
        Set s = m.entrySet();
        Iterator it = s.iterator();

        while(it.hasNext()){

            Map.Entry<String,String[]> entry = (Map.Entry<String,String[]>)it.next();

            String key             = entry.getKey();
            String[] value         = entry.getValue();

            System.out.println("Key is "+key);

                if(value.length>1){    
                    for (int i = 0; i < value.length; i++) {
                        System.out.println("value is "+value[i].toString());
                    }
                }else
                    System.out.println("Value is "+value[0].toString());


        }

    List<Shapes> allShapes = shapesDao.getAllShapes();
        for (Shapes shapeobj : allShapes)
        {
            if(shapeobj.getShapeId() == id)
            {
                formula = shapeobj.getShapeFormula();
            }
        }

here i have values in map and formula is in formula varialbe
now how to store values in this polynomial answered bt Bohemian in the way its formula is ? 






        System.out.println("Formula is "+formula);
        shape.setShapeId(id);
        shape.setShapeCount(count);
        shape.setShapeArea(area);
        shapesDao.updateShapeArea(shape);

        List<Shapes> shapes = shapesDao.getAllShapes();
        System.out.println("size in server "+shapes.size());
        request.setAttribute("data", shapes);
        request.getRequestDispatcher("/index.jsp").forward(request, response);
    }

formulas may be different for different shapes

• circle (3.14*radius*radius)
• elippse(3.14*aaxis*baxis)
• rectangle(width*length)
• square(a*a)

and so on

Answer 1

All areas a 2nd order polynomial expressions, which have only 3 coefficients:

Given (up to) 2 dimensions, x and y (y is zero for single dimensioned shapes):

area = ax² + bxy + cy²

so all you need to do is store these 3 numbers.

Here are some sets:

shape    |  a  |  b  |  c  
circle   | pi  |  0  |  0
square   |  1  |  0  |  0
triangle |  0  |  .5 |  0
ellipse  |  0  | pi  |  0

etc

Answer 2

direct sense of "formula saved in database" lead to interpreters. There are many https://en.wikipedia.org/wiki/List_of_JVM_languages

I don't have contact with jexl but here is code: http://commons.apache.org/proper/commons-jexl/reference/examples.html What is hard to understand?

Personally I integrate groovy with (big) servlet project for such dynamic computation.

Java 8 (SDK, not JRE) has dynamic compiler built-in. How do you dynamically compile and load external java classes?

You can declare interface Shape in main code, implement few overrides in database.

EDIT: question is not clear, in one place say "formula in database" is the other negate this requirement.

EDIT2: here is interface for dynamic count of sizes, ie Circle has one "Radius" etc ...

interface Shape {
 String getSahpeName(); // "Circle"
 String[] getNamesOfSizes(); // { "Radius" }
 void setSize(String name, double val);
 double getSize(String name);

 double compute(); // Pi*r^2
}

Answer 3

i have done it with apache JEXL Library

here is the code

List<Shapes> allShapes = shapesDao.getAllShapes();
        for (Shapes shapeobj : allShapes)
        {
            if(shapeobj.getShapeId() == id)
            {
                formula = shapeobj.getShapeFormula();
            }
        }
        final JexlEngine jexl = new 

    JexlBuilder().cache(512).strict(true).silent(false).create();
            String jexlExp = formula;

            JexlExpression  e = jexl.createExpression(formula);
            JexlContext context = new MapContext();
            for(int i = 0; i<params; i++)
            {
                for(int j=0;j<completed.length;j++)
                {
                    System.out.println("-------------------------");
                    System.out.println(completed[j]);
                    System.out.println("-------------------------");
                    context.set(completed[j], request.getParameter(completed[j]));
                }
            }


            Object o = e.evaluate(context);