So I think I have a pretty basic question. Say there's an open source Java program called com.cow.moo that you include in your project com.bee.buzz.
moo has a bunch of great classes, most of which you don't want to touch, but there are a couple you do. Now at this point, the best thing to do would be to extend the classes you want to modify, right? (I know there's been a lot said of extends vs. implements, but none of these classes are interfaces, so that's kind of out of the question.)
My question is, say this is the class in moo:
package com.cow.moo;
public class Milk {
private float currentMilk;
public int getMilk() { /* Stuff */ }
public float convertToGallons (float liquid) { /* More Stuff */ }
}
Now, say I want to just use getMilk in my new class that extends Milk. However, getMilk in Milk relies on private variables (like currentMilk) and other functions I won't be including (like convertToGallons.) Will I have to include those other variables and functions if I want my new function to work correctly? I don't want to heavily modify the function, just add a little bit to it. What's the best way to do this?
Tips in general in building off a larger project would be useful, too. I figure it won't even take five seconds for some of the Java experts here to come up with an answer. Thanks for your time.
The general recommendation is to favor composition over inheritance.
Say, you have an interface and an existing implementation that mostly fits you needs, like
public interface MilkProvider { public float getMilk(); }
public class Milk implements MilkProvider { // same as you example }
and need another custom implementation, you could code it like that:
public class MyMilk implements MilkProvider {
private MilkProvider milk;
public MyMilk(int someValue) {
milk = new Milk(someValue); // unfortunatly we can't get rid of a depencency
// to the concrete class because we need to create
// something. An existing factory could help, but
// but usually there's none implemented.
}
public float getMilk() {
float result = milk.getMilk();
// do somethink with the result
return float;
}
}
Now, say I want to just use getMilk in my new class that extends Milk. However, getMilk in Milk relies on private variables (like currentMilk) and other functions I won't be including (like convertToGallons.) Will I have to include those other variables and functions if I want my new function to work correctly?
You won't have to include the public functions and variables. The core concept of inheritance is that, as a subclass, you get all of your parent class's public (and protected) members included in your subclass for free. So your subclass (let's say HoneyMilk) can call convertToGallons right from the get-go.
Overriding getMilk in this case is a lot trickier, since it relies on a private variable (which your subclass cannot access). My advice is to shift your mindset from treating the class as a "white box" to a "black box". What I mean by that is that you should implement your overridden version of getMilk as if you weren't actually able to see Milk's source code. While it may seem like a roundabout solution (I mean, why can't I just go tweak this line here?!), this will force you to implement your subclass using only what the parent class exposes publicly. It also heavily emphasizes the importance of abstraction, which is absolutely crucial to utilize when developing large-scale projects.
I think in this case better solution will be polymorphism (static polymorphism), or you can use reflection (do not use this way) to reach to the private variable.
You can extend the class and access instance variables throught method accessors (getters & setters) if they are public.
You can use AOP (Aspect Oriented Programming) to change your moo classes at runtime without changing its sources.
Consider too read some Composition vs. Inheritance topics.
Hope this will help you.
You won't be able to use private class members unless you use Java reflection which will be kind of ugly. If I were you (and the changes are not too heavy, in which case I'd fork the original project), I'd look at modifying the code at runtime or statically using aspect weaving (aspect oriented programming). AspectJ may look as if it had a sharp learning curve, but it's a great tool to have in your toolbox and perfectly matches your needs here.
Related
I would like to know if it safe and a good practice to keep common code in a separate class and make method static.
I have a class Car, that is constructed based on inputs from other classes. I need to apply some post construct processing after the Car object is created. Example below.
Class Travel uses Car and calls postConstructProcessing method.
CarProcessor is simillary used in other classes whenever car object is creates.
My question is should I make method process Static in CarProcessor.
Class car{
Type type;
Int model
Car(Type t, int m){
...
...
}
;
....
...}
Below class of code uses Car and calls postConstructProcessing method
public class Travel {
public void go(){
....
....
Car c = new Car(t,m);
new CarProcessor().process(c);
}
}
class CarProcessor{
public Car process(Car c){
If(c.type.value.equals("ABC"){
c.type.version=1.1;
}
if(c.model=5.7){
c.price=50k
}
}
}
My question is , is it safe and a good practice to make method process in CarProcessor static.
In general it's not great.
The most obvious problem is, if you are testing the go method, how do you replace/mock out CarProcessor::process?
The real problem is organizational though. When you are coding next time and looking for the functionality you'd expect to see in "Car" or "go", you type "car." or "go." into your IDE and hit ctrl-space, you'd expect to see all the interesting methods shown to you. How do you know to create a CarProcessor to proceed?
Some things are difficult to implement in OO though--in particular utilities. Look at the entire Math package in the java library. It's full of static methods that you just call. An oo fanatic would say these all belong in the Number class (maybe something like "Number.math.sqrt()?", but java didn't take that route--in fact they don't even have a good common number class (We have one, it's not good)--
But even when we have real classes like String, we lean towards "StringUtil" and such. This has led to a HUGE number of conflicting "Util" implementations of String. In this case part of the problem is that String is immutable and we can't really back-fill it with methods (probably a good thing). but in general, OO just isn't great for general-purpose utility methods.
Functions (which is what you are proposing) are not awesome, but are heavily used. If you have the ability to modify your business classes then that's almost always a better fit for this type of code.
Just to clarify: A Function is different from a Method--methods work on members (class variables), functions are stand-alone (Might as well be static).
Functions are a very old approach at organization. OO is a somewhat newer approach invented for when the sheer number of functions become too difficult to manage (conceptually).
Let's take an example:
public interface Testerface {
default public String example() {
return "Hello";
}
}
public class Tester implements Testerface {
#Override
public String example() {
return Testerface.super.example() + " world!";
}
}
public class Internet {
public static void main(String[] args) {
System.out.println(new Tester().example());
}
}
Simply enough, this would print Hello world!. But say I was doing something else with the return value of Testerface#example, for instance initializing a data file and returning a sensitive internal value that shouldn't leave the implementing class. Why does Java not allow access modifiers on default interface methods? Why can't they be protected/private and potentially elevated by a subclass (similar in how a class that extends a parent class can use a more visible modifier for an overridden method)?
A common solution is moving to an abstract class however in my specific case, I have an interface for enums, so that does not apply here. I imagine it was either overlooked or because the original idea behind interfaces that they are a "contract" of available methods, but I suppose I want input as to what's going on with this.
I've read "Why is “final” not allowed in Java 8 interface methods?", which states:
The basic idea of a default method is: it is an interface method with a default implementation, and a derived class can provide a more specific implementation
And it sounds to me like visibility wouldn't break that aspect at all.
As with the linked question since it looks like it had trouble being closed, an authoritative answer would be appreciated in this matter, rather than opinion-based ones.
As we saw in What is the reason why “synchronized” is not allowed in Java 8 interface methods? and Why is "final" not allowed in Java 8 interface methods?, extending interfaces to define behavior is more subtle than it might first appear. It turns out that each of the possible modifiers has their own story; its not simply a matter of blindly copying from how classes work. (This is at least obvious in hindsight, as tools for OO modeling that work for single inheritance do not automatically work for multiple inheritance.)
Let's start with the obvious answer: interfaces have always been restricted to only having public members, and while we added default methods and static methods to interfaces in Java 8, that doesn't mean we have to change everything just to be "more like" classes.
Unlike with synchronized and final, which would have been serious mistakes to support for default methods, weaker accessibilities, especially private, are reasonable features to consider. Private interface methods, whether static or instance (note that these would not be defaults, since they do not participate in inheritance) are a perfectly sensible tool (though they can be easily simulated by nonpublic helper classes.)
We actually did consider doing private interface methods in Java 8; this was mostly something that just fell off the bottom of the list due to resource and time constraints. It is quite possible this feature might reappear on the to-do list some day. (UPDATE: private methods in interfaces were added in Java 9.)
Package and protected methods, however, are more complicated than they look; the complexity of multiple inheritance and the complexity of the true meaning of protected would interact in all sorts of no-so-fun ways. So I wouldn't hold your breath for that.
So, the short answer is, private interface methods is something we could have done in 8, but we couldn't do everything that could have been done and still ship, so it was cut, but could come back.
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Background:
As a Java programmer, I extensively inherit (rather: implement) from interfaces, and sometimes I design abstract base classes. However, I have never really felt the need to subclass a concrete (non-abstract) class (in the cases where I did it, it later turned out that another solution, such as delegation would have been better).
So now I'm beginning to feel that there is almost no situation where inheriting from a concrete class is appropriate. For one thing, the Liskov substitution principle (LSP) seems almost impossible to satisfy for non-trivial classes; also many other questions here seem to echo a similar opinion.
So my question:
In which situation (if any) does it actually make sense to inherit from a concrete class?
Can you give a concrete, real-world example of a class that inherits from another concrete class, where you feel this is the best design given the constraints? I'b be particularly interested in examples that satisfy the LSP (or examples where satisfying LSP seems unimportant).
I mainly have a Java background, but I'm interested in examples from any language.
You often have a skeletal implementations for an interface I. If you can offer extensibility without abstract methods (e.g. via hooks), it is preferable to have a non-abstract skeletal class because you can instantiate it.
An example would be a forwarding wrapper classes, to be able to forward to another object of a concrete class C implementing I, e.g. enabling decoration or simple code-reuse of C without having to inherit from C. You can find such an example in Effective Java item 16, favor composition over inheritance. (I do not want to post it here because of copyrights, but it is really simply forwarding all method calls of I to the wrapped implementation).
I think the following is a good example when it can be appropriate:
public class LinkedHashMap<K,V>
extends HashMap<K,V>
Another good example is inheritance of exceptions:
public class IllegalFormatPrecisionException extends IllegalFormatException
public class IllegalFormatException extends IllegalArgumentException
public class IllegalArgumentException extends RuntimeException
public class RuntimeException extends Exception
public class Exception extends Throwable
One very common case I can think of is to derive from basic UI controls, such as forms, textboxes, comboboxes, etc. They are complete, concrete, and well able to stand on their own; however, most of them are also very basic, and sometimes their default behavior isn't what you want. Virtually nobody, for instance, would use an instance of an unadulterated Form, unless possibly they were creating an entirely dynamic UI layer.
For example, in a piece of software I wrote that recently reached relative maturity (meaning I ran out of time to focus primarily on developing it :) ), I found I needed to add "lazy loading" capability to ComboBoxes, so it wouldn't take 50 years (in computer years) for the first window to load. I also needed the ability to automatically filter the available options in one ComboBox based on what was shown in another, and lastly I needed a way to "mirror" one ComboBox's value in another editable control, and make a change in one control happen to the other as well. So, I extended the basic ComboBox to give it these extra features, and created two new types: LazyComboBox, and then further, MirroringComboBox. Both are based on the totally serviceable, concrete ComboBox control, just overriding some behaviors and adding a couple others. They're not very loosely-coupled and therefore not too SOLID, but the added functionality is generic enough that if I had to, I could rewrite either of these classes from scratch to do the same job, possibly better.
Generally speaking, the only time I derive from concrete classes is when they're in the framework. Deriving from Applet or JApplet being the trivial example.
This is an example of a current implementation that I'm undertaking.
In OAuth 2 environment, since the documentation is still in draft stage, the specification keeps changing (as of time of writing, we're in version 21).
Thus, I had to extend my concrete AccessToken class to accommodate the different access tokens.
In earlier draft, there was no token_type field set, so the actual access token is as follows:
public class AccessToken extends OAuthToken {
/**
*
*/
private static final long serialVersionUID = -4419729971477912556L;
private String accessToken;
private String refreshToken;
private Map<String, String> additionalParameters;
//Getters and setters are here
}
Now, with Access tokens that returns token_type, I have
public class TokenTypedAccessToken extends AccessToken {
private String tokenType;
//Getter and setter are here...
}
So, I can return both and the end user is none the wiser. :-)
In Summary: If you want a customized class that has the same functionality of your concrete class without changing the structure of the concrete class, I suggest extending the concrete class.
I mainly have a Java background, but I'm interested in examples from any language.
Like many frameworks, ASP.NET makes heavy use of inheritance to share behaviour between classes. For example, HtmlInputPassword has this inheritance hierarchy:
System.Object
System.Web.UI.Control
System.Web.UI.HtmlControls.HtmlControl // abstract
System.Web.UI.HtmlControls.HtmlInputControl // abstract
System.Web.UI.HtmlControls.HtmlInputText
System.Web.UI.HtmlControls.HtmlInputPassword
in which can be seen examples of concrete classes being derived from.
If you're building a framework - and you're sure you want to do that - you may well finding yourself wanting a nice big inheritance hierarchy.
Other use case would be the to override the default behavior:
Lets say there is a class which uses standard Jaxb parser for parsing
public class Util{
public void mainOperaiton(){..}
protected MyDataStructure parse(){
//standard Jaxb code
}
}
Now say I want to use some different binding (Say XMLBean) for the parsing operation,
public class MyUtil extends Util{
protected MyDataStructure parse(){
//XmlBean code code
}
}
Now I can use the new binding with code reuse of super class.
The decorator pattern, a handy way of adding additional behaviour to a class without making it too general, makes heavy use of inheritance of concrete classes. It was mentioned here already, but under somewhat a scientific name of "forwarding wrapper class".
Lot of answers but I though I'd add my own $0.02.
I override concreate classes infrequently but under some specific circumstances. At least 1 has already been mentioned when framework classes are designed to be extended. 2 additional ones come to mind with some examples:
1) If I want to tweak the behavior of a concrete class. Sometimes I want to change how the concrete class works or I want to know when a certain method is called so I can trigger something. Often concrete classes will define a hook method whose sole usage is for subclasses to override the method.
Example: We overrode MBeanExporter because we need to be able to unregister a JMX bean:
public class MBeanRegistrationSupport {
// the concrete class has a hook defined
protected void onRegister(ObjectName objectName) {
}
Our class:
public class UnregisterableMBeanExporter extends MBeanExporter {
#Override
protected void onUnregister(ObjectName name) {
// always a good idea
super.onRegister(name);
objectMap.remove(name);
}
Here's another good example. LinkedHashMap is designed to have its removeEldestEntry method overridden.
private static class LimitedLinkedHashMap<K, V> extends LinkedHashMap<K, V> {
#Override
protected boolean removeEldestEntry(Entry<K, V> eldest) {
return size() > 1000;
}
2) If a class shares a significant amount of overlap with the concrete class except for some tweaks to functionality.
Example: My ORMLite project handles persisting Long object fields and long primitive fields. Both have almost the identical definition. LongObjectType provides all of the methods that describe how the database deals with long fields.
public class LongObjectType {
// a whole bunch of methods
while LongType overrides LongObjectType and only tweaks a single method to say that handles primitives.
public class LongType extends LongObjectType {
...
#Override
public boolean isPrimitive() {
return true;
}
}
Hope this helps.
Inheriting concrete class is only option if you want to extend side-library functionality.
For example of real life usage you can look at hierarchy of DataInputStream, that implements DataInput interface for FilterInputStream.
I'm beginning to feel that there is almost no situation where inheriting from a concrete class is appropriate.
This is one 'almost'. Try writing an applet without extending Applet or JApplet.
Here is an e.g. from the applet info. page.
/* <!-- Defines the applet element used by the appletviewer. -->
<applet code='HelloWorld' width='200' height='100'></applet> */
import javax.swing.*;
/** An 'Hello World' Swing based applet.
To compile and launch:
prompt> javac HelloWorld.java
prompt> appletviewer HelloWorld.java */
public class HelloWorld extends JApplet {
public void init() {
// Swing operations need to be performed on the EDT.
// The Runnable/invokeLater() ensures that happens.
Runnable r = new Runnable() {
public void run() {
// the crux of this simple applet
getContentPane().add( new JLabel("Hello World!") );
}
};
SwingUtilities.invokeLater(r);
}
}
Another good example would be data storage types. To give a precise example: a red-black tree is a more specific binary tree, but retrieving data and other information like size can be handled identical. Of course, a good library should have that already implemented but sometimes you have to add specific data types for your problem.
I am currently developing an application which calculates matrices for the users. The user can provide settings to influence the calculation. There are several types of matrices that can be calculated, but there is a clear similarity, especially in the configurability: matrix A can use all the settings of matrix B but has additional parameters which can be used. In that case, I inherited from the ConfigObjectB for my ConfigObjectA and it works pretty good.
In general, it is better to inherit from an abstract class than from a concrete class. A concrete class must provide a definition for its data representation, and some subclasses will need a different representation. Since an abstract class does not have to provide a data representation, future subclasses can use any representation without fear of conflicting with the one that they inherited.
Even i never found a situation where i felt concrete inheritence is neccessary. But there could be some situations for concrete inheritence specially when you are providing backward compatibility to your software. In that case u might have specialized a class A but you want it to be concrete as your older application might be using it.
Your concerns are also echoed in the classic principle "favor composition over inheritance", for the reasons you stated. I can't remember the last time I inherited from a concrete class. Any common code that needs to be reused by child classes almost always needs to declare abstract interfaces for those classes. In this order I try to prefer the following strategies:
Composition (no inheritance)
Interface
Abstract Class
Inheriting from a concrete class really isn't ever a good idea.
[EDIT] I'll qualify this statement by saying I don't see a good use case for it when you have control over the architecture. Of course when using an API that expects it, whaddaya gonna do? But I don't understand the design choices made by those APIs. The calling class should always be able to declare and use an abstraction according to the Dependency Inversion Principle. If a child class has additional interfaces to be consumed you'd either have to violate DIP or do some ugly casting to get at those interfaces.
from the gdata project:
com.google.gdata.client.Service is designed to act as a base class that can be customized for specific types of GData services.
Service javadoc:
The Service class represents a client connection to a GData service. It encapsulates all protocol-level interactions with the GData server and acts as a helper class for higher level entities (feeds, entries, etc) that invoke operations on the server and process their results.
This class provides the base level common functionality required to access any GData service. It is also designed to act as a base class that can be customized for specific types of GData services. Examples of supported customizations include:
Authentication - implementing a custom authentication mechanism for services that require authentication and use something other than HTTP basic or digest authentication.
Extensions - define expected extensions for feed, entry, and other types associated with a the service.
Formats - define additional custom resource representations that might be consumed or produced by the service and client side parsers and generators to handle them.
I find the java collection classes as a very good example.
So you have an AbstractCollection with childs like AbstractList, AbstractSet, AbstractQueue...
I think this hierarchy has been well designed.. and just to ensure there's no explosion there's the Collections class with all its inner static classes.
You do that for instance in GUI libraries. It makes not much sense to inherit from a mere Component and delegate to a Panel. It is likely much easyer to inherit from the Panel directly.
Just a general thought. Abstract classes are missing something. It makes sense if this, what is missing, is different in each derived class. But you may have a case where you don't want to modify a class but just want to add something. To avoid duplication of code you would inherit. And if you need both classes it would be inheritance from a concrete class.
So my answer would be: In all cases where you really only want to add something. Maybe this just doesn't happen very often.
When I create complex type hierarchies (several levels, several types per level), I like to use the final keyword on methods implementing some interface declaration. An example:
interface Garble {
int zork();
}
interface Gnarf extends Garble {
/**
* This is the same as calling {#link #zblah(0)}
*/
int zblah();
int zblah(int defaultZblah);
}
And then
abstract class AbstractGarble implements Garble {
#Override
public final int zork() { ... }
}
abstract class AbstractGnarf extends AbstractGarble implements Gnarf {
// Here I absolutely want to fix the default behaviour of zblah
// No Gnarf shouldn't be allowed to set 1 as the default, for instance
#Override
public final int zblah() {
return zblah(0);
}
// This method is not implemented here, but in a subclass
#Override
public abstract int zblah(int defaultZblah);
}
I do this for several reasons:
It helps me develop the type hierarchy. When I add a class to the hierarchy, it is very clear, what methods I have to implement, and what methods I may not override (in case I forgot the details about the hierarchy)
I think overriding concrete stuff is bad according to design principles and patterns, such as the template method pattern. I don't want other developers or my users do it.
So the final keyword works perfectly for me. My question is:
Why is it used so rarely in the wild? Can you show me some examples / reasons where final (in a similar case to mine) would be very bad?
Why is it used so rarely in the wild?
Because you should write one more word to make variable/method final
Can you show me some examples / reasons where final (in a similar case to mine) would be very bad?
Usually I see such examples in 3d part libraries. In some cases I want to extend some class and change some behavior. Especially it is dangerous in non open-source libraries without interface/implementation separation.
I always use final when I write an abstract class and want to make it clear which methods are fixed. I think this is the most important function of this keyword.
But when you're not expecting a class to be extended anyway, why the fuss? Of course if you're writing a library for someone else, you try to safeguard it as much as you can but when you're writing "end user code", there is a point where trying to make your code foolproof will only serve to annoy the maintenance developers who will try to figure out how to work around the maze you had built.
The same goes to making classes final. Although some classes should by their very nature be final, all too often a short-sighted developer will simply mark all the leaf classes in the inheirance tree as final.
After all, coding serves two distinct purposes: to give instructions to the computer and to pass information to other developers reading the code. The second one is ignored most of the time, even though it's almost as important as making your code work. Putting in unnecessary final keywords is a good example of this: it doesn't change the way the code behaves, so its sole purpose should be communication. But what do you communicate? If you mark a method as final, a maintainer will assume you'd had a good readon to do so. If it turns out that you hadn't, all you achieved was to confuse others.
My approach is (and I may be utterly wrong here obviously): don't write anything down unless it changes the way your code works or conveys useful information.
Why is it used so rarely in the wild?
That doesn't match my experience. I see it used very frequently in all kinds of libraries. Just one (random) example: Look at the abstract classes in:
http://code.google.com/p/guava-libraries/
, e.g. com.google.common.collect.AbstractIterator. peek(), hasNext(), next() and endOfData() are final, leaving just computeNext() to the implementor. This is a very common example IMO.
The main reason against using final is to allow implementors to change an algorithm - you mentioned the "template method" pattern: It can still make sense to modify a template method, or to enhance it with some pre-/post actions (without spamming the entire class with dozens of pre-/post-hooks).
The main reason pro using final is to avoid accidental implementation mistakes, or when the method relies on internals of the class which aren't specified (and thus may change in the future).
I think it is not commonly used for two reasons:
People don't know it exists
People are not in the habit of thinking about it when they build a method.
I typically fall into the second reason. I do override concrete methods on a somewhat common basis. In some cases this is bad, but there are many times it doesn't conflict with design principles and in fact might be the best solution. Therefore when I am implementing an interface, I typically don't think deeply enough at each method to decide if a final keyword would be useful. Especially since I work on a lot of business applications that change frequently.
Why is it used so rarely in the wild?
Because it should not be necessary. It also does not fully close down the implementation, so in effect it might give you a false sense of security.
It should not be necessary due to the Liskov substitution principle. The method has a contract and in a correctly designed inheritance diagram that contract is fullfilled (otherwise it's a bug). Example:
interface Animal {
void bark();
}
abstract class AbstractAnimal implements Animal{
final void bark() {
playSound("whoof.wav"); // you were thinking about a dog, weren't you?
}
}
class Dog extends AbstractAnimal {
// ok
}
class Cat extends AbstractAnimal() {
// oops - no barking allowed!
}
By not allowing a subclass to do the right thing (for it) you might introduce a bug. Or you might require another developer to put an inheritance tree of your Garble interface right beside yours because your final method does not allow it to do what it should do.
The false sense of security is typical of a non-static final method. A static method should not use state from the instance (it cannot). A non-static method probably does. Your final (non-static) method probably does too, but it does not own the instance variables - they can be different than expected. So you add a burden on the developer of the class inheriting form AbstractGarble - to ensure instance fields are in a state expected by your implementation at any point in time. Without giving the developer a way to prepare the state before calling your method as in:
int zblah() {
prepareState();
return super.zblah();
}
In my opinion you should not close an implementation in such a fashion unless you have a very good reason. If you document your method contract and provide a junit test you should be able to trust other developers. Using the Junit test they can actually verify the Liskov substitution principle.
As a side note, I do occasionally close a method. Especially if it's on the boundary part of a framework. My method does some bookkeeping and then continues to an abstract method to be implemented by someone else:
final boolean login() {
bookkeeping();
return doLogin();
}
abstract boolean doLogin();
That way no-one forgets to do the bookkeeping but they can provide a custom login. Whether you like such a setup is of course up to you :)
I'm writing (well, completing) an "extension" of Java which will help role programming.
I translate my code to Java code with javacc. My compilers add to every declared class some code. Here's an example to be clearer:
MyClass extends String implements ObjectWithRoles { //implements... is added
/*Added by me */
public setRole(...){...}
public ...
/*Ends of stuff added*/
...//myClass stuff
}
It adds Implements.. and the necessary methods to EVERY SINGLE CLASS you declare. Quite rough, isnt'it?
It will be better if I write my methods in one class and all class extends that.. but.. if class already extends another class (just like the example)?
I don't want to create a sort of wrapper that manage roles because i don't want that the programmer has to know much more than Java, few new reserved words and their use.
My idea was to extends java.lang.Object.. but you can't. (right?)
Other ideas?
I'm new here, but I follow this site so thank you for reading and all the answers you give! (I apologize for english, I'm italian)
If it is only like a "research" project in which you want to explore how such extension would work, you could provide your own implementation of the Object class. Simply copy the existing object implementation, add your setRole method etc, and give -Xbootclasspath:.:/usr/lib/jvm/java-6-sun/jre/lib/rt.jar as parameter to the java command. (I will look for api-classes in . before looking in the real rt.jar.)
You should consider using composition rather than inheritence to solve this problem; that way you can provide the functionality you need without using up your "one-shot" at inheritence.
For example, the JDK provides a class PropertyChangeSupport, which can be used to manage PropertyChangeListeners and the firing of PropertyChangeEvents. In situations where you wish to write a class that fires PropertyChangeEvents you could embed a PropertyChangeSupport instance variable and delegate all method calls to that. This avoids the need for inheritence and means you can supplement an existing class hierarchy with new functionality.
public class MyClass extends MySuperClass {
private final PropertyChangeSupport support;
public MyClass() {
this.support = new PropertyChangeSupport(this);
}
public void addPropertyChangeListener(PropertyChangeListener l) {
support.addPropertyChangeListener(l);
}
protected void firePropertyChangeEvent() {
PropertyChangeEvent evt = new ...
support.firePropertyChangeEvent(evt);
}
}
you can extend Object - every class extends it.
you seem to need something like multiple inheritance - there isn't such a thing in Java
if you want to add functionality, use object composition. I.e.,
YourClass extends Whatever implements ObjectWithRoles {
private RoleHandler roleHandler;
public RoleHandler getRoleHandler() {..} // defined by the interface
}
And then all of the methods are placed in the RoleHandler
If you're talking about adding a role to all your objects I would also consider an annotation-based solution. You'd annotate your classes with something like #Role("User"). In another class you can extract that role value and use it.
I think it would need an annotation with runtime retention and you can check, run-time, whether the annotation is present using reflection and get that annotation using getAnnotation. I feel that this would be a lot cleaner than extending all your classes automatically.
I believe there are some frameworks which use exactly such a solution, so there should be example code somewhere.
If you are doing what you are doing, then inheritance is probably not the correct idiom. You may want to consider the decorator pattern, whereby you construct a class that takes as its parameter some other class with less functionality, and adds some additional functionality to it, delegating to the existing class for functionality that already exists. If the implementation is common to many of your decorators, you may want to consider putting that functionality in class that can be shared and to which you can delegate for all your decorators. Depending on what you need, double-dispatch or reflection may be appropriate in order to make similar but not quite the same decorators for a large variety of classes.
Also, as has been pointed out in the comments, String is declared "final" and, therefore, cannot be extended. So, you should really consider a solution whereby you delegate/decorate objects. For example, you might have some object that wraps a string and provides access to the string via getString() or toString(), but then adds the additional functionality on top of the String class.
If you just want to associate some objects with additional attributes, use a Map (e.g. HashMap).
What you really want to do would be monkey patching, i.e. changing the behaviour of existing classes without modifying their code.
Unfortunately, Java does not support this, nor things like mixins that might be used alternatively. So unless you're willing to switch to a more dynamic language like Groovy, you'll have to live with less elegant solutions like composition.