A wonderful APCS project in Java. Basically ed for a spreadsheet

Feel free to do PRs against this document if you feel something should be different

Remember, avoid stupid getter/setters, instead be clever!

Spreadsheet contains a matrix of Values that represent it's cells
Can obtain the Value at a index with valueAt

Value: base class, all values can be refine()'d in the context of some Spreadsheet, which by default will simpily return the value itself. In addition all Values should have a useful toString method
Number: a decimal value, represented by a double
StringValue: a string value
CellReference: a value that holds a cell's row and column, and when it is refine()'d it returns the value in that cell
DeferredExpression: a value that, when refine()'d, returns the evaluation of it's child expression
ErrorValue: a value that represents a error resulting from a evaluation/refinement

Expression: base class, all expressions can be evaluate()'d in the context of some Spreadsheet
ValueExpression: a expression that represents a basic value (number, string, cellref or deferred expr)
AddExpression: a expression that evaluates to the addition of the evaluation of its two arguments
SubExpression: same as AddExpression with subtraction
Term: another base class
MulTerm: a expression that evaluates to the multiplication of the evalution of its two arguments
DivTerm: same as MulTerm with division
AssignmentExpression: a expression that assigns the value on the left of a = to the value on the right

Parser class will have a public method that takes a String and returns a Expression named parse

Main class will run the basic command line, which will read a line of input, see if it is a special command (print or quit), then invoke either the special command or parse the line and evaluate the expression, printing the resulting value