While working on a project, I came across the following code segment which appears to provide code, entirely contained inside a new variable declaration, which appears to override a method. I've, come across code of this form before but admittedly, I do not fully understand it. If anyone could explain the programming mechanisms upon which this code is based, I'd be very truly grateful. Particularly, when are overridden methods of this sort permitted inside of variable declarations. What other sorts of data structures allow such behavior? When is it advantageous to write code of such nature? Why not override the method outside of a variable declaration?
tempRequests.sort(new Comparator<Integer>()
{
#Override
public int compare(Integer integer1, Integer integer2)
{
return integer1.compareTo(integer2);
}
});
What other sorts of data structures allow such behavior?
-> You can sort objects by implements interface Comparable.
For example:
public class Car implements Comparable<Car> {
private String name;
#Override
public int compareTo(Car b) {
return name.compareTo(b.name);
}
}
->You can also use Comparator without override method compare inside the inner class.
public class Car implements Comparator<Car> {
private String name;
private double price;
#Override
public int compare(Car b1, Car b2) {
return b1.price - b2.price;
}
}
When is it advantageous to write code of such nature? Why not override the method outside of a variable declaration?
-> Image that after use sort object Car by name, you want to sort by something else (like by price, by weight).How to do this when you want to sort objects in different ways at different times? We use Comparator with define inside the inner class to do this.
*Additionally, Comparator is a functional interface since an only abstract method to implement. You can rewrite using a funky syntax in one line of code:
Ex:
Compareator<Car> byPrice = (b1,b2) -> b1.price - b2.price;
This mechanism has been explained well in the comments.
As an aside: ever since Java 8, this usage of anonymous classes is considered somewhat old fashioned, as it can be replaced with a simple Lambda expression:
tempRequests.sort((l, r) -> l.compareTo(r));
This applies to all "Functional Interfaces", which is defined as an interface with exactly one non-static and non-default method.
All:
I am on first day reading team's code(the one wrote this left...):
There is one enum definition confused me so much:
/**
* Enum defines the processing stages and the order
*
*/
public enum ProcessStage {
/*
* Individual stages in the process.
* Order of processing is based on the order of listing.
*/
EXTRACT("Extraction", "EXTRACTED", "EXTRACTION_FAILED"),
ROUTE("Routing", "ROUTED", "ROUTE_FAILED"),
PUBLISH("Publishing", "PUBLISHED", "PUBLISH_FAILED");
private String detailedName;
private String successState;
private String failedState;
private ProcessStage(String detailedName, String successState, String failedState) {
this.detailedName = detailedName;
this.successState = successState;
this.failedState = failedState;
}
public String getSuccessState() {
return successState;
}
public String getFailedState() {
return failedState;
}
/**
* Factory method to provide the ProcessStage from its success or failed state value stored in DB.
* #param state
* #return ProcessStage
*/
public static ProcessStage getProcessStage(String state) {
for(ProcessStage ps: ProcessStage.values()) {
if(ps.getSuccessState().equals(state) || ps.getFailedState().equals(state)) {
return ps;
}
}
return null;
}
public String toString() {
return detailedName;
}
}
I wonder if anyone give me some simple introduction about how to read this(like what kinda syntax it uses)? The most confused part is:
EXTRACT("Extraction", "EXTRACTED", "EXTRACTION_FAILED"),
ROUTE("Routing", "ROUTED", "ROUTE_FAILED"),
PUBLISH("Publishing", "PUBLISHED", "PUBLISH_FAILED");
I do not quite understand what this means and how to use this.
And why there are a lot of methods defined inside it and how to use method with a enum variable?
Thanks
Enum
The enum declaration defines a class (called an enum type). The enum
class body can include methods and other fields. The compiler
automatically adds some special methods when it creates an enum.
enums are special type of class. Instead of creating singleton pattern using regular class or to create constants, like WeekDays, we can use enum in such places. Here
EXTRACT("Extraction", "EXTRACTED", "EXTRACTION_FAILED"),
Here EXTRACT is an enum constant meaning it is an instance of the classProcessStage and also all other enum constants(ROUTE, PUBLISH). All costants of enum are unique objects, meaning they are singleton instance created in the jvm and enum makes sure the instances are unique. You need not to put additional effort to create singleton pattern.
The above code is not only declaration, it is also calling the constructor with three String parameters to create the instance.
private ProcessStage(String detailedName, String successState, String failedState) {
this.detailedName = detailedName;
this.successState = successState;
this.failedState = failedState;
}
why there are a lot of methods defined inside it?
Since it is also a class, it can have methods like any other classes. But the restriction is, it cannot be inherited, because internally enum extens the class Enum<E extends Enum<E>> class.
how to use method with a enum variable?
EXTRACT.getFailedState() //returns "EXTRACTION_FAILED"
Keep in mind, without seeing more of the code, I can't be exactly sure what this particular enum is being used for.
So, Let's say we have a method somewhere, where a process is passed through.
public void doSomething(Process process) {}
Now, let's assume that the purpose of this method is to check the status of the process and then do some logic based upon that result. This would entail doing something like the following
public void doSomething(Process process) {
if(ProcessStage.EXTRACT.equals(process.getStage()) {
//do something here...you will have access to the methods within
//the enum
}
}
Without knowing more, this is all I can give you. I hope this gives you a slightly better understanding of what that enum is doing
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.
This is a design question. The design is pseudo-code and represents a small example but I may add a lot more methods, data, logic in the future.
In this example, I am considering two approaches. In the execute method below, should I return an immutable "data/bean/model" object with the output of the execute method or update the state of the BusinessLogic class.
The both accomplish the same goal, I want the result of execute and either the data should be contained in a bean container or in the internally in the BusinessLogic class.
I am kind of favoring just having the BusinessLogic class because SomeObject is just a useless bean that doesn't do anything.
What are your thoughts?
public class SomeObject {
private String data1;
private String data2;
}
public class BusinessLogic {
private final IWebObject webObject;
/* String data1; String data2 */
public BusinessLogic(final IWebObject webObject) {
this.webObject = webObject;
}
// Approach 1
public SomeObject execute() {
return new SomeObject();
}
or
...
...
// Approach 2
public void execute() {
// Do something
this.data1 = "data1";
this.data2 = "data2";
}
public String getData1() { }
public String getData2() { }
} // End of the Class //
My only problem with approach 2 is that data1 and data2 won't be immutable so. I can call execute arbitrarily and change those values.
If the main purpose of the method would be to execute and not return anything, I would personally go with your second approach and keep the data internally inside the BusinessLogic class.
For the most part, I'd say it depends on the nature of the code that's calling the execute() method. If it's going to just be reading the results as-is, then keeping it in the BusinessLogic class would be fine. If you're going to be passing the results around to different methods, you should put the results in a separate class (you could have BusinessLogic implement an interface that contains just the result methods, but that could blur the line between where the BusinessLogic implementation ends and where the result implementation begins).
To start with, it'd probably be best to keep it internal -- following the "You Ain't Gonna Need It" principle. When the time comes where you can see that you are going to need to pass the results around to other methods and other objects, you can refactor it to suit your needs at the time.
This isn't a general object-oriented design question - it depends on the domain you're trying to model.
Should execute change the state of BusinessLogic or not? (Option 2)
How is that class being consumed? Are data1 and data2 used in a different way? In a different context? (Option 1)
Do data1 and data2 have different lifetimes than BusinessLogic? (Option 1)
Sorry for the stupid question.
I'm very sure, that the Java API provides a class which wraps a reference,
and provides a getter and a setter to it.
class SomeWrapperClass<T> {
private T obj;
public T get(){ return obj; }
public void set(T obj){ this.obj=obj; }
}
Am I right? Is there something like this in the Java API?
Thank you.
Yes, I could write it y myself, but why should I mimic existing functionality?
EDIT: I wanted to use it for reference
parameters (like the ref keyword in C#), or more specific,
to be able to "write to method parameters" ;)
There is the AtomicReference class, which provides this. It exists mostly to ensure atomicity, especially with the getAndSet() and compareAndSet() methods, but I guess it does what you want.
When I started programming in Java after years of writing C++, I was concerned with the fact that I could not return multiple objects from a function.
It turned out that not only was it possible but it was also improving the design of my programs.
However, Java's implementation of CORBA uses single-element arrays to pass things by reference. This also works with basic types.
I'm not clear what this would be for, but you could use one of the subclasses of the Reference type. They set the reference in the constructor rather than setter.
It' worth pointing out that the Reference subclasses are primarily intended to facilitate garbage collection, for example when used in conjunction with WeakHashMap.
I'm tempted to ask why you'd want one of these, but I assume it's so you can return multiple objects from a function...
Whenever I've wanted to do that, I've used an array or a container object...
bool doStuff(int params, ... , SomeObject[] returnedObject)
{
returnedObject[0] = new SomeObject();
return true;
}
void main(String[] args)
{
SomeObject myObject;
SomeObject[1] myObjectRef = new SomeObject[1];
if(doStuff(..., myObjectRef))
{
myObject = myObjectRef[0];
//handle stuff
}
//could not initialize.
}
... good question, but have not come across it. I'd vote no.
.... hm, after some reflection, reflection might be what comes close to it:
http://java.sun.com/developer/technicalArticles/ALT/Reflection/
there is java.lang.ref.Reference, but it is immutable (setter is missing). The java.lang.ref documentation says:
Every reference object provides methods for getting and clearing the reference. Aside from the clearing operation reference objects are otherwise immutable, so no set operation is provided. A program may further subclass these subclasses, adding whatever fields and methods are required for its purposes, or it may use these subclasses without change.
EDIT
void refDemo(MyReference<String> changeMe) {
changeMe.set("I like changing!");
...
the caller:
String iWantToChange = "I'm like a woman";
Reference<String> ref = new MyReference<String>(iWantToChange)
refDemo(myRef);
ref.get();
I don't like it however, too much code. This kind of features must be supported at language level as in C#.
If you are trying to return multiple values from a function, I would create a Pair, Triple, &c class that acts like a tuple.
class Pair<A,B> {
A a;
B b;
public void Pair() { }
public void Pair(A a,B b) {
this.a=a;
this.b=b;
}
public void Pair( Pair<? extends A,? extends B> p) {
this.a=p.a;
this.b=p.b;
}
public void setFirst(A a) { this.a=a; }
public A getFirst() { return a; }
public void setSecond(B b) { this.b=b; }
public B getSecond() { return b; }
}
This would allow you to return 2 (techically infinite) return values
/* Reads a line from the provided input stream and returns the number of
* characters read and the line read.*/
public Pair<Integer,String> read(System.in) {
...
}
I think there is no Java API Class designed for your intent, i would also prefer your example (the Wrapper Class) then using this "array-trick" because you could insert later some guards or can check several thinks via aspects or reflection and you're able to add features which are cross-cutting-concerns functionality.
But be sure that's what you want to do! Maybe you could redesign and come to another solutions?