OO设计建议 - toString

Question

So I got the Address class:

class Address 
{
    private String streetAddress;
    private int number;
    private String postalCode;
    private City city;
    private State state;
    private Country country;
}

And I want to get its readable version to, lets say, show in a grid column.

Whats the best and concise way to implement this?

1. toString method inside class Address ( I personally don't like this approach, as 'toString' is not directly related to an Address )
2. class ReadableAddressFormatter
   • ReadableAddressFormatter ( Address addressToFormat )
   • public String getFormatted()
3. Previous class but getFormmated would be static, receiving the Address instance and returning the string
4. Other? Suggestions please.

I'm looking for a good design, focusing also in Clean Code , Decoupling and Maintainability .

Answer 1

All of these methods have been used, and there's no way to offer a "context independent" best practice. The best answer in Software Engineering is usually "it depends." That's said, let's analyze each:

1. The KISS approach at its finest. I do this for all my basic "print to console, make sure things are working" kind of thing. If you have a specific format you can expect for addresses, this is the low-hanging fruit/easy win solution. You can always override this or print out the object differently in one off situations.
2. This is the most extensible solution, in that it will nicely allow for localization and custom formatting. Whether it is appropriate depends on how often you expect addresses to be shown in different formats. Do you really need that Death Star to knock out a fly, or is the ability to change to all uppercase or change between languages pivotal to your app?
3. I would not suggest this approach, as it generally started to bleed "view level" logic into the Domain, which is usually best handled by other tiers (in a class MVC approach). One could argue that toString() does the same thing, but toString() can also be thought of as the "name" or "essence" of how an object appears to the external world, so I'd say it's more than just presentational.

Hope this helps, and kudos for thinking about Clean Code, Decoupling, and Maintainability from the beginning.

For an example of principle #2 in action--using the Strategy Pattern , adhering to the Single Responsibility Principle , the Open/Closed Principle and allowing for Inversion of Control via Dependency Injection -- compare the following approach (graciously provided by @SteveJ):

public class Address {
        private String streetAddress;
        private int number;
        private String postalCode;
        private String city;
        private String state;
        private String country;

        public String toLongFormat(){
            return null; // stitch together your long format
        }

        public String toShortFormat(){
            return null; // stitch together your short format
        }

        public String toMailingLabelFormat(){
            return null; // stitch together your mailing label format
        }

        @Override
        public String toString(){
            return toShortFormat(); // your default format
        }
    }

}

With this one (in "mostly correct" Groovy):

public interface AddressFormatter {
   String format(Address toFormat)
}

public class LongAddressFormatter implements AddressFormatter {
    @Override
    public String format(Address toFormat){
         return String.format("%sBLAHBLAHBLAHBLAHBLAHBLAHBLAHBLAHBLAHBLAHBLAHBLAHBLAHBLAHBLAHBLAHBLAH%n%s", toFormat.streetAddress, toFormat.postalCode)
    }
}


public class ShortAddressFormatter implements AddressFormatter {
    @Override
    public String format(Address toFormat){
         return String.format("%d", toFormat.number)
    }
}

public  class Address {
        private String streetAddress;
        private int number;
        private String postalCode;
        private String city;
        private String state;
        private String country;
        public  AddressFormatter formatter = new ShortAddressFormatter(); // just to avoid NPE

        public void setFormatter(AddressFormatter fr) { this.formatter = fr; }



        @Override
        public String toString(){
            return formatter.format(this); // your default format
        }
    }

def addrr = new Address(streetAddress:"1234 fun drive", postalCode:"11223", number:1)
addr.setFormatter(new LongAddressFormatter());
println "The address is ${addrr}"
addr.setFormatter(new ShortAddressFormatter());
println "The address is ${addrr}"

As @SteveJ has observed:

" So the you have different formatting "strategies" and you can switch between them...I had this idea that you would set the formatting once and be stuck with it...AND if you want to add another formatting style, you don't have to open up and rewrite the address class, but write a new separate style and inject it when you want to use it."

Answer 2

Overriding Object.ToString() seems to be the most logical solution. This makes it clean to use in situations such as: Console.WriteLine("Home Address: {0}", homeAddress);

If you wish to provide additional , the Address class should implement IFormattable .

Also, you should create an AddressFormatter class that implements from IFormatProvider and ICustomFormatter .

The MSDN links provide very well put examples (a BinaryFormatter and a AcctNumberFormat), but if those aren't enough also look at this good example: PhoneFormatter

---

Additionally, if you do decide to go full out on this and implement IFormattable and a custom IFormatProvider/ICustomFormatter then I'd suggest having your ToString() simply call to your ToString(String format, IFormatProvider formatProvider) with a default provider. That way you can account for things like localization and types of addresses (short, long, etc).

Answer 3

Using toString requires no additional baggage outside the function itself; seems like the simplest solution. It's there for a reason, right?

Answer 4

Usually i divide presentation layer from data layer. Presenting it to a GUI seems to me something related to the presentation layer, not the data layer.

I would suggest you to put a function somewhere in your presentation layer that will convert the address to string.

Presentation of data is not related to data!

A static method is good. A converter class would be better, you can keep one single instance for your application but you can replace it or write another if you are moving your application from GUI to WEB with another format, or if in one window you want to show everything and in another window you want to show only part of the informations or informations formatted in another way.

There are several model you can follow, for example Microsoft WPF uses totally another approach, the MVVM, Model View View Model, that will allow you to divide very well data layer from business logic from presentation layer.

I usually override ToString in C# or toString in java only for debugging purposes (presenting a string that i can use for debug) or for some kind of simple serialization to string, usually putting also a FromString (or fromString method in java). One example is custom types like Point, Vector, Matrix and so on.

Talking about C# world..

public class AddressToStringConverter
{
    public virtual string ToString(Address address)
    {
        return address.Street + ", " + address.City
    }
}

Then in your form (for example).

AddressToStringConverter myConverter = new AddressToStringConverter();

public Address CurrentSelectedAddress { get { ... } }

public button1_click(object sender, EventArgs e)
{
    button1.Text = myConverter.Convert(address);
}

If you want you can imlpement other useful interfaces like for example ITypeConverter

Answer 5

toString() is the most flexible and convenient, being implicitly called when you combine an object of the Address class with a String, as in System.out.println("My address is " + objectOfAddressClass).

The only reason I can think of to not override toString() is if you need to alter the formatting. Then you would need different methods (as in toMailingString() and toShortFormString() and so on) or a parameterized method (as in toMailingString(boolean useShortForm) or whatever), but either way, toString() won't cut it.

Of course, you can (and should) do both. Have toString() as your default, probably calling one of your specific format methods, and then have your other helper methods for alternate formats.

public class TestClass {

    class City{};

    class State{};

    class Country{};

    class Address {
        private String streetAddress;
        private int number;
        private String postalCode;
        private City city;
        private State state;
        private Country country;

        public String toLongFormat(){
            return null; // stitch together your long format
        }

        public String toShortFormat(){
            return null; // stitch together your short format
        }

        public String toMailingLabelFormat(){
            return null; // stitch together your mailing label format
        }

        @Override
        public String toString(){
            return toShortFormat(); // your default format
        }
    }

}

Answer 6

I think that a toString() method that returns a string is your best approach. If you have an Address instance, let's say address , then it is obvious what address.toString() does. The fact that toString() isn't directly associated with Address doesn't really change anything.

Answer 7

You've tagged your post with Java, so I'll answer for Java (and Swing, more specifically). This task is normally the task of a specific TableCellRenderer. If the same format must be used for other visual components, I would indeed extract the formatting inside an instantiatable class (solution 2). This would allow subclasses to customize the format if needed.