Related
I have studying Java, and wanna understand how to realize idea of such factory of products?
(or other way structure of code)
public interface VirtualBD <E extends Enum<E>> {
void addInStorage(Class<E> type, Product product, int amount);
}
1 Question: how to use generic to get any type of Enum class as parameter
Root Categorie
public abstract class Product {
...
}
public enum AlchogolType {
Beer, Vodka;
}
public enum nonAlchogolType {
FreshJuise, Lemonade;
}
SubCategories
public abstract class Alchogol extends Product {
...
}
public abstract class nonAlchogol extends Product {
...
}
public class Beer extends Alchogol {
...
}
And, here is a problem begin's:
public class AlchogolTables implements VirtualBD{
HashMap<Alchogol, Integer> beer = new HashMap<Alchogol, Integer>();
HashMap<Alchogol, Integer> vodka = new HashMap<Alchogol, Integer>();
#Override
public void addInStorage(AlchogolType type, Product product, int amount) {
switch (type) {
case Beer:
beer.put((Alchogol) product,amount);
break;
case Vodka:
vodka.put((Alchogol) product,amount);
break;
default:
break;
}
}
}
in my idea - i want to use addInStorage method for different products, like:
public class OtherBeveragesTables implements VirtualBD{
HashMap<nonAlchogol, Integer> orangeFresh = new HashMap<nonAlchogol, Integer>();
HashMap<nonAlchogol, Integer> soda = new HashMap<nonAlchogol, Integer>();
#Override
public void addInStorage(nonAlchogolType type, Product product, int amount) {
switch (type) {
case FreshJuise:
orangeFresh.put((nonAlchogol) product,amount);
break;
case Lemonade:
soda.put((nonAlchogol) product,amount);
break;
default:
break;
}
}
}
how can i use Enum AlchogolType/nonAlchogolType as parametr?
I doubt the correctness of the organization of the code for such a task as: writing a factory for many products with categories, some differences and similarities.
Is a CAST: beer.put((Alchogol) product,amount); normal way?
Question 1
"how can i use Enum AlchogolType/nonAlchogolType as parametr?"
When reading your question, I assume that you are looking for a way on how you can pass both AlchogolType and NonAlchogolType as parameter in the same method. Unfortunately this is not possible, since a method asks for a specific type. Unlike classes, enums cannot override from other classes/enums. This means that the parameter type given in the method cannot be anything else than that.
Let's say there is EnumOne (APPLE, BANANA, PINEAPPLE) and EnumTwo (AUDI, BWM, FORD). If a method asks for a parameter of type EnumOne there is no way of passing anything else that is not defined in the EnumOne enum. You will get compiler errors when trying to pass values from, let's say, EnumTwo.
If you would like to have a work-around for this, you can try and put the values in one enum class. Only do this if your code can still remain maintainable and efficient.
Question 2
"I doubt the correctness of the organization of the code for such a task as: writing a factory for many products with categories, some differences and similarities."
Even though you are using abstraction to divide logic from your models in your code, you are still using separate classes (VirtualDB classes in your case) that prevent your program from using the benefits of abstraction in Object-oriented-programming.
Using abstractions in your model classes (Beer in your case), is good. I would recommend keeping it like that and not go further with applying abstraction in combination with your factory classes.
I believe you only need 1 factory class that holds all of your products. You can make a HashMap for every type of drink, but that makes it so you always have to update your factory class when you make a new type of drink. What you can do is make a single HashMap with as key Product and as value Integer and then save your enum of the type of drink in the Product class. The reason is because the type of drink is a property of a product. Now the only way to tell the type of drink is by knowing in what HashMap it is saved in. That seems inefficient considering later in your code you might have access to a Product instance, without having access to the HashMap in your factory. When saving the type of drink in the Product instance itself, you can make a method that returns the type of drink and that's it.
Question 3
"Is a CAST: beer.put((Alchogol) product,amount); normal way?"
Whether this is ok or not really depends on what you did beforehand. This all has to do with the "A dog is an animal, but an animal is not always a dog" story.
Let's say you have an abstract class Animal and two classes Dog and Cat that both inherit from the Animal class. If you have an instance of an Animal inherited class that you want to cast to Dog and you did not check before if this instance is actually a dog, you are asking for errors to be thrown. The instance could be an instance of a Dog class, but it could also be an instance of a Cat class. If you try to cast it and you are casting it to the wrong inherited type, you will get a java.lang.ClassCastException.
To solve the probable situation where this error could be thrown, always check beforehand if the variable you are trying to cast is of a specific inherited type. You should have a look at the instanceof keyword.
FYI, modern IDEs will most of the time give you a warning when you try to cast a variable if you did not check it beforehand.
My main language is not English. I tried to explain it the best I could. If you do have comments or anything that you would like to let me know, feel free to edit my comment or put a comment under it.
As I proceed through my online tutorial, I came across this lesson. I have an interface and two classes that implements that interface.
public interface Payable {
double getPaymentAmount();
}
and
class Invoice that implements the above interface Payable
class SalariedEmployee that extends an abstract class Employee which implements Payable interface.
A test class that contains the main method to test this.
Now in the test class, when creating an array of objects, the type of object was referred to as Payable[] rather than SalariedEmployee[] or Invoice[], like
public static void main(String[] args) {
Payable[] payableObjects = new Payable[4];
payableObjects[0] = new Invoice("0000", "abc", 1,2);
payableObjects[1] = new SalariedEmployee("test", "user", "000-000", 35);
Is this because all the classes implements the interface Payable[] ?
If an interface is defined at the top level hierarchy, is it always possible to create objects of all the classes that implements that interface?
Your question title is not grammatical, but the word choice suggests there's a subtle misunderstanding of concepts going on.
You can only create a class as that class. That is, you can't write new Payable and somehow expect there to be any Invoice or SalariedEmployee objects being created.
But you can make a reference to a Payable point to any object that implements Payable. That's pretty much the fundamental idea of subtype polymorphism.
What's going on in the example is that an array of four Payable references is created. These references don't yet point at anything (they're null) and there are no Invoice or SalariedEmployee objects around yet, only the array object.
Then, the code creates two objects and assigns them to two of the references. This works the same as if the array elements were independent variables. (Almost, but the difference is due to array covariance, which isn't important at this point.) In particular, the result of the new operator has the type Invoice in the first case, not Payable. You can assign the Invoice to the Payable reference, but the conversion happens at assignment, not at creation.
So technically, the answer to your title question is "no, objects are always created as the type you specify in the new expression, but you can convert the references afterwards".
for your first question: yes, you would get a compiler error if it wasn´t the case.
for the second case, take the List or the Map as an example. Look at the following example. We declare a list but depending on the the flag, we want the this specific List to act different, since it represents a different class
public class BookList{
List <String> list;
public BookList(boolean flag) {
if(flag) {
list = new ArrayList<>();
} else {
list = new LinkedList<>();
}
}
}
Since we declared it just as List we are able to assign different type of lists that implement this interface. You could change the usecase of this class very simple, while you are still able to acces every method that is provided by the interface.
That´s what your array of Payable is doing aswell. You want to assign different types of classes into this array, which all implement this interface.
This would make it more easy, to create methods for this specific interface. Take a summing method as an example for your case.
public int sumPayable(Payable[] payables) {
int sum = 0;
for(Payable p : payables) {
sum += p.getPaymentAmount();
}
return sum;
}
In this case, it wouldn´t matter what the actuall class of each of this classes that do implements Payable is, since you could simply pass an array, like you did create, to this method.
I have came across the following point the advantage of object composition over class inheritance. But I often see the following sentence in many articles
In object composition, functionality is acquired dynamically at run-time by objects collecting references to other
objects. The advantage of this approach is that implementations can be replaced at run-time. This is possible because
objects are accessed only through their interfaces, so one object can be replaced with another just as long as they
have the same type.
But doubt may be naive, since i am a beginner. How the implementation can be replaced in runtime? If we write a new line of code, don't we need to compile to reflect the change? Then what is meant be replacing at runtime? quite confusing.
Or any other magic, behind the scene activities happen. Can anyone please reply.
Think of an implementation of Stack. A simple implementation of Stack utilizes a List behind the scenes. So naively, you could extend ArrayList. But now if you want a separate Stack backed by a LinkedList instead, you would have to have two classes: ArrayListStack and LinkedListStack. (This approach also has the disadvantage of exposing List methods on a Stack, which violates encapsulation).
If you used composition instead, the List to back the Stack could be provided by the caller, and you could have one Stack class that could take either a LinkedList or an ArrayList, depending on the runtime characteristics desired by the user.
In short, the ability of the implementation to "change at runtime" refers not to an instance of the class being able to change its implementation at runtime, but rather that the class does not know at compile time what its precise implementation will be.
Also note that a class using composition need not allow the delegate implementation to be chosen at runtime (by the caller). Sometimes doing so would violate encapsulation, as it would give the caller more information about the internals of the class than is desirable. In these cases, composition still carries the benefits of only exposing the methods of the abstraction, and allowing the concrete implementation to be changed in a later revision.
Real-life examples
By the way, I use the example of Stack because it's not purely hypothetical. Java's Stack class in fact extended Vector, which made it forever carry the baggage of synchronization and the performance characteristics of an array-backed list. As a result, using that class is heavily discouraged.
A perfect example of correctly using composition for a collection can also be found in the Java library, in Collections.newSetFromMap(Map). Since any Map can be used to represent a Set (by using dummy values), this method returns a Set composed of the passed-in Map. The returned Set then inherits the characteristics of the Map it wraps, for example: mutability, thread safety, and runtime performance--all without having to create parallel Set implementations to ConcurrentHashMap, ImmutableMap, TreeMap, etc.
There are two strong reasons to prefer composition over inheritance:
Avoids combinatorial explosions in class hierarchy.
Can be modified at run-time
Let's say that you are writing an ordering system for a pizza parlor. You would almost certainly have a class Pizza...
public class Pizza {
public double getPrice() { return BASE_PIZZA_PRICE; }
}
And, all else being equal, the pizza parlor probably sells a lot of pepperoni pizza. You can use inheritance for this - PepperoniPizza satisfies an "is-a" relationship with pizza, so that sounds valid.
public class PepperoniPizza extends Pizza {
public double getPrice() { return super.getPrice() + PEPPERONI_PRICE; }
}
Okay, so far so good, right? But you can probably see that things we haven't considered. What if a customer wants pepperoni and mushrooms, for instance? Well, we can add a PepperoniMushroomPizza class. Already we have a problem - should PepperoniMushroomPizza extend Pizza, PepperoniPizza, or MushroomPizza?
But things get even worse. Let's say our hypothetical pizza parlor offers sizes Small, Medium, and Large. And crust varies too - they offer a thick, thin, and regular crust. If we are just using inheritance, suddenly we have classes like MediumThickCrustPepperoniPizza, LargeThinCrustMushroomPizza, SmallRegularCrustPepperoniAndMushroomPizza, et cetera...
public class LargeThinCrustMushroomPizza extends ThinCrustMushroomPizza {
// This is not good!
}
In short, using inheritance to handle diversity along multiple axes causes a combinatorial explosion in the class hierarchy.
The second problem (modification at run-time) derives from this as well. Suppose that a customer looks at the price of their LargeThinCrustMushroomPizza, gawks, and decides they'd rather get a MediumThinCrustMushroomPizza instead? Now you are stuck making a whole new object just to change that one attribute!
This is where composition comes in. We observe that a "pepperoni pizza" does indeed have an "is-a" relationship with Pizza, but it also satisfies a "has-a" relationship with Pepperoni. And it also satisfies "has-a" relationships with crust type, and size. So you re-define Pizza using composition:
public class Pizza {
private List<Topping> toppings;
private Crust crust;
private Size size;
//...omitting constructor, getters, setters for brevity...
public double getPrice() {
double price = size.getPrice();
for (Topping topping : toppings) {
price += topping.getPriceAtSize(size);
}
return price;
}
}
With this composition-based Pizza, the customer can choose a smaller size (pizza.setSize(new SmallSize())) and the price (getPrice()) will respond appropriately - that is, the run-time behavior of the method may vary according to the run-time composition of the object.
This is not to say that inheritance is bad. But where it is possible to use composition instead of inheritance to express a diversity of objects (like pizzas), composition should usually be preferred.
The other answers speak a bit about this, but I thought that an example of how the behaviour can change at runtime would be helpfull. Suppose you have a interface Printer:
interface Printer {
void print(Printable printable);
}
class TestPrinter implements Printer {
public void print(Printable printable) {
// set an internal state that can be checked later in a test
}
}
class FilePrinter implements Printer {
public void print(Printable printable) {
// Do stuff to print the printable to a file
}
}
class NetworkPrinter implements Printer {
public void print(Printable printable) {
// Connects to a networked printer and tell it to print the printable
}
}
All of the Printer classes can now be used for different purposes. TestPrinter can be used as a mock or stubb when we run tests. FilePrinter and NetworkPrinter each handle a specific case when printing. So assume we have a UI widget where the user can press a button to print something:
class PrintWidget {
// A collection of printers that keeps track of which printer the user has selected.
// It could contain a FilePrinter, NetworkPrinter and any other object implementing the
// Printer interface
private Selectable<Printer> printers;
// A reference to a printable object, could be a document or image or something
private Printable printable;
public void onPrintButtonPressed() {
Printer printer = printers.getSelectedPrinter();
printer.print(printable);
}
// other methods
}
Now at runtime when the user selects another printer and presses the a print button the onPrintButtonPressed method gets called and the selected Printer used.
That is the Polymorphism which is the core concept of OOP.
It means ‘a state of having many shapes’ or ‘the capacity to take on different forms’. When applied to object oriented programming languages like Java, it describes a language’s ability to process objects of various types and classes through a single, uniform interface.
As mark said List is a Uniform interface and its Different implementations are like ArrayList.....etc
This is interesting to answer. I am not sure whether you have used factory pattern or not. But if yo have then understanding this with that example should be good. Let me try to put it here:
Suppose you have a parent class called Pet as defined here
package com.javapapers.sample.designpattern.factorymethod;
//super class that serves as type to be instantiated for factory method pattern
public interface Pet {
public String speak();
}
And there are few subclasses such as Dog, Duck etc, sample here:
package com.javapapers.sample.designpattern.factorymethod;
//sub class 1 that might get instantiated by a factory method pattern
public class Dog implements Pet {
public String speak() {
return "Bark bark...";
}
}
package com.javapapers.sample.designpattern.factorymethod;
//sub class 2 that might get instantiated by a factory method pattern
public class Duck implements Pet {
public String speak() {
return "Quack quack...";
}
}
And there is a factory class which returns you a Pet depending on the input type, sample here:
package com.javapapers.sample.designpattern.factorymethod;
//Factory method pattern implementation that instantiates objects based on logic
public class PetFactory {
public Pet getPet(String petType) {
Pet pet = null;
// based on logic factory instantiates an object
if ("bark".equals(petType))
pet = new Dog();
else if ("quack".equals(petType))
pet = new Duck();
return pet;
}
}
Now lets see how at the run time we can have different kind of Pets created depending on the input, sample here
//using the factory method pattern
public class SampleFactoryMethod {
public static void main(String args[]) {
// creating the factory
PetFactory petFactory = new PetFactory();
System.out.println("Enter a pets language to get the desired pet");
String input = "";
try {
BufferedReader bufferRead = new BufferedReader(
new InputStreamReader(System.in));
input = bufferRead.readLine();
// factory instantiates an object
Pet pet = petFactory.getPet(input);
// you don't know which object factory created
System.out.println(pet.speak());
} catch (IOException e) {
e.printStackTrace();
}
}
}
Now if you run the program for different kind of inputs such as "bark" or "quack", you will get a differnt pet. You can change the above program to take different inputs and create different Pets.
Here it answers your question that without changing the code, just depending on type of input you get different behavioral pets.
Hope it helps!
How the implementation can be replaced in runtime?
Let's use some code example to brighten the day (a loop that reads a new line everytime, and reprint all lines read so far):
List<String> myList = new ArrayList<String>(); // chose first "implementation"
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
while (true) {
String line = br.readLine(); // do something like read input from the user
myCounter.resetChronometer(); // hypothetical time counter
myList.add(line); // add the line (make use my "implementation")
// and then do some final work, like printing...
for(String s: myList) {
System.out.println(s); // print it all...
}
//But, hey, I'm keeping track of the time:
myCounter.stopChronometer();
if (myCounter.isTimeTakenTooLong())
// this "implementation" is too slow! I want to replace it.
// I WILL replace it at runtime (no recompile, not even stopping)
List<String> swapList = myList; // just to keep track...
myList = new LinkedList<String>(); // REPLACED implementation! (!!!) <---
myList.addAll(swapList); // so I don't lose what I did up until now
// from now on, the loop will operate with the
// new implementation of the List<String>
// was using the ArrayList implementation. Now will use LinkedList
}
}
Just as you said: This is [only] possible because objects [myList] are accessed only through their interfaces (List<String>). (If we declared myList as ArrayList<String> myList, this would never be possible...)
In English, a homograph pair is two words that have the same spelling but different meanings.
In software engineering, a pair of homographic methods is two methods with the same name but different requirements. Let's see a contrived example to make the question as clear as possible:
interface I1 {
/** return 1 */
int f()
}
interface I2 {
/** return 2*/
int f()
}
interface I12 extends I1, I2 {}
How can I implement I12? C# has a way to do this, but Java doesn't. So the only way around is a hack. How can it be done with reflection/bytecode tricks/etc most reliably (i.e it doesn't have to be a perfect solution, I just want the one that works the best)?
Note that some existing closed source massive piece of legacy code which I cannot legally reverse engineer requires a parameter of type I12 and delegates the I12 both to code that has I1 as a parameter, and code that has I2 as a parameter. So basically I need to make an instance of I12 that knows when it should act as I1 and when it should act as I2, which I believe can be done by looking at the bytecode at runtime of the immediate caller. We can assume that no reflection is used by the callers, because this is straightforward code. The problem is that the author of I12 didn't expect that Java merges f from both interfaces, so now I have to come up with the best hack around the problem. Nothing calls I12.f (obviously if the author wrote some code that actually calls I12.f, he would have noticed the problem before selling it).
Note that I'm actually looking for an answer to this question, not how to restructure the code that I can't change. I'm looking for the best heuristic possible or an exact solution if one exists. See Gray's answer for a valid example (I'm sure there are more robust solutions).
Here is a concrete example of how the problem of homographic methods within two interfaces can happen. And here is another concrete example:
I have the following 6 simple classes/interfaces. It resembles a business around a theater and the artists who perform in it. For simplicity and to be specific, let's assume they are all created by different people.
Set represents a set, as in set theory:
interface Set {
/** Complements this set,
i.e: all elements in the set are removed,
and all other elements in the universe are added. */
public void complement();
/** Remove an arbitrary element from the set */
public void remove();
public boolean empty();
}
HRDepartment uses Set to represent employees. It uses a sophisticated process to decode which employees to hire/fire:
import java.util.Random;
class HRDepartment {
private Random random = new Random();
private Set employees;
public HRDepartment(Set employees) {
this.employees = employees;
}
public void doHiringAndLayingoffProcess() {
if (random.nextBoolean())
employees.complement();
else
employees.remove();
if (employees.empty())
employees.complement();
}
}
The universe of a Set of employees would probably be the employees who have applied to the employer. So when complement is called on that set, all the existing employees are fired, and all the other ones that applied previously are hired.
Artist represents an artist, such as a musician or an actor. An artist has an ego. This ego can increase when others compliment him:
interface Artist {
/** Complements the artist. Increases ego. */
public void complement();
public int getEgo();
}
Theater makes an Artist perform, which possibly causes the Artist to be complemented. The theater's audience can judge the artist between performances. The higher the ego of the performer, the more likely the audience will like the Artist, but if the ego goes beyond a certain point, the artist will be viewed negatively by the audience:
import java.util.Random;
public class Theater {
private Artist artist;
private Random random = new Random();
public Theater(Artist artist) {
this.artist = artist;
}
public void perform() {
if (random.nextBoolean())
artist.complement();
}
public boolean judge() {
int ego = artist.getEgo();
if (ego > 10)
return false;
return (ego - random.nextInt(15) > 0);
}
}
ArtistSet is simply an Artist and a Set:
/** A set of associated artists, e.g: a band. */
interface ArtistSet extends Set, Artist {
}
TheaterManager runs the show. If the theater's audience judges the artist negatively, the theater talks to the HR department, which will in turn fire artists, hire new ones, etc:
class TheaterManager {
private Theater theater;
private HRDepartment hr;
public TheaterManager(ArtistSet artists) {
this.theater = new Theater(artists);
this.hr = new HRDepartment(artists);
}
public void runShow() {
theater.perform();
if (!theater.judge()) {
hr.doHiringAndLayingoffProcess();
}
}
}
The problem becomes clear once you try to implement an ArtistSet: both superinterfaces specify that complement should do something else, so you have to implement two complement methods with the same signature within the same class, somehow. Artist.complement is a homograph of Set.complement.
New idea, kinda messy...
public class MyArtistSet implements ArtistSet {
public void complement() {
StackTraceElement[] stackTraceElements = Thread.currentThread().getStackTrace();
// the last element in stackTraceElements is the least recent method invocation
// so we want the one near the top, probably index 1, but you might have to play
// with it to figure it out: could do something like this
boolean callCameFromHR = false;
boolean callCameFromTheatre = false;
for(int i = 0; i < 3; i++) {
if(stackTraceElements[i].getClassName().contains("Theatre")) {
callCameFromTheatre = true;
}
if(stackTraceElements[i].getClassName().contains("HRDepartment")) {
callCameFromHR = true;
}
}
if(callCameFromHR && callCameFromTheatre) {
// problem
}
else if(callCameFromHR) {
// respond one way
}
else if(callCameFromTheatre) {
// respond another way
}
else {
// it didn't come from either
}
}
}
Despite Gray Kemmey's valiant attempt, I would say the problem as you have stated it is not solvable. As a general rule given an ArtistSet you cannot know whether the code calling it was expecting an Artist or a Set.
Furthermore, even if you could, according to your comments on various other answers, you actually have a requirement to pass an ArtistSet to a vendor-supplied function, meaning that function has not given the compiler or humans any clue as to what it is expecting. You are completely out of luck for any sort of technically correct answer.
As practical programming matter for getting the job done, I would do the following (in this order):
File a bug report with whoever created an interface requiring ArtistSet and whoever generated the ArtistSet interface itself.
File a support request with the vendor supplying the function requiring an ArtistSet and ask them what they expect the behavior of complement() to be.
Implement the complement() function to throw an exception.
public class Sybil implements ArtistSet {
public void complement() {
throw new UnsupportedOperationException('What am I supposed to do');
}
...
}
Because seriously, you don't know what to do. What would be the correct thing to do when called like this (and how do you know for sure)?
class TalentAgent {
public void pr(ArtistSet artistsSet) {
artistSet.complement();
}
}
By throwing an exception you have a chance at getting a stack trace that gives you a clue as to which of the two behaviors the caller is expecting. With luck nobody calls that function, which is why the vendor got as far as shipping code with this problem. With less luck but still some, they handle the exception. If not even that, well, at least now you will have a stack trace you can review to decide what the caller was really expecting and possibly implement that (though I shudder to think of perpetuation a bug that way, I've explained how I would do it in this other answer).
BTW, for the rest of the implementation I would delegate everything to actual Artist and Set objects passed in via the constructor so this can be easily pulled apart later.
How to Solve For Your Specific Case
ArtistSet is simply an Artist and a Set:
/** A set of associated artists, e.g: a band. */
interface ArtistSet extends Set, Artist { }
From an OO perspective, that's not a useful declaration. An Artist is a type of noun, a "thing" that has defined properties and actions (methods).
A Set is an aggregate of things - a collection of unique elements. Instead, try:
ArtistSet is simply a Set of Artists.
/** A set of associated artists, e.g: a band. */
interface ArtistSet extends Set<Artist> { };
Then, for your particular case, the homonym methods are on interfaces that are never combined within the one type, so you have no clash and can program away...
Further, you don't need to declare ArtistSet because you aren't actually extending Set with any new declarations. You're just instantiating a type parameter, so you can replace all usage with Set<Artist>.
How to Solve For the More General Case
For this clash the method names don't even need to be homographic in the english language sense - they can be the same word with same english meaning, used in different contexts in java. Clash occurs if you have two interfaces that you wish to apply to a type but they contain the same declaration (e.g. method signature) with conflicting semantic/processing definitions.
Java does not allow you to implement the behaviour you request - you must have an alternative work-around. Java doesn't allow a class to provide multiple implementations for the same method signature from multiple different interfaces (implementing the same method multiple times with some form of qualification/alias/annotation to distinguish). See Java overriding two interfaces, clash of method names,
Java - Method name collision in interface implementation
Avoid use of Inheritence (extends or implements) and instead use Object Composition (see http://en.wikipedia.org/wiki/Composition_over_inheritance)
E.g. If you have the following
interface TV {
void switchOn();
void switchOff();
void changeChannel(int ChannelNumber);
}
interface Video {
void switchOn();
void switchOff();
void eject();
void play();
void stop();
}
Then if you have an object that is both of these things, you can combine the two in a new interface (optional) or type:
interface TVVideo {
TV getTv();
Video getVideo();
}
class TVVideoImpl implements TVVideo {
TV tv;
Video video;
public TVVideoImpl() {
tv = new SomeTVImpl(....);
video = new SomeVideoImpl(....);
}
TV getTv() { return tv };
Video getVideo() { return video };
}
How can I implement a class which has two superinterfaces having homographic methods?
In Java, a class which has two superinterfaces having homographic methods is considered to have only one implementation of this method. (See the Java Language Specification section 8.4.8). This allows classes to conveniently inherit from multiple interfaces that all implement the same other interface and only implement the function once. This also simplifies the language because this eliminates the need for syntax and method dispatching support for distinguishing between homographic methods based on which interface they came from.
So the correct way to implement a class which has two superinterfaces having homographic methods is to provide a single method that satisfies the contracts of both superinterfaces.
C# has a way to do this. How can it be done in Java? Is there no construct for this?
C# defines interfaces differently than Java does and therefore has capabilities that Java does not.
In Java, the language construct is defined to mean that all interfaces get the same single implementation of homographic methods. There is no Java language construct for creating alternate behaviors of multiply-inherited interface functions based on the compile time class of the object. This was a conscious choice made by the Java language designers.
If not, how can it be done with reflection/bytecode tricks/etc most reliably?
"It" cannot be done with reflection/bytecode tricks because the information needed to decide which interface's version of the homographic method to choose is not necessarily present in the Java source code. Given:
interface I1 {
// return ASCII character code of first character of String s
int f(String s); // f("Hello") returns 72
}
interface I2 {
// return number of characters in String s
int f(String s); // f("Hello") returns 5
}
interface I12 extends I1, I2 {}
public class C {
public static int f1(I1 i, String s) { return i.f(s); } // f1( i, "Hi") == 72
public static int f2(I2 i, String s) { return i.f(s); } // f2( i, "Hi") == 2
public static int f12(I12 i, String s) { return i.f(s);} // f12(i, "Hi") == ???
}
According to the Java language specification, a class implementing I12 must do so in such a way that C.f1(), C.f2(), and C.f12() return the exact same result when called with the same arguments. If C.f12(i, "Hello") sometimes returned 72 and sometimes returned 5 based on how C.f12() were called, that would be a serious bug in the program and a violation of the language specification.
Furthermore, if the author of class C expected some kind of consistent behavior out of f12(), there is no bytecode or other information in class C that indicates whether it should be the behavior of I1.f(s) or I2.f(s). If the author of C.f12() had in mind C.f("Hello") should return 5 or 72, there's no way to tell from looking at the code.
Fine, so I cannot in general provide different behaviors for homographic functions using bytecode tricks, but I really have a class like my example class TheaterManager. What should I do to implement ArtistSet.complement()?
The actual answer to the actual question you asked is to create your own substitute implementation of TheaterManager that does not require an ArtistSet. You do not need to change the library's implementation, you need to write your own.
The actual answer to the other example question you cite is basically "delegate I12.f() to I2.f()" because no function that receives an I12 object goes on to pass that object to a function expecting an I1 object.
Stack Overflow is only for questions and answers of general interest
One of the stated reasons to reject a question here is that "it is only relevant to an extraordinarily narrow situation that is not generally applicable to the worldwide audience of the internet." Because we want to be helpful, the preferred way to handle such narrow questions is to revise the question to be more broadly applicable. For this question I have taken the approach of answering the broadly applicable version of the question rather than actually editing the question to remove what makes it unique to your situation.
In the real world of commercial programming any Java library that has a broken interface like I12 would not accumulate even dozens of commercial clients unless it could be used by implementing I12.f() in one of these ways:
delegate to I1.f()
delegate to I2.f()
do nothing
throw an exception
pick one of the above strategies on a per-call basis based on the values of some members of the I12 object
If thousands or even only a handful of companies are using this part of this library in Java then you can be assured they have used one of those solutions. If the library is not in use by even a handful of companies then the question is too narrow for Stack Overflow.
OK, TheaterManager was an oversimplification. In the real case it is too hard for me to replace that class and I don't like any of the practical solutions you've outlined. Can't I just fix this with fancy JVM tricks?
It depends on what you want to fix. If you want to fix your specific library by mapping all the calls to I12.f() and then parsing the the stack to determine the caller and choosing a behavior based on that. You can access the stack via Thread.currentThread().getStackTrace().
If you run across a caller you do not recognize you may have a hard time figuring out which version they want. For example you may be called from a generic (as was the actual case in the other specific example you gave), like:
public class TalentAgent<T extends Artist> {
public static void butterUp(List<T> people) {
for (T a: people) {
a.complement()
}
}
}
In Java, generics are implemented as erasures, meaning all type information is thrown away at compile time. There is no class or method signature difference between a TalentAgent<Artist> and a TalentAgent<Set> and the formal type of the people parameter is just List. There is nothing in the class interface or method signature of the caller to tell you what to do by looking at the stack.
So you would need to implement multiple strategies, one of which would be decompiling the code of the calling method looking for clues that the caller is expecting one class or another. It would have to be very sophisticated to cover all the ways this could happen, because among other things you have no way of knowing in advance what class it actually expecting, only that it is expecting a class that implements one of the interfaces.
There are mature and extremely sophisticated open source bytecode utilities, including one that automatically generates a proxy for a given class at runtime (written long before there was support for that in the Java language), so the fact that there isn't an open source utility for handling this case speaks volumes about the ratio of effort to usefulness in pursuing this approach.
Okay, after much research, I have another idea to fully accommodate the situation. Since you can't directly modify their code... you can force the modifications yourself.
DISCLAIMER: The example code below is very simplified. My intention is to show the general method of how this might be done, not to produce functioning source code to do it (since that's a project in itself).
The issue is that the methods are homographic. So to solve it, we can just rename the methods. Simple, right? We can use the Instrument package to achieve this. As you'll see in the linked documentation, it allows you to make an "agent" which can directly modify classes as they're loaded or re-modify them even if they've already been loaded.
Essentially, this requires you to make two classes:
An agent class which preprocesses and reloads classes; and,
A ClassFileTransformer implementation which specifies the changes you want to make.
The agent class must have either a premain() or agentmain() method defined, based on whether you want it to begin its processing as the JVM starts up or after it is already running. Examples of this are in the package documentation above. These methods give you access to an Instrumenation instance, which will allow you to register your ClassFileTransformer. So it might look something like this:
InterfaceFixAgent.java
public class InterfaceFixAgent {
public static void premain(String agentArgs, Instrumentation inst) {
//Register an ArtistTransformer
inst.addTransformer(new ArtistTransformer());
//In case the Artist interface or its subclasses
//have already been loaded by the JVM
try {
for(Class<?> clazz : inst.getAllLoadedClasses()) {
if(Artist.class.isAssignableFrom(clazz)) {
inst.retransformClasses(clazz);
}
}
}
catch(UnmodifiableClassException e) {
//TODO logging
e.printStackTrace();
}
}
}
ArtistTransformer.java
public class ArtistTransformer implements ClassFileTransformer {
private static final byte[] BYTES_TO_REPLACE = "complement".getBytes();
private static final byte[] BYTES_TO_INSERT = "compliment".getBytes();
#Override
public byte[] transform(ClassLoader loader, String className,
Class<?> classBeingRedefined, ProtectionDomain protectionDomain,
byte[] classfileBuffer) throws IllegalClassFormatException {
if(Artist.class.isAssignableFrom(classBeingRedefined)) {
//Loop through the classfileBuffer, find sequences of bytes
//which match BYTES_TO_REPLACE, replace with BYTES_TO_INSERT
}
else return classfileBuffer;
}
This is, of course, simplified. It will replace the word "complement" with "compliment" in any class which extends or implements Artist, so you will very likely need to further conditionalize it (for example, if Artist.class.isAssignableFrom(classBeingRedefined) && Set.class.isAssignableFrom(classBeingRedefined), you obviously don't want to replace every instance of "complement" with "compliment", as the "complement" for Set is perfectly legitimate).
So, now we've corrected the Artist interface and its implementations. The typo is gone, the methods have two different names, so there is no homography. This allows us to have two different implementations in our CommunityTheatre class now, each of which will properly implement/override the methods from the ArtistSet.
Unfortunately, we've now created another (possibly even bigger) issue. We've just broken all the previously-legitimate references to complement() from classes implementing Artist. To fix this, we will need to create another ClassFileTransformer which replaces these calls with our new method name.
This is somewhat more difficult, but not impossible. Essentially, the new ClassFileTransformer (let's say we call it the OldComplementTransformer) will have to perform the following steps:
Find the same string of bytes as before (the one representing the old method name, "complement");
Get the bytes before this which represent the object reference calling the method;
Convert those bytes into an Object;
Check to see if that Object is an Artist; and,
If so, replace those bytes with the new method name.
Once you've made this second transformer, you can modify the InterfaceFixAgent to accommodate it. (I also simplified the retransformClasses() call, since in the example above we perform the needed check within the transformer itself.)
InterfaceFixAgent.java (modified)
public class InterfaceFixAgent {
public static void premain(String agentArgs, Instrumentation inst) {
//Register our transformers
inst.addTransformer(new ArtistTransformer());
inst.addTransformer(new OldComplementTransformer());
//Retransform the classes that have already been loaded
try {
inst.retransformClasses(inst.getAllLoadedClasses());
}
catch(UnmodifiableClassException e) {
//TODO logging
e.printStackTrace();
}
}
}
And now... our program is good to go. It certainly wouldn't be easy to code, and it will be utter hell to QA and test. But it's certainly robust, and it solves the issue. (Technically, I suppose it avoids the issue by removing it, but... I'll take what I can get.)
Other ways we might have solved the problem:
The Unsafe API
A native method written in C
Both of these would allow you to directly manipulate bytes in memory. A solution could certainly be designed around these, but I believe it would be much more difficult and much less safe. So I went with the route above.
I think this solution could even be made more generic into an incredibly useful library for integrating code bases. Specify which interface and which method you need refactored in a variable, a command line argument, or a configuration file, and let her loose. The library that reconciles conflicting interfaces in Java at runtime. (Of course, I think it would still be better for everyone if they just fixed the bug in Java 8.)
Here's what I'd do to remove the ambiguity:
interface Artist {
void complement(); // [SIC] from OP, really "compliment"
int getEgo();
}
interface Set {
void complement(); // as in Set Theory
void remove();
boolean empty(); // [SIC] from OP, I prefer: isEmpty()
}
/**
* This class is to represent a Set of Artists (as a group) -OR-
* act like a single Artist (with some aggregate behavior). I
* choose to implement NEITHER interface so that a caller is
* forced to designate, for any given operation, which type's
* behavior is desired.
*/
class GroupOfArtists { // does NOT implement either
private final Set setBehavior = new Set() {
#Override public void remove() { /*...*/ }
#Override public boolean empty() { return true; /* TODO */ }
#Override public void complement() {
// implement Set-specific behavior
}
};
private final Artist artistBehavior = new Artist() {
#Override public int getEgo() { return Integer.MAX_VALUE; /* TODO */ }
#Override public void complement() {
// implement Artist-specific behavior
}
};
Set asSet() {
return setBehavior;
}
Artist asArtist() {
return artistBehavior;
}
}
If I were passing this object to the HR department, I'd actually give it the value returned from asSet() to hire/fire the entire group.
If I were passing this object to the Theater for a performance, I'd actually give it the value returned from asArtist() to be treated as talent.
This works as long as YOU are in control of talking to the different components directly...
But I realize that your problem is a single third-party vendor has created a component, TheaterManager, that expects one object for both of these functions and it won't know about the asSet and asArtist methods. The problem is not with the vendors that created Set and Artist, it is the vendor that combined them instead of using a Visitor pattern or just specifying an interface that would mirror the asSet and asArtist methods I made above. If you can convince your one vendor "C" to fix that interface, your world will be a lot happier.
Good luck!
Dog, I have a strong feeling you are leaving out some details that are crucial to the solution. This often happens on SO because
people need to leave out a lot of details to get the question to a reasonable size and scope,
people do not fully understand the problem and the solution (which is why they are asking for help) so they cannot be sure which details are important and which are not, and
the reason the person cannot solve the problem on their own is because they do not understand the importance of this detail, which is the same reason they left it out.
I've said in another answer what I would do about ArtistSet. But keeping the above in mind I will give you another solution to a slightly different problem. Lets say I had code from a bad vendor:
package com.bad;
public interface IAlpha {
public String getName();
// Sort Alphabetically by Name
public int compareTo(IAlpha other);
}
This is bad because you should declare a function returning a Comparator<IAlpha> to implement the sorting strategy, but whatever. Now I get code from a worse company:
package com.worse;
import com.bad.IAlpha;
// an Alpha ordered by name length
public interface ISybil extends IAlpha, Comparable<IAlpha> {}
This is worse, because it is totally wrong, in that it overrides behavior incompatibly. An ISybil orders itself by name length, but an IAlpha orders itself alphabetically, except an ISybil is an IAlpha. They were mislead by the anti-pattern of IAlpha when they could and should have done something like:
public interface ISybil extends IAlpha {
public Comparator<IAlpha> getLengthComparator();
}
However, this situation is still much better than ArtistSet because here the expected behavior is documented. There is no confusion about what ISybil.compareTo() should do. So I would create classes as follows. A Sybil class that implements compareTo() as com.worse expects and delegates everything else:
package com.hack;
import com.bad.IAlpha;
import com.worse.ISybil;
public class Sybil implements ISybil {
private final Alpha delegate;
public Sybil(Alpha delegate) { this.delegate = delegate; }
public Alpha getAlpha() { return delegate; }
public String getName() { return delegate.getName(); }
public int compareTo(IAlpha other) {
return delegate.getName().length() - other.getName().length();
}
}
and an Alpha class that works exactly like com.bad said it should:
package com.hack;
import com.bad.IAlpha;
public class Alpha implements IAlpha {
private String name;
private final Sybil sybil;
public Alpha(String name) {
this.name = name;
this.sybil = new Sybil(this);
}
// Sort Alphabetically
public int compareTo(IAlpha other) {
return name.compareTo(other.getName());
}
public String getName() { return name; }
public Sybil getSybil() { return sybil; }
}
Note that I included type conversion methods: Alpha.getSybil() and Sybil.getAlpha(). This is so I could create my own wrappers around any com.worse vendor's methods that take or return Sybils so I can avoid polluting my code or any other vendor's code with com.worse's breakage. So if com.worse had:
public ISybil breakage(ISybil broken);
I could write a function
public Alpha safeDelegateBreakage(Alpha alpha) {
return breakage(alpha.getSybil).getAlpha();
}
and be done with it, except I would still complain vociferously to com.worse and politely to com.bad.
Let's say I have an object of the class Car, with like 30 variables ranging from top speed to color. I want to make a MyCar object (class MyCar extends Car) that is basically the same thing except it stores some more information.
It is not possible for me to create MyCar objects right away (since out of thousands of Car objects only a few will become MyCar objects), unless I leave the additional fields blank, but that doesn't seem too professional. And neither does creating a constructor which takes 30 arguments, or setting 30 arguments through method calls.
So, is there any way to easily inherit all the variables from a superclass object?
PS: My program is not about cars, but I figured it would be an easier example.
EDIT
Thanks for the replies. They are all helpful for my program, but not for this particular problem. Builders don't seem beneficial because these cars of mine don't have default values for their variables. Every time a car is made, all the variables are filled out (which is needed to construct a sort of "fact-sheet").
Envelopes are an interesting design, but still require me to copy all the variables in the subclass constructor. I was hoping there would be a way to circumvent this. Templates would also require me to copy all the variables one by one.
In my program the subclass acts as a sort of "wrapper class" in a search engine. The subclasses are the same as the normal cars, but they have a "ranking score". My program is designed to display regular cars, and by extending those I can easily display the subclasses and order them by score at the same time.
I have to create new objects anyway, because multiple searches can be performed on the same list of cars. So editing variables in the original cars was not an option.
Maybe there is a better solution to this problem, but for now I guess I'll have to pass the superclass object into the constructor and copy all the variables there.
Thanks for the help!
PS: I'm just thinking, perhaps I could throw all the variables into a HashMap. That way I could access them by using .get(varname), and I would only have to pass one HashMap variable into the subclass. Downside is I would have to cast a lot, since the vars are a mixture of Strings, ints, doubles etc. What do you think, is it acceptable coding style?
Effective Java 2nd Edition, Item 2: Consider a builder when faced with many constructor parameters
And neither does creating a
constructor which takes 30 arguments,
or setting 30 arguments through method
calls.
If you are facing a constructor with too many parameters then you might want to have a look at: The Builder Pattern.
The idea is to set only the field you want/know into the builder, without bothering with the optional ones, or ones that you'd want to use default values, then calling build() to construct the actual object. See the Java examples in that article.
Once you have setup that pattern, you can construct Cars this way (notice the clean structure):
Car car = new Car.Builder(required_param1, required_param2)
.color(RED) // optional params
.topSpeed(300)
.zeroToHundred(2)
.build();
You can add a constructor that gets a Car object and copy the values from the Car to the new MyCar.
It is not possible for me to create MyCar objects right away (since out of thousands of Car objects only a few will become MyCar objects),
So, you'll have lots of objects of Car type, a few of which you'd like to, at runtime, "promote" to SpecialCar type?
Do SpecialCars have exactly the same interface as Cars?
You might want to read up on Coplien's Envelope-Letter Pattern, it's a way of "changing" object type at runtime. The object doesn't really change type, of course; instead, a different "Letter" goes into the existing "Envelope". The Envelope is the handle that other code references, but method calls on the Envelope are delegated to the Letter:
class CarEnvelope { // an envelope
Car c ; // the letter
CarEnvelope( Car c ) { this.c = c ; }
int someMethod() {
return c.someMethod(); // delegate
}
void promoteToSpecialType() {
if( ! c.isSpecialCar() ) {
c = new SpecialCar( c ) ;
}
}
class Car {
Car() {}
int someMethod() { do Car stuff }
boolean isSpecial() { return false; }
}
class SpecialCar extends Car {
SpecialCar( Car c ) { /*copy c's attributes...*/ }
int someMethod() { do SpecialCar stuff}
boolean isSpecial() { return true; }
}
CarEnvelope c = new CarEnvelope( new Car() ) ;
// do stuff with c
c.someMethod(); // indirectly calls Car.someMethod();
// promote
c.promoteToSpecialType();
c.someMethod(); // indirectly calls SpecialCar.someMethod
OP here.
I understand that this looks like laziness. But I already have it working by manually copying 30 variables in the constructor. Not that big of a task, I know.
The thing is, I have been taught to code with style. When I see mindless code blocks that look like copypasta my instincts tell me that there is probably a better way. So my desire to learn and strive for perfection has driven me here.
But if there really is no other way than to copy the variables (or override all the get&set methods) then I don't have to look any further.
Needless to say, all the replies in this topic have given me new insights. Thanks guys.
I have a feeling that what you are looking for is the notion of a template; e.g.
public class Car {
private final int attr1;
private final int attr2;
...
public Car() {
super();
}
/* copy constructor */
public Car(Car template) {
this.attr1 = template.attr1;
this.attr2 = template.attr2;
...
}
/* setters and getters */
}
Then ...
Car myTemplate = new Car();
myTemplate.setAttr1(3);
myTemplate.setAttr2(11);
...
Car car1 = new Car(myTemplate);
car1.setAttr1(4);
Car car2 = new Car(myTemplate);
car1.setAttr1(5);
I don't get it. What's wrong with a regular inheritance?
class Car {
private int attributeOne;
.....
private boolean attrbuteThirty;
public void methodOne(){...}
...
public void methodThirty(){...}
}
And then just subclass it:
class SubCar extends Car {
private int extraAttribute;
}
All the 30+ attributes and methods are already inherited, that's what the extends is all about.
If what you need is to create a new object based on the data of an existing one, but you're resisting to code it! ( ¬¬ ) , then you may just create a small script to create the code for you. It's very easy. You copy/paste the generated code and you're done.
If you don't want to do it, because you don't want to duplicate the data, you may override interesting methods, and delegate the code to the original, that's what the Decorator design pattern is all about:
class SubCar extends Car {
private Car wrapped;
private String extraAttribute;
public SubCar( Car original, String newAttributeOne ) {
wrapped = original;
this.extraAttribute = newAttributeOne;
}
public void someMethod() {
wrapped.someMethod();
}
public String getName() { return wrapped.getName(); }
... and so on
// new method
public String extraAttribute() { return extraAttribute; }
}
That way you won't duplicate the data, but just decorate it.
We can create an interface ICar that has all the getters and setters for all the 30 columns.
Car can implement ICar and can contain all the 30 fields with their corresponding getters and setters.
MyCar can also implement ICar and can use composition. It exposes Car's methods as delegate methods (which can automatically be generated in an IDE like eclipse)
public interface ICar {
// getter and setter methods
}
public Car implements ICar {
private String color;
// getters and setters for each of the fields
}
public MyCar implements ICar {
private Car car;
public MyCar(Car car){
this.car = car;
}
public String getColor() {
return car.getColor();
}
}
All consumers could then use the ICar interface for their manipulation. Will that work?