Related
I have just started to learn Java and is curious is it any good practice in Java for good object decomposition? Let me describe a problem. In big software project it's always a big classes like 'core' or 'ui' that tends to have a lot of methods and are intended as a mediators between smaller classes. For example, if user clicks a button on some window, this window's class sends a message to 'ui' class. This 'ui' class catches this message and acts accordingly by doing something with application user interface ( via calling method of one of it's member objects ) or by posting message to application 'core' if it's something like 'exit application' or 'start network connection'.
Such objects is very hard to break apart since they are a mere mediators between a lots of small application objects. But having a classes in application with hundreds and thousands of methods is not very handy, event if such methods are trivial task delegation from one object to another. C# solves such problem by allowing to break class implementation into multiple source files: you can divide god object any way you choose, and it will work.
Any practices by dividing such objects in Java?
One way to begin breaking such a large object apart is to first find a good subset of fields or properties managed by the large object that are related to each other and that don't interact with other fields or properties of the object. Then, create a new, smaller object using only those fields. That is, move all logic from the large class to the new smaller class. In the original large class, create a delegation method that simply passes the request along. This is a good first step that only involves changing the big object. It doesn't reduce the number of methods, but it can greatly reduce the amount of logic needed in the large class.
After a few rounds of doing this, you can begin to remove some of the delegation by pointing other objects directly at the newer, smaller objects, rather than going through the previously-huge object that was in the middle of everything.
See Wikipedia's Delegation pattern discussion for example.
As a simple example, if you have a personnel object to represent staff at a company, then you could create a payroll object to keep track of payroll-related values, a ratings object to keep track of employee ratings, an awards object to keep track of awards that the person has won, and so on.
To wit, if you started out with one big class containing the following methods, each containing business logic, among many other methods:
...
public boolean isManagement() { ... }
public boolean isExecutive() { ... }
public int getYearsOfService() { ... }
public Date getHireDate() { ... }
public int getDepartment() { ... }
public BigDecimal getBasePay() { ... }
public BigDecimal getStockShares() { ... }
public boolean hasStockSharePlan() { ... }
...
then this big object could, in its constructor, create a newly created object StaffType and a newly created object PayInformation and a newly created object StaffInformation, and initially these methods in the big object would look like:
// Newly added variables, initialized in the constructor (or as appropriate)
private final StaffType staffType;
private final StaffInformation staffInformation;
private final PayInformation payInformation;
...
public boolean isManagement() { return staffType.isManagement(); }
public boolean isExecutive() { return staffType.isExecutive(); }
public int getYearsOfService() { return staffInformation.getYearsOfService(); }
public Date getHireDate() { return staffInformation.getHireDate(); }
public int getDepartment() { return staffInformation.getDepartment(); }
public BigDecimal getBasePay() { return payInformation.getBasePay(); }
public BigDecimal getStockShares() { return payInformation.getStockShares(); }
public boolean hasStockSharePlan() { return payInformation.hasStockSharePlan(); }
...
where the full logic that used to be in the big object has been moved to these three new smaller objects. With this change, you can break the big object into smaller parts without having to touch anything that makes use of the big object. However, as you do this over time, you'll find that some clients of the big object may only need access to one of the divisible components. For these clients, instead of them using the big object and delegating to the specific object, they can make direct use of the small object. But even if this refactoring never occurs, you've improved things by separating the business logic of unrelated items into different classes.
The next logical step may be to change the BigClass into a java package. Next create new objects for each group of related functionality (noting in each class that the object is part of the new package).
The benefits of doing this are dependency reduction and performance.
No need to import the entire
package/BigClass just to get a few
methods.
Code changes to related
functionality don't require a
recompile/redeploy of the entire
package/BigClass.
Less memory used
for allocating/deallocating objects,
since you are using smaller classes.
I've seen some cases where this is solved by inheritance: let's say class Big takes care of 5 different things, and (for various reasons) they all have to be in the same class. So you pick an arbitrary inheritance order, and define:
BigPart1 // all methods dealing with topic #1
BigPart2 extends BigPart1 // all methods dealing with topic #2
...
Big extends BigPart4 // all methods dealing with the last topic.
If you can really layer things up, so that the breakage makes sense (Part2 actually uses stuff from Part1, but not vice versa, etc.) then maybe it makes some sense.
The place where I've seen this is in WebWorks, where a single class had tons of getter/setter methods -- the setters used for dependency injection (e.g., URL args passed to the object upon execution) and the getters for making values accessible to various page templates (I think it was JSPs).
So, the breakdown grouped stuff logically, e.g., assuming the class was called MyAction, there was MyActionBasicArgs (fields and setters for basic CGI arguments), extended by MyActionAdvancedArgs (advanced-option args), extended by MyActionExposedValues (getters), extended by MyActionDependencies (setters used by Spring dependency injection, non-CGI args), extended by MyAction (which contained the actual execute() method).
Because of the way dependency injection in WebWorks works (or at least, used to work, back then), it had to be one huge class, so breaking it down this way made things more maintainable. But first, please, please, see if you can simply avoid having a single huge class; think carefully about your design.
Yes, C# provides partial classes. I assume this is what you are referring to when you say:
C# solves such problem by allowing to break class implementation into multiple source
files: you can divide god object any way you choose, and it will work.
This does help make huge classes more manageable. However, I find partial classes best used when one needs to extend code created by a code generator. When a class is as large as you're talking about, it can almost always be divided into smaller classes by proper object oriented design. Using a partial class sidesteps the more correct object oriented design, which is sometimes OK as the end goal is stable, reliable, maintainable code, and not a textbook example of OO code. However, many times, putting the code of a large object into a large number of smaller partial class instances of the same class is not the ideal solution.
If you can possibly find subsets of the properties of the "god" object that do not interact with one another, then each one of those sets would logically make a good candidate for a new object type. However, if all properties of this "god" object depend on one another, then there is not much you can do to decompose the object.
I don't know why you would ever have such a large class.
I suppose if you were using a gui builder code generation and being lazy about it, you might end up in such a situation, but codegen usually ends up nasty unless you take control yourself.
Splitting a single class arbitrarily is a terrible solution to a terrible manufactured problem. (Code reuse, for one thing will become virtually impossible)
If you have to use a GUI builder, have it build smaller components, then use the small components to build up a bigger GUI. Each component should do exactly one job and do it well.
Try not to EVER edit generated code if you can avoid it. Putting business logic into a genned "frame" is just a horrid design pattern. Most code generators aren't very helpful with this, so try to just make a single, minimal edit to get at what you need from external classes (think MVC where the genned code is your View and the code you edit should be in your Model and Controller).
Sometimes you can just expose the getComponents method from the Frame object, get all the components out by iterating through the containers and then dynamically bind them to data and code (often binding to the name property works well), I've been able to safely use form editors this way, and all the binding code tends to be very easily abstracted and reused.
If you're not talking about generated code--Well in your "God" class, does it do exactly one small job and do it well? If not, pull out a "Job", put it in it's own class, and delegate to it.
Is your GOD class fully factored? When I've seen huge classes like this, I've usually seen a lot of copy/paste/edit lines. If there is enough of a similarity to copy and past and edit some section, then there is enough to factor these lines into a single chunk of code.
If your big class is a GUI class, consider decorators--reusable and moves stuff out of your main class. A double win.
I guess the answer to your question is that in Java we just use good OO to ensure that the problem doesn't arise in the first place (or we don't--Java's certainly not immune to the problems you are talking about any more than any other language)
I am curious about java project structure, as well as best practices in regards to classes, interfaces, etc.
If anyone knows of a good open source project that follows good best practices I would appreciate it; it seems like every one is slightly different, with some downright contradicting Oracle documentation on the topic. If anyone could give me a breakdown (or a critique of my structure for a theoretical project it would be appreicated). I understand the /src, bin, lib, doc, etc. as well as com.* structure reasonably well I believe. My issue is exactly WHAT should be in each class, in each file, etc.
My biggest issue is how to exactly break up functionality between classes. For example; I have two classes:
Person.java
Runner.java (This is entry point; is there a naming convention for the entry points? It also seems that classes with main(), run(), etc. are a different 'tier'....how to decide where the entry point should be? Should a class with only main () be made (as well as the actual execution of necessary calculations?)
Person has all the the common variables you would expect....
int height, weight;
String ethnicity;
boolean gender; etc.etc.
Person(int h, int w.....) {this.height=h....}
public getters/setters for all variables
Now I am looking for a program that will do two things:
Take all the attributes of every Person (Say we have instantiated an array of Person()), concatanate them to a String and add that to a new Array.
Order the people by height, then weight, and put into a List.
So for the first thing; should Person class have a method "String concatToString(){}", or should the code that does the concatanetion be in Runner...for example:
Runner.java:
public class Runner {
String getPersonString(Person p) {
StringBuffer Sb = new StringBuffer();
Sb.append(p.get(height));
etc.
return Sb.toString();
}
main() {
for(int i=0; i<arr.len; i++) {
getPersonString(arr[i]);
}
//more code that we will be further executing etc...; mostly just function calls in
class Runner
}
}
Now for the second problem how should I approach this...create a new class; create a priorityQueue and comparator in runner.java? Create another class PersonPQueue that has the comparator in it?
These issues consistently pop up in my code and I am never certain how to split my code correctly. Any great and clear examples would be much appreciated. I have checked out some open source projects and many were too large for me to wrap my head around the design decisions in a reasonable amount of time or had contradictory design choices.
Thanks!
There's a lot going on in your posting. It's a huge topic for a Q&A site. Anyway, I'll answer some specific points.
Take all the attributes of every Person (Say we have instantiated an
array of Person()), concatanate them to a String and add that to a new
Array.
Implement toString() in your Person class, that formats a Person in a certain way. The thing that calls toString() would probably not be in the Person class. Without more context, I can't say for sure.
Order the people by height, then weight, and put into a List.
If this were to be the only way to sort Person, I'd make the getter for the sorted list a method in Person and add the Comparator class to the bottom of the Person class. If the application gets more complex, where several sorts could be used, I'd consider moving the Comparators to their own class files.
My final advice would be to not sweat it too much and to do it the easiest/quickest way. The reason is that many of these decisions are arbitrary and don't really matter in the scheme of things.
I would avoid making the Person.PersonComparator class public outside of Person. The reason is that if something changes and you decide the comparators should be outside of Person, you could eventually have applications out there referring to the internals of Person. Now when you want to re-org Person, a bunch of code has to be changed. What a pain!
What you have to do is to understand the business and their needs going forward. If they are not sure how to sort Person then program defensively, perhaps, and put the comparators outside of it. Or implement an Interface or Facade design pattern to further insulate your code from the fickleness of the users. For items that are well defined, do the easiest thing. This affords more time for testing, reduces unhelpful unneeded abstractions, and increases the chance of project success.
I am programming in Java but this is a more of a design question so any OO programmer could probably answer this question. I have a question concerning the Strategy design pattern. Here are several inks I have found useful:
Strategy Pattern Explained-OO Design.
I am using the strategy pattern twice with one group of four strategies and one group of three. In each case I am deciding which strategy to use by maintaining a decaying counter. if the strategy the software decides to use is successful then the counter is incremented by one. If the strategy used is not successful then the counter is decremented by one. Regardless of success or failure ALL counters are multiplied by a number around .9 to "decay" the counters over time. The software will choose which strategy to use based on which strategy has the highest counter. An example of my very simple UML is shown below:
.
And in Link form (for easier reading):
Example UML
The UML above is the mockup I would like to use. If you can't tell from the above UML, I am writing a Rock, Paper, Scissors game with the intention of beating all of my friends.
Now, on to the problem:
I cannot decide how to implement the "counter system" for deciding which strategy to use. I was thinking about some kind of "data" class where all counters and history strings could be stored, but that just seemed clunky to me. At all times I am maintaining about 2 strings and about eight counters (maybe more maybe less). That is why I was thinking about a "data" class where everything could be stored. I could just instantiate the class to be used in the chooseStrategy() and chooseMetaStrategy() methods, but I just don't know. This is my first project that I will be working on my own for and I just cannot decide on anything. I feel like there is definitely a better solution but I am not experienced enough to know.
Thanks!
------------------------------------follow-up 1--------------------------------------------
Thank you so very much on everyone's answers and kind words. I do have a few follow up questions though. I am new to StackOverflow (and loving it) so if this is not the correct place for a followup question please let me know. I am editing my original post because my follow-up is a little lengthy.
I was looking into Paul Sonier's advice about using the composite pattern and it looked very interesting (thanks Paul!). For the purpose of the HistoryMatching and "intelligent" AntiRotation strategies I want to implement a string of all opponent plays accessible to both classes. Also, I want the history string to be edited no matter what strategy my program played so that I can keep an accurate record of the opponent's plays. The more comprehensive the string (actually I will probably use a LinkedList but if anyone knows of a better (sub-String/sub-List) search method/collection please let me know) the better the strategy can predict the opponent's behavior.
I was wondering how I could implement this "string" or collection while still using the composite pattern.
Also, TheCapn brought up that it would be a good idea to store different counters and history collections for each opponent. Any thought on how to implement this with the composite pattern?
Ideally, the intent is to have the counters be associated with the strategies, because they're counting the successes of the strategies. However, you don't necessarily want the strategies to know anything about the fact that they're being counted. To me, this indicates a Composite pattern, whereby you wrap your Strategy class in a class which has logic for tracking / degrading / modifying the count of usages.
This gives you locality (the count is stored with the strategy it's counting) and functional composition (count functionality is encapsulated in the composition class). As well, it maintains the isolation of the strategy class from other influences.
Your design breakdown so far looks good; you're certainly on a good and interesting path. Hope this helps!
Firstly, i'd suggest you try something a bit simpler: move-to-front. Keep your strategies in a list, initially in an arbitrary order. Work through them like this:
List<Strategy> strategies;
Strategy successfulStrategy = null;
for (Strategy strategy: strategies) {
boolean success = strategy.attempt();
if (success) {
break;
successfulStrategy = strategy;
}
}
if (successfulStrategy == null) throw new NoSuccessfulStrategyException();
// now move the successful strategy to the front of the list
strategies.remove(successfulStrategy);
strategies.add(0, successfulStrategy);
Basically, a successful strategy moves straight to the head of the queue; over time, good strategies accumulate near the head. It's not as subtle as something based on counts, but it's simple, and in practice, for all sorts of uses, it works very well.
However, if you're dead set on a count, then what i'd do is create a Decorator which wraps a strategy and keeps a count, and which can be compared with other such objects. Code is the easiest explanation:
public class ScoredStrategy implements Strategy, Comparable<ScoredStrategy> {
private final Strategy delegate;
private float score;
public ScoredStrategy(Strategy delegate) {
this.delegate = delegate;
}
public boolean attempt() {
boolean success = delegate.attempt();
score = (score * 0.9f) + (success ? 1 : -1);
return success;
}
public int compareTo(ScoredStrategy that) {
return -Float.compare(this.score, that.score);
}
}
In the main object, take your actual strategies, and wrap each in a ScoredStrategy. Put those in a list. When you need a strategy, work over the list, calling each strategy until you hit one that works. Then, simply sort the list. The strategies will then be in order, from best to worst.
Although "clunky" I think your intuition is correct. Store all data in a collection that can be accessed by the strategy you choose to implement. Keep separate data objects for each of your opponents and create new data objects when you define new opponents, storing them in a collection such as a Map will provide easy accessability.
The reason behind this is if/when you decide to change "MetaStrategies" you'll need the relevent information accessible to your objects, by storing them in other, abstract objects you may find the searching/parsing/gathering of data more difficult then it should be. This also closely matches the mental model you've generated for yourself so attempting to go against that flow will possibly lead to design mistakes.
The only other logical away around this (from my brief brainstorming) is to better define the surrounding logic or heuristics you plan to implement. If you create a more concrete way of picking a MetaStrategy you'll have a better understanding of how the data needs to be collected for access. Tom Anderson's method looks promising for your project!
So I'm working on creating a visualization for a data structure in Java. I already have the implementation of the data structure (Binary Search Tree) to start with, but I need to add some additional functionality to the included node class. As far as conventions and best practices are concerned, should I create a subclass of the node with this added functionality or should I just modify what I have and document it there?
My question is similar to what's asked here but that's a little over my head.
I know it probably doesn't matter much for what I'm doing, so I'm asking this more as a general thing.
Edit: I probably should have been more clear. My modifications don't actually change the original implementation other than to add a couple of extra fields (x and y coords plus a boolean to set whether that node is highlighted) and functions to access/modify those fields. Also the node class I'm working with is included in the BST implementation
From reading your answers it seems like there's arguments to be made for either case. I agree that creating a separate class or interface is probably the best thing to do in general. Creating another class seems like it could get tricky since you'd still need a way to extract the data out of the node. The BST implementation I'm using is generic and doesn't have any such functionality by itself in the Node class or the BST class to just return the data so at minimum I have to add that.
Thanks for the informative replies.
The question to answer is, is the 'base functionality' useful, even disirable, when you're not visualizing the data structure?
You might not even want to extend the class at all. Without more detail, it seems to me that you have a datastructure that works. You could create a NEW class that knows how to vizualise it.
That is, instead of a datastructure than knows how to visualize itself, you have a datastructure, and another class that knows how to visualize the datastructure. Heck - you may find that that evolves into another whole class hierarchy because you might need to visualize queues, stacks, etc. etc. NOTHING to do wiht your binary search tree.
Since you're asking in general, here's the short answer: it really depends on the situation.
First off, subclasses are assumed to have an "IS-A" relationship with their parent classes. If you can't say that your new subclass IS A specific kind of the original class, you're asking the wrong question, and should be making a new, unrelated class.
If the new code is closely related to the core purpose of the class, and applies to all members of the class (e.g. all BSTs), it may be better to modify. High cohesion is good.
If your new code is related to the core purpose of the class but has to do with only some objects of that type (e.g. only BSTs that are balanced), subclassing is probably the way to go.
Depending on what you're changing, how many places your code is used, how many different people/organizations are using it, &c., your changes might lead to unexpected behavior in other code, so you should think twice before modifying existing code. That doesn't mean automatically subclassing commonly used things; that would often be wrong for the reasons described above.
In your specific case, I agree with n8wrl; since visualization has nothing to do with data structures, it's probably better to implement a whole separate Visualizable interface than make a DrawableBSTNode subclass.
I would say that in the general case of adding functionality to an existing implementation, you should extend the existing implementation rather than modify it.
And here's my reasoning. If that node is used anywhere aside from the Binary Search Tree implementation, then when you modify it you'll need to find everywhere it is used to ensure that none of those places conflict with your modifications. While just adding functionality in the form of new methods generally won't cause problems, it could cause problems. You never know how an object is used.
Second, even if it is only used in the Binary Search Tree, you'll still need to make sure that the BST's implementation will play nice with your modifications.
Finally, if you do extend it, you don't have to worry about points one and two. And you get the added bonus of having your modifications kept separate from the original implementation for all time. This will make it easier to track what you have done and comment on it.
There's no simple answer, knowing when and how to add functionality is a something you have to learn over time.
Just adding to the base class seems like the easy solution, but it's polluting your base class. If this is a class you could reasonably expect another program (or even part of your program) to use does the functionality you are adding make sense in the context of your class's responsibility? If it doesn't this is probably a bad move. Are you adding dependencies linking your base class to your specific use? Because if you are that's throwing code reuse right out the window.
Inheriting is the solution a lot of engineers gravitate to, and it's a seductive route. But as I've grown as an engineer it's one that I use sparingly. Inheritance should only be used in true is-a relationships, and you need to respect behavioral subtyping
or you are going to regret it later on. And since Java only allows single inheritance it means you only get one shot at subtyping.
Composition (especially with interfaces) is often a better idea. Often what looks like a is-a relationship is really a has-a one. Or sometimes all you really need is a helper class, that has many functions that take your original class as an argument.
However with composition there is one issue, want to store these objects in your tree. The solution here is interfaces. You don't want a tree that stores Nodes. You want to objects that have an interface that can give you a node.
public interface HasNode {
public Node getNode();
}
Your node class is a HasNode with getNode just returning this. Your NodeVisualizer class is also a HasNode, and now you can store NodeVisualizers in your tree as well. Of course now you have another problem, your tree could contain NodeVisualizers and Nodes, and that wouldn't be good. Plus when you get a HasNode back from the tree functions you have to cast them to the right instance and that's ugly. You'll want to use templates for that, but that's another answer.
Mixing up logically independent functionalities will cause a mess. Subclassing is a very special relationship, often overused. Subclassing is for Is-a-Kind relationships.
If you want to visualize something, why not create a fully independent Class for that? You could simply pass your Node object to this. (Or even better, use an Interface.)
I keep hearing the statement on most programming related sites:
Program to an interface and not to an Implementation
However I don't understand the implications?
Examples would help.
EDIT: I have received a lot of good answers even so could you'll supplement it with some snippets of code for a better understanding of the subject. Thanks!
You are probably looking for something like this:
public static void main(String... args) {
// do this - declare the variable to be of type Set, which is an interface
Set buddies = new HashSet();
// don't do this - you declare the variable to have a fixed type
HashSet buddies2 = new HashSet();
}
Why is it considered good to do it the first way? Let's say later on you decide you need to use a different data structure, say a LinkedHashSet, in order to take advantage of the LinkedHashSet's functionality. The code has to be changed like so:
public static void main(String... args) {
// do this - declare the variable to be of type Set, which is an interface
Set buddies = new LinkedHashSet(); // <- change the constructor call
// don't do this - you declare the variable to have a fixed type
// this you have to change both the variable type and the constructor call
// HashSet buddies2 = new HashSet(); // old version
LinkedHashSet buddies2 = new LinkedHashSet();
}
This doesn't seem so bad, right? But what if you wrote getters the same way?
public HashSet getBuddies() {
return buddies;
}
This would have to be changed, too!
public LinkedHashSet getBuddies() {
return buddies;
}
Hopefully you see, even with a small program like this you have far-reaching implications on what you declare the type of the variable to be. With objects going back and forth so much it definitely helps make the program easier to code and maintain if you just rely on a variable being declared as an interface, not as a specific implementation of that interface (in this case, declare it to be a Set, not a LinkedHashSet or whatever). It can be just this:
public Set getBuddies() {
return buddies;
}
There's another benefit too, in that (well at least for me) the difference helps me design a program better. But hopefully my examples give you some idea... hope it helps.
One day, a junior programmer was instructed by his boss to write an application to analyze business data and condense it all in pretty reports with metrics, graphs and all that stuff. The boss gave him an XML file with the remark "here's some example business data".
The programmer started coding. A few weeks later he felt that the metrics and graphs and stuff were pretty enough to satisfy the boss, and he presented his work. "That's great" said the boss, "but can it also show business data from this SQL database we have?".
The programmer went back to coding. There was code for reading business data from XML sprinkled throughout his application. He rewrote all those snippets, wrapping them with an "if" condition:
if (dataType == "XML")
{
... read a piece of XML data ...
}
else
{
.. query something from the SQL database ...
}
When presented with the new iteration of the software, the boss replied: "That's great, but can it also report on business data from this web service?" Remembering all those tedious if statements he would have to rewrite AGAIN, the programmer became enraged. "First xml, then SQL, now web services! What is the REAL source of business data?"
The boss replied: "Anything that can provide it"
At that moment, the programmer was enlightened.
An interface defines the methods an object is commited to respond.
When you code to the interface, you can change the underlying object and your code will still work ( because your code is agnostic of WHO do perform the job or HOW the job is performed ) You gain flexibility this way.
When you code to a particular implementation, if you need to change the underlying object your code will most likely break, because the new object may not respond to the same methods.
So to put a clear example:
If you need to hold a number of objects you might have decided to use a Vector.
If you need to access the first object of the Vector you could write:
Vector items = new Vector();
// fill it
Object first = items.firstElement();
So far so good.
Later you decided that because for "some" reason you need to change the implementation ( let's say the Vector creates a bottleneck due to excessive synchronization)
You realize you need to use an ArrayList instad.
Well, you code will break ...
ArrayList items = new ArrayList();
// fill it
Object first = items.firstElement(); // compile time error.
You can't. This line and all those line who use the firstElement() method would break.
If you need specific behavior and you definitely need this method, it might be ok ( although you won't be able to change the implementation ) But if what you need is to simply retrieve the first element ( that is , there is nothing special with the Vector other that it has the firstElement() method ) then using the interface rather than the implementation would give you the flexibility to change.
List items = new Vector();
// fill it
Object first = items.get( 0 ); //
In this form you are not coding to the get method of Vector, but to the get method of List.
It does not matter how do the underlying object performs the method, as long as it respond to the contract of "get the 0th element of the collection"
This way you may later change it to any other implementation:
List items = new ArrayList(); // Or LinkedList or any other who implements List
// fill it
Object first = items.get( 0 ); // Doesn't break
This sample might look naive, but is the base on which OO technology is based ( even on those language which are not statically typed like Python, Ruby, Smalltalk, Objective-C etc )
A more complex example is the way JDBC works. You can change the driver, but most of your call will work the same way. For instance you could use the standard driver for oracle databases or you could use one more sophisticated like the ones Weblogic or Webpshere provide . Of course it isn't magical you still have to test your product before, but at least you don't have stuff like:
statement.executeOracle9iSomething();
vs
statement.executeOracle11gSomething();
Something similar happens with Java Swing.
Additional reading:
Design Principles from Design Patterns
Effective Java Item: Refer to objects by their interfaces
( Buying this book the one of the best things you could do in life - and read if of course - )
My initial read of that statement is very different than any answer I've read yet. I agree with all the people that say using interface types for your method params, etc are very important, but that's not what this statement means to me.
My take is that it's telling you to write code that only depends on what the interface (in this case, I'm using "interface" to mean exposed methods of either a class or interface type) you're using says it does in the documentation. This is the opposite of writing code that depends on the implementation details of the functions you're calling. You should treat all function calls as black boxes (you can make exceptions to this if both functions are methods of the same class, but ideally it is maintained at all times).
Example: suppose there is a Screen class that has Draw(image) and Clear() methods on it. The documentation says something like "the draw method draws the specified image on the screen" and "the clear method clears the screen". If you wanted to display images sequentially, the correct way to do so would be to repeatedly call Clear() followed by Draw(). That would be coding to the interface. If you're coding to the implementation, you might do something like only calling the Draw() method because you know from looking at the implementation of Draw() that it internally calls Clear() before doing any drawing. This is bad because you're now dependent on implementation details that you can't know from looking at the exposed interface.
I look forward to seeing if anyone else shares this interpretation of the phrase in the OP's question, or if I'm entirely off base...
It's a way to separate responsibilities / dependancies between modules.
By defining a particular Interface (an API), you ensure that the modules on either side of the interface won't "bother" one another.
For example, say module 1 will take care of displaying bank account info for a particular user, and module2 will fetch bank account info from "whatever" back-end is used.
By defining a few types and functions, along with the associated parameters, for example a structure defining a bank transaction, and a few methods (functions) like GetLastTransactions(AccountNumber, NbTransactionsWanted, ArrayToReturnTheseRec) and GetBalance(AccountNumer), the Module1 will be able to get the needed info, and not worry about how this info is stored or calculated or whatever. Conversely, the Module2 will just respond to the methods call by providing the info as per the defined interface, but won't worry about where this info is to be displayed, printed or whatever...
When a module is changed, the implementation of the interface may vary, but as long as the interface remains the same, the modules using the API may at worst need to be recompiled/rebuilt, but they do not need to have their logic modified in anyway.
That's the idea of an API.
At its core, this statement is really about dependencies. If I code my class Foo to an implementation (Bar instead of IBar) then Foo is now dependent on Bar. But if I code my class Foo to an interface (IBar instead of Bar) then the implementation can vary and Foo is no longer dependent on a specific implementation. This approach gives a flexible, loosely-coupled code base that is more easily reused, refactored and unit tested.
Take a red 2x4 Lego block and attach it to a blue 2x4 Lego block so one sits atop the other. Now remove the blue block and replace it with a yellow 2x4 Lego block. Notice that the red block did not have to change even though the "implementation" of the attached block varied.
Now go get some other kind of block that does not share the Lego "interface". Try to attach it to the red 2x4 Lego. To make this happen, you will need to change either the Lego or the other block, perhaps by cutting away some plastic or adding new plastic or glue. Notice that by varying the "implementation" you are forced to change it or the client.
Being able to let implementations vary without changing the client or the server - that is what it means to program to interfaces.
An interface is like a contract between you and the person who made the interface that your code will carry out what they request. Furthermore, you want to code things in such a way that your solution can solve the problem many times over. Think code re-use. When you are coding to an implementation, you are thinking purely of the instance of a problem that you are trying to solve. So when under this influence, your solutions will be less generic and more focused. That will make writing a general solution that abides by an interface much more challenging.
Look, I didn't realize this was for Java, and my code is based on C#, but I believe it provides the point.
Every car have doors.
But not every door act the same, like in UK the taxi doors are backwards. One universal fact is that they "Open" and "Close".
interface IDoor
{
void Open();
void Close();
}
class BackwardDoor : IDoor
{
public void Open()
{
// code to make the door open the "wrong way".
}
public void Close()
{
// code to make the door close properly.
}
}
class RegularDoor : IDoor
{
public void Open()
{
// code to make the door open the "proper way"
}
public void Close()
{
// code to make the door close properly.
}
}
class RedUkTaxiDoor : BackwardDoor
{
public Color Color
{
get
{
return Color.Red;
}
}
}
If you are a car door repairer, you dont care how the door looks, or if it opens one way or the other way. Your only requirement is that the door acts like a door, such as IDoor.
class DoorRepairer
{
public void Repair(IDoor door)
{
door.Open();
// Do stuff inside the car.
door.Close();
}
}
The Repairer can handle RedUkTaxiDoor, RegularDoor and BackwardDoor. And any other type of doors, such as truck doors, limousine doors.
DoorRepairer repairer = new DoorRepairer();
repairer.Repair( new RegularDoor() );
repairer.Repair( new BackwardDoor() );
repairer.Repair( new RedUkTaxiDoor() );
Apply this for lists, you have LinkedList, Stack, Queue, the normal List, and if you want your own, MyList. They all implement the IList interface, which requires them to implement Add and Remove. So if your class add or remove items in any given list...
class ListAdder
{
public void PopulateWithSomething(IList list)
{
list.Add("one");
list.Add("two");
}
}
Stack stack = new Stack();
Queue queue = new Queue();
ListAdder la = new ListAdder()
la.PopulateWithSomething(stack);
la.PopulateWithSomething(queue);
Allen Holub wrote a great article for JavaWorld in 2003 on this topic called Why extends is evil. His take on the "program to the interface" statement, as you can gather from his title, is that you should happily implement interfaces, but very rarely use the extends keyword to subclass. He points to, among other things, what is known as the fragile base-class problem. From Wikipedia:
a fundamental architectural problem of object-oriented programming systems where base classes (superclasses) are considered "fragile" because seemingly safe modifications to a base class, when inherited by the derived classes, may cause the derived classes to malfunction. The programmer cannot determine whether a base class change is safe simply by examining in isolation the methods of the base class.
In addition to the other answers, I add more:
You program to an interface because it's easier to handle. The interface encapsulates the behavior of the underlying class. This way, the class is a blackbox. Your whole real life is programming to an interface. When you use a tv, a car, a stereo, you are acting on its interface, not on its implementation details, and you assume that if implementation changes (e.g. diesel engine or gas) the interface remains the same. Programming to an interface allows you to preserve your behavior when non-disruptive details are changed, optimized, or fixed. This simplifies also the task of documenting, learning, and using.
Also, programming to an interface allows you to delineate what is the behavior of your code before even writing it. You expect a class to do something. You can test this something even before you write the actual code that does it. When your interface is clean and done, and you like interacting with it, you can write the actual code that does things.
"Program to an interface" can be more flexible.
For example, we are writing a class Printer which provides print service. currently there are 2 class (Cat and Dog) need to be printed. So we write code like below
class Printer
{
public void PrintCat(Cat cat)
{
...
}
public void PrintDog(Dog dog)
{
...
}
...
}
How about if there is a new class Bird also needs this print service? We have to change Printer class to add a new method PrintBird(). In real case, when we develop Printer class, we may have no idea about who will use it. So how to write Printer? Program to an interface can help, see below code
class Printer
{
public void Print(Printable p)
{
Bitmap bitmap = p.GetBitmap();
// print bitmap ...
}
}
With this new Printer, everything can be printed as long as it implements Interface Printable. Here method GetBitmap() is just a example. The key thing is to expose an Interface not a implementation.
Hope it's helpful.
Essentially, interfaces are the slightly more concrete representation of general concepts of interoperation - they provide the specification for what all the various options you might care to "plug in" for a particular function should do similarly so that code which uses them won't be dependent on one particular option.
For instance, many DB libraries act as interfaces in that they can operate with many different actual DBs (MSSQL, MySQL, PostgreSQL, SQLite, etc.) without the code that uses the DB library having to change at all.
Overall, it allows you to create code that's more flexible - giving your clients more options on how they use it, and also potentially allowing you to more easily reuse code in multiple places instead of having to write new specialized code.
By programming to an interface, you are more likely to apply the low coupling / high cohesion principle.
By programming to an interface, you can easily switch the implementation of that interface (the specific class).
It means that your variables, properties, parameters and return types should have an interface type instead of a concrete implementation.
Which means you use IEnumerable<T> Foo(IList mylist) instead of ArrayList Foo(ArrayList myList) for example.
Use the implementation only when constructing the object:
IList list = new ArrayList();
If you have done this you can later change the object type maybe you want to use LinkedList instead of ArrayList later on, this is no problem since everywhere else you refer to it as just "IList"
It's basically where you make a method/interface like this: create( 'apple' ) where the method create(param) comes from an abstract class/interface fruit that is later implemented by concrete classes. This is different than subclassing. You are creating a contract that classes must fulfill. This also reduces coupling and making things more flexible where each concrete class implements it differently.
The client code remains unaware of the specific types of objects used and remains unaware of the classes that implement these objects. Client code only knows about the interface create(param) and it uses it to make fruit objects. It's like saying, "I don't care how you get it or make it I, just want you to give it to me."
An analogy to this is a set of on and off buttons. That is an interface on() and off(). You can use these buttons on several devices, a TV, radio, light. They all handle them differently but we don't care about that, all we care about is to turn it on or turn it off.
Coding to an interface is a philosophy, rather than specific language constructs or design patterns - it instructs you what is the correct order of steps to follow in order to create better software systems (e.g. more resilient, more testable, more scalable, more extendible, and other nice traits).
What it actually means is:
===
Before jumping to implementations and coding (the HOW) - think of the WHAT:
What black boxes should make up your system,
What is each box' responsibility,
What are the ways each "client" (that is, one of those other boxes, 3rd party "boxes", or even humans) should communicate with it (the API of each box).
After you figure the above, go ahead and implement those boxes (the HOW).
Thinking first of what a box' is and what its API, leads the developer to distil the box' responsibility, and to mark for himself and future developers the difference between what is its exposed details ("API") and it's hidden details ("implementation details"), which is a very important differentiation to have.
One immediate and easily noticeable gain is the team can then change and improve implementations without affecting the general architecture. It also makes the system MUCH more testable (it goes well with the TDD approach).
===
Beyond the traits I've mentioned above, you also save A LOT OF TIME going this direction.
Micro Services and DDD, when done right, are great examples of "Coding to an interface", however the concept wins in every pattern from monoliths to "serverless", from BE to FE, from OOP to functional, etc....
I strongly recommend this approach for Software Engineering (and I basically believe it makes total sense in other fields as well).