Difference between an instance of a class and a cl

问题:

I use Java as an example but this is more of a general OOP design related question.

Lets take the IOExceptions in Java as an example. Why is there a class FileNotFoundException for example? Should not that be an instance of a IOException where the cause is FileNotFound? I would say FileNotFoundException is an instance of IOException. Where does this end? FileNotFoundButOnlyCheckedOnceException, FileNotFoundNoMatterHowHardITriedException..?

I have also seen code in projects I worked in where classes such as FirstLineReader and LastLineReader existed. To me, such classes actually represent instances, but I see such design in many places. Look at the Spring Framework source code for example, it comes with hundreds of such classes, where every time I see one I see an instance instead of a blueprint. Are not classes meant to be blueprints?

What I am trying to ask is, how does one make the decision between these 2 very simple options:

Option 1:

enum DogBreed {
    Bulldog, Poodle;
}

class Dog {
    DogBreed dogBreed;
    public Dog(DogBreed dogBreed) {
        this.dogBreed = dogBreed;
    }
}

Option 2:

class Dog {}

class Bulldog extends Dog {
}

class Poodle extends Dog {
}

The first option gives the caller the requirement to configure the instance it is creating. In the second option, the class represents the instance itself already (as I see it, which might be totally wrong ..).

If you agree that these classes represent instances instead of blueprints, would you say it is a good practice to create classes that represents instances or is it totally wrong the way I am looking at this and my statement "classes representing instances" is just load of nonsense?

回答1:

Edited

First of all: We know the Inheritance definition and we can find a lot of examples in SO and internet. But, I think we should look in-depth and a little more scientific.

Note 0:
Clarification about Inheritance and Instance terminology.
First let me name Development Scope for development life cycle, when we are modeling and programming our system and Runtime Scope for sometimes our system is running.

We have Classes and modeling and developing them in Development Scope. And Objects in Runtime Scope. There is no Object in Development Scope.

And in Object Oriented, the definition of Instance is: Creating an Object from a Class.

On the other hand, when we are talking about classes and object, we should clarify our Viewpoint about Development Scope and Runtime Scope.

So, with this introduction, I want to clarify Inheritance:
Inheritance is a relationship between Classes, NOT Objects.
Inheritance can exist in Development Scope, not in Runtime Scope. There is no Inheritance in Runtime Scope.

After running our project, there is no relationship between parent and child (If there is only Inheritance between a child class and parent class). So, the question is: What is super.invokeMethod1() or super.attribute1 ?, they are not the relationship between child and parent. All attributes and methods of a parent are transmitted to the child and that is just a notation to access the parts that transmitted from a parent.

Also, there are not any Objects in Development Scope. So there are not any Instances in Development scope. It is just Is-A and Has-A relationship.

Therefore, when we said:

I would say FileNotFoundException is a instance of an IOException

We should clarify about our Scope (Development and Runtime).
For example, If FileNotFoundException is an instance of IOException, then what is the relationship between a specific FileNotFoundException exception at runtime (the Object) and FileNotFoundException. Is it an instance of instance?

Note 1:
Why we used Inheritance? The goal of inheritance is to extending parent class functionalities (based on the same type).

• This extension can happen by adding new attributes or new methods.
• Or overriding existing methods.
• In addition, by extending a parent class, we can reach to reusability too.
• We can not restrict the parent class functionality (Liskov Principle)
• We should be able to replace the child as parent in the system (Liskov Principle)
• and etc.

Note 2:
The Width and Depth of Inheritance Hierarchies
The Width and Depth of Inheritance can be related to many factors:

• The project: The complexity of the project (Type Complexity) and it's architecture and design. The size of the project, the number of classes and etc.
• The team: The expertise of a team in controlling the complexity of the project.
• and etc.

However, we have some heuristics about it. (Object-Oriented Design Heuristics, Arthur J. Riel)

In theory, inheritance hierarchies should be deep—the deeper, the better.

In practice, inheritance hierarchies should be no deeper than an average person can keep in his or her short-term memory. A popular value for this depth is six.

Note that they are heuristics and based on short-term memory number (7). And maybe the expertise of a team affect this number. But in many hierarchies like organizational charts is used.

Note 3:
When we are using Wrong Inheritance?
Based on :

• Note 1: the goal of Inheritance (Extending parent class functionalities)
• Note 2: the width and depth of Inheritance

In this conditions we use wrong inheritance:

1. We have some classes in an inheritance hierarchy, without extending parent class functionalities. The extension should be reasonable and should be enough to make a new class. The reasonable means from Observer's point of view. The observer can be Project Architect or Designer (Or other Architects and Designers).

2. We have a lot of classes in the inheritance hierarchy. It calls Over-Specialization. Some reasons may cause this:

   • Maybe we did not consider Note 1 (Extending parent functionalities)
   • Maybe our Modularization (packaging) is not correct. And we put many system use cases in one package and we should make Design Refactoring.

3. They are other reasons, but not exactly related this answer.

Note 4:
What should we do? When we are using Wrong Inheritance?

Solution 1: We should perform Design Refactoring to check the value of classes in order to Extending parent Functionality. In this refactoring, maybe many classes of system deleted.

Solution 2: We should perform Design Refactoring to modularization. In this refactoring, maybe some classes of our package transmitted to other packages.

Solution 3: Using the Composition over Inheritance.
We can use this technique for many reasons. Dynamic Hierarchy is one of popular reasons that we prefer Composition instead of Inheritance.
see Tim Boudreau (of Sun) notes here:

Object hierarchies don't scale

Solution 4: use instances over Subclasses

This question is about this technique. Let me named it instances over Subclasses.

When we can use it:

1. (Tip 1): Consider Note 1, when we do not exactly extend the parent class functionalities. Or the extensions are not reasonable and enough.

2. (Tip 2:) Consider Note 2, If we have a lot of subclasses (semi or identical classes) that extends the parent class a little and we can control this extension without inheritance. Note that it is not easy to say that. We should prove that it is not violating other Object Oriented Principles like Open-Close Principle.

What should we do?
Martin Fowler recommend (Book 1 page 232 and Book 2 page 251):

Replace Subclass with Fields, Change the methods to superclass fields and eliminate the subclasses.

We can use other techniques like enum as the question mentioned.

回答2:

First, by including the exceptions question along with a general system design issue, you're really asking two different questions.

Exceptions are just complicated values. Their behaviors are trivial: provide the message, provide the cause, etc. And they're naturally hierarchical. There's Throwable at the top, and other exceptions repeatedly specialize it. The hierarchy simplifies exception handling by providing a natural filter mechanism: when you say catch (IOException..., you know you'll get everything bad that happened regarding i/o. Can't get much clearer than that. Testing, which can be ugly for big object hierarchies, is no problem for exceptions: There's little or nothing to test in a value.

It follows that if you are designing similar complex values with trivial behaviors, a tall inheritance hierarchy is a reasonable choice: Different kinds of tree or graph nodes constitute a good example.

Your second example seems to be about objects with more complex behaviors. These have two aspects:

• Behaviors need to be tested.
• Objects with complex behaviors often change their relationships with each other as systems evolve.

These are the reasons for the often heard mantra "composition over inheritance." It's been well-understood since the mid-90s that big compositions of small objects are generally easier to test, maintain, and change than big inheritance hierarchies of necessarily big objects.

Having said that, the choices you've offered for implementation are missing the point. The question you need to answer is "What are the behaviors of dogs I'm interested in?" Then describe these with an interface, and program to the interface.

interface Dog {
  Breed getBreed();
  Set<Dog> getFavoritePlaymates(DayOfWeek dayOfWeek);
  void emitBarkingSound(double volume);
  Food getFavoriteFood(Instant asOfTime);
}

When you understand the behaviors, implementation decisions become much clearer.

Then a rule of thumb for implementation is to put simple, common behaviors in an abstract base class:

abstract class AbstractDog implements Dog {
  private Breed breed;
  Dog(Breed breed) { this.breed = breed; }
  @Override Breed getBreed() { return breed; }
}

You should be able to test such base classes by creating minimal concrete versions that just throw UnsupportedOperationException for the unimplemented methods and verify the implemented ones. A need for any fancier kind of setup is a code smell: you've put too much into the base.

Implementation hierarchies like this can be helpful for reducing boilerplate, but more than 2 deep is a code smell. If you find yourself needing 3 or more levels, it's very likely you can and should wrap chunks of common behavior from the low-level classes in helper classes that will be easier to test and available for composition throughout the system. For example, rather than offering a protected void emitSound(Mp3Stream sound); method in the base class for inheritors to use, it would be far preferable to create a new class SoundEmitter {} and add a final member with this type in Dog.

Then make concrete classes by filling in the rest of the behavior:

class Poodle extends AbstractDog {
  Poodle() { super(Breed.POODLE); }
  Set<Dog> getFavoritePlaymates(DayOfWeek dayOfWeek) { ... }
  Food getFavoriteFood(Instant asOfTime) { ... }
}

Observe: The need for a behavior - that the dog must be able to return its breed - and our decision to implement the "get breed" behavior in an abstract base class resulted in a stored enum value.

We ended up adopting something closer to your Option 1, but this wasn't an a priori choice. It flowed from thinking about behaviors and the cleanest way to implement them.

回答3:

Following comments are on the condition where sub-classes do not actually extend the functionality of their super class.

From Oracle doc:

Signals that an I/O exception of some sort has occurred. This class is the general class of exceptions produced by failed or interrupted I/O operations.

It says IOException is a general exception. If we have a cause enum:

enum cause{
    FileNotFound, CharacterCoding, ...;
}

We will not be able to throw an IOException if the cause in our custom code is not included in the enum. In another word, it makes IOException more specific instead of general.

Assuming we are not programming a library, and the functionality of class Dog below is specific in our business requirement:

enum DogBreed {
    Bulldog, Poodle;
}

class Dog {
    DogBreed dogBreed;
    public Dog(DogBreed dogBreed) {
       this.dogBreed = dogBreed;
    }
}

Personally I think it is good to use enum because it simplifies the class structure (less classes).

回答4:

The first code you cite involves exceptions.

Inheritance is a natural fit for exception types because the language-provided construct to differentiate exceptions of interest in the try-catch statement is through use of the type system. This means we can easily choose to handle just a more specific type (FileNotFound), or the more general type (IOException).

Testing a field's value, to see whether to handle an exception, means stepping out of the standard language construct and writing some boiler plate guard code (e.g. test value(s) and rethrow if not interested).

(Further, exceptions need to be extensible across DLL (compilation) boundaries. When we use enums we may have problems extending the design without modifying the source that introduces (and other that consumes) the enum.)

When it comes to things other than exceptions, today's wisdom encourages composition over inheritance as this tends to result in less complex and more maintainable designs.

Your Option 1 is more of a composition example, whereas your Option 2 is clearly an inheritance example.

If you agree that these classes represent instances instead of blueprints, would you say it is a good practice to create classes that represents instances or is it totally wrong the way I am looking at this and my statement "classes representing instances" is just load of nonsense?

I agree with you, and would not say this represents good practice. These classes as shown are not particularly customizable and don't represent added value.

A class that has offers no overrides, no new state, no new methods, is not particularly differentiated from its base. So there is little merit in declaring such a class, unless we seek to do instance-of tests on it (like the exception handling language construct does under the covers). We can't really tell from this example, which is contrived for the purposes of asking the question, whether there is any added value in these subclasses but it doesn't appear so.

To be clear, though, there are lots of worse example of inheritance, such as when an (pre) occupation like Teacher or Student inherits from Person. This means that a Teacher cannot a be Student at the same time unless we engage in adding even more classes, e.g. TeacherStudent, perhaps using multiple inheritance..

We might call this class explosion, as sometimes we end up needing a matrix of classes because of inappropriate is-a relationships. (Add one new class, and you need a whole new row or column of exploded classes.)

Working with a design that suffers class explosion actually creates more work for clients consuming these abstractions, so it is a loose-loose situation.
Here at issue, is in our trust of natural language because when we say someone is-a Student, this is not, from a logical perspective, the same permanent "is-a"/instance-of relationship (of subclassing), but rather a potentially-temporary role being played that the Person: one of many possible roles a Person might play concurrently at that. In these cases composition is clearly superior to inheritance.

In your scenario, however, the BullDog is unlikely to be able to be anything other than the BullDog, so the permanent is-a relationship of subclassing holds, and while adding little value, at least this hierarchy does not risk class explosion.

Note that the main drawback to with the enum approach is that the enum may not be extensible depending on the language you're using. If you need arbitrary extensibility (e.g. by others and without altering your code), you have the choice of using something extensible but more weakly typed, like strings (typos aren't caught, duplicates aren't caught, etc..), or you can use inheritance, as it offers decent extensibility with stronger typing. Exceptions need this kind of extensibility by others without modification and recompilation of the originals and others since they are used across DLL boundaries.

If you control the enum and can recompile the code as a unit as needed to handle new dog types, then you don't need this extensibility.

回答5:

Option 1 has to list all known causes at declaration time.

Option 2 can be extended by creating new classes, without touching the original declaration.

This is important when the base/original declaration is done by the framework. If there were 100 known, fixed, reasons for I/O problems, an enum or something similar could make sense, but if new ways to communicate can crop up that should also be I/O exceptions, then a class hierarchy makes more sense. Any class library that you add to your application can extend with more I/O exceptions without touching the original declaration.

This is basically the O in the SOLID, open for extension, closed for modification.

But this is also why, as an example, DayOfWeek type of enumerations exists in many frameworks. It is extremely unlikely that the western world suddenly wakes up one day and decides to go for 14 unique days, or 8, or 6. So having classes for those is probably overkill. These things are more fixed in stone (knock-on-wood).

回答6:

The two options you present do not actually express what I think you're trying to get at. What you're trying to differentiate between is composition and inheritance.

Composition works like this:

class Poodle {
    Legs legs;
    Tail tail;
}

class Bulldog {
    Legs legs;
    Tail tail;
}

Both have a common set of characteristics that we can aggregate to 'compose' a class. We can specialize components where we need to, but can just expect that "Legs" mostly work like other legs.

Java has chosen inheritance instead of composition for IOException and FileNotFoundException.

That is, a FileNotFoundException is a kind of (i.e. extends) IOException and permits handling based on the identity of the superclass only (though you can specify special handling if you choose to).

The arguments for choosing composition over inheritance are well-rehearsed by others and can be easily found by searching for "composition vs. inheritance."