Java对象和内部值

Question

I am doing some diving into the object oriented world and trying to apply the principles of how the authors of the book Clean Code think a rich object should be constructed.

However, I have run into a somewhat philosophical problem now and I am not too sure about how to deal with representation of an object.

In the book Clean Code you can read that a rich object should only expose behaviors and never its internal variables. This statement has other sources as well.

But if you only expose behaviors, how do you then show a representation of the object in a GUI, if you can't reach the values of the internal variables?

Example:

public class Todo {

    private final TodoID id;
    private final String createdBy;
    private final String currentOwner;
    private final String description;
    private final Status status;

    private Todo(String createdBy, DateTime creationDate, String description, Status status, String todoName) {
        this.id = new TodoID(creationDate, todoName);
        this.createdBy = createdBy;
                this.currentOwner = createdBy;
        this.description = description;
        this.status = status;

    }

    public void startWorking(String name){
        if(Status.IN_PROGRESS > this.status){
        // Set status to IN_PROGRESS
        // Set current owner to name    
        } else {
          throw new RuntimeException(this.currentOwner + " has already started working on this task or it might be finished already.");
    }

    public static TodoID create(TodoValues values) {
        Todo todo = new Todo(values.createdBy, 
                             values.creationDate, 
                             values.description, 
                             values.status, 
                             values.todoName);
        // Operations to save the Todo-instance in a database
        return todo.id;
    }

}

I might have a method on my Todo-aggregate root that returns a list of all new Todo's public List<Todo> listAllNewTodos(){} . All the Todo's in the list have id's that represents the object. But how do I make a representation of the id's if an object is only allowed to expose behaviors?

EDIT: Right after posting this question I got a revelation. The book Clean Code also talks about that there is another type of structure in OOP. Not only objects with behaviors, but also data structures. In this case I should probably view the Todo-class/instances as objects and the TodoID-instances as data structures that exposes its internal variables. And thanks to that I CAN represent the Todo, with the TodoID-variables, in a GUI.

Answer 1

You are doing it right, you should use Getters and Setters to reach the internal variables

See http://docs.oracle.com/javaee/6/tutorial/doc/gjbbp.html

Answer 2

You may also want to look into property change events. Using this mechanism your UI doesn't have to read the values, they are sent to the UI by the object whenever they change.

Answer 3

Not all class variables might be "internal variables". You can consider a variable internal if it is used by some behavior in your class but not needed by the consumers of your class.

In your example, suppose that, you need to keep total time for each ToDo item and users can start a task multiple times. Then, you would need to declare an internal variable:

private Date latestStartTime;

Clearly, this variable should be used by startWorking and pause methods (which I didn't include) but will not be provided to calling methods. For other fields, you can provide getter methods (and setter methods when your fields are not final) as @Gynnad has recommended. You can use them in UI.

There might be other (and better) solutions, such as the one you mentioned in your edit. Just remember, these are all heuristics to create better code, not hard rules.

Answer 4

Have you taken a look at CQRS ?

You would split up the tasks the user can do and the information he needs on a screen:

When a user wants to do something, you don't need to have internal state:

public class StartWorkingOnTodoCommandHandler : CommandHandler<StartWorkingOnTodoCommand>
{
    public void handle(StartWorkingOnTodoCommand command)
    {
        Todo todo = session.get<Todo>(command.getId());
        User user = session.get<User>(command.getUserId());
        todo.startWorking(user);
    }
}

and when you want to get query for information, you can create a separate layer:

public class UnfinishedTodo
{
    public String getTitle()
    {
    }

    public TodoId getId()
    {
    }

    public String getCreatedBy()
    {
    }
}

(My Java is getting a bit rusty)

Here it will be possible (I don't know if it is wanted) to have a User domain object in your Todo, but still have a String back to the UI.

Answer 5

In the book Clean Code you can read that a rich object should only expose behaviors and never its internal variables. This statement has other sources as well.

But if you only expose behaviors, how do you then show a representation of the object in a GUI, if you can't reach the values of the internal variables?

This is because that the rich objects in the book Clean code are not designed for displaying in GUI, but for protecting invariants. In your case, the business rule is not very complex and the TODO class is also responsible for query requirements, so it's fine to have some getter methods for now.