About classes and their valid states

Question

I get stuck each time I need to write a constructor for any class I design. The reason is that I am not sure what should go into a constructor and what should not.

On googling for this, I got the answer that the constructor need to do only as much as to make sure that the opbect created is valid. But my query is

How do you know if an object is in the valid state?

My class represents a chess board:

public class ChessBoard
{
    List<ICellRow> lstRow;
    int cellOffset;

    //Other function

    public ChessBoard()
    {
        //What should go in here?
    }
}

How do you decide when a class has the minimum information to be in the valid state? In this specific example, makes the board class valid?

Answer 1

Think of an object as a value, just like a number or a string. That value represents a concept in the problem you’re trying to solve. For example, you might use an int to denote a distance in miles, or a number of points, or a set of Boolean flags. The value has no meaning on its own, but it does have an interpretation.

Whenever you create a value, you want to make sure its interpretation makes sense. It should be valid by construction—3 is a valid number, and “hello” is a valid string. In particular, there is no way to construct a string with a value that isn’t a string.

In addition, operations on values should be designed in such a way that every operation leads to a valid internal state, or else reports an error—whether at compile time through static typing, or at runtime through exceptions. Division by zero, for example, is not allowed on integers.

What this means for you is that the constructor of your ChessBoard class should produce a legal board state according to the game rules. For example, you could use the constructor to set the initial configuration of pieces for the start of a game. Operations on the board should then only allow you to move to other legal game states. By thinking along these lines, you can ensure that your ChessBoard accurately represents how a game of chess works in real life.

Answer 2

The constructor should also make sure any structures (colloeections for example) are initialized according to your conventions.

For example, if you expect a method or a property to always return a non-null collection (even in the case there were never any items added to it) the constructor should initialize the collection. That way any calls to methods or properties dependent on the collection will be able to rely on a valid collection.

Answer 3

The purpose of the constructor is for you to setup the class into a user friendly default state.

E.g. If the chessboard was a graphical object for a video game then it would be reasonable to expect that the colors of the tiles would be defaulted to black and white so as to take some burden away from the programmer having to manually do this using extra function calls.