java traits or mixins pattern? - java

Is there a way to emulate mixins or traits in java? basically, I need a way to do multiple inheritance so I can add common business logic to several classes

Not the way you want to do it. Effective Java recommends that you "Favor composition over inheritance". Meaning you move the common logic to other classes and delegate. This is how you get around the lack of multiple inheritance in java.

I would encapsulate all of the business logic into a new class BusinessLogic and have each class that needs BusinessLogic make calls to the class. If you need a single rooted heirarchy for your classes that make calls to BusinessLogic, you'll have to create an interface as well (BusinessLogicInterface?)
In pseudo-code:
interface BusinessLogicInterace
{
void method1();
void method2();
}
class BusinessLogic implements BusinessLogicInterface
{
void method1() { ... }
void method2() { ... }
}
class User
extends OtherClass
implements BusinessLogicInterface
{
BusinessLogic logic = new BusinessLogic();
#Override
void method1() { logic.method1(); }
#Override
void method2() { logic.method2(); }
}
This isn't the prettiest implementation to work around a lack of multiple inheritance and it becomes quite cumbersome when the interface has a lot of methods. Most likely, you'll want to try and redesign your code to avoid needing mixins.

Is the object-purist stirring in you today?
Think you could do with a little composite oriented programming?
Then you, sir, are looking for Apache Polygene (formerly named Qi4J, then it renamed to Zest and/or Apache-Zest) ;)
Update 2022; It's discontinued currently, but useful anyway.

Java's answer to multiple inheritance is the ability to implement multiple interfaces. Of course, this means you'll get the method declarations, but not the logic.
You could try emulating mixins by composition: your Java class could define member variables that represent other classes that perform some common business logic.
In designing Java classes, I have not found the lack of C++ style multiple inheritance to inhibit the design of my architecture. You will find a way to achieve what you want to do.

QI4J allows you to use mixins

You can exploit the fact that interfaces allow nested classes (automatically public static) to keep the default implementation of the interface methods encapsulated within the interface itself. I.e. move the BusinessLogic class of Alex B's example inside the interface.
This is similar to the way Scala generates the JVM code for traits as explained here How are Scala traits compiled into Java bytecode?
When doing this the example becomes:
interface BusinessLogicInterface {
void method0();
class DefaultImpl {
private DefaultImpl() {
}
public static void method1(BusinessLogicInterface self) { ... }
public static void method2(BusinessLogicInterface self) { ... }
}
void method1();
void method2();
}
class User extends OtherClass implements BusinessLogicInterface {
#Override
void method0() { ... }
#Override
void method1() { BusinessLogic.defaultImpl.method1(this); }
#Override
void method2() { BusinessLogic.defaultImpl.method2(this); }
}
Note that we pass an object of the interface type as the "self" parameter. This means the business logic can use other abstract methods (method0). This can be very useful for creating a trait with abstract methods that are all orthogonal to each other and utility "extension" methods that may be implemented in terms of these orthogonal methods.
The drawback is that each interface must copy/paste the boilerplate delegation code. Another often used pattern in Java without this drawback (but with less cohesion and less OO way to call the methods) is to create a class with the plural name as the interface containing the static methods, this is used in the Collections utility class.

As of Java-8, default interface methods were added. This, together with multiple inheritance of interfaces in Java should allow some sort of mixin. Clearly the interfaces have to operate independently. So, there will be significant limitations.

Implementing simple mixin/traits support in java using CGLib/javassit is quite easy.
You can take a look for instance here for small example.
More complete, ready to use solution might be found: here

Related

Why the Class Adapter Design Pattern Can't Use an Interface Instead of Multiple Inheritance?

I've recently learned the Class Adapter pattern. In order to implement it, the language used must support multiple inheritance since the adapter class must inherit two classes, the Target, and the Adaptee. So in a language like Java, it could not be done.
But why couldn't it use an interface Target instead of a class Target? More inline with the Object Adapter pattern as well. Just switching from object composition (Adapter having the Adaptee) to single inheritance (Adapter inheriting the Adaptee).
By using an interface, I don't see the design difference, and as a result, the pattern can be used in Java.
Link to object adapter and class adapter class diagram
There's generally no reason you can't create a Class Adapter between two interfaces in Java. But first you have to be lucky enough to be in a scenario where the two APIs that need adapting are both interfaces to begin with.
An Adapter is mostly useful to bridge the gap between two existing APIs that don't belong to you. If you control one or both APIs, you can simply change one to match the other. When you need two separate APIs to be compatible, but the APIs don't belong to you, then you have to adapt what you're given, which may not be interfaces.
But why couldn't it use an interface Target instead of a class Target?
you can use interface. But then you will have duplication of code, but multiple inheritance removes duplication of code.
Let me show an example.
Our abstractions:
public interface IDuck
{
void Quack();
}
public interface ITurkey
{
void Gobble();
}
And concrete implementations:
public class Duck : IDuck
{
public void Quack()
{
Console.WriteLine("Quack");
}
}
public class Turkey : ITurkey
{
public void Gobble()
{
Console.WriteLine("Gobble");
}
}
And class adapter would look like this:
public class ClassAdapter : IDuck, ITurkey
{
public void Gobble()
{
// duplication of code
Console.WriteLine("Gobble");
}
public void Quack()
{
Gobble();
}
}
The above ClassAdapter has duplications of code. Sure we can extract this code and provide it through composition or inject Duck and Turkey. However, it brings additional dependencies and some complexity. So it is better to use object adapter pattern. Your code will be simpler. Simple code is almost always the best choice.

Why does Java not allow multiple inheritance but does allow conforming to multiple interfaces with default implementations

I am not asking this -> Why is there no multiple inheritance in Java, but implementing multiple interfaces is allowed?
In Java, multiple inheritance isn't allowed, but, after Java 8, Interfaces can have default methods (can implement methods itself), just like abstract classes. Within this context, it multiple inheritance should also be allowed.
interface TestInterface
{
// abstract method
public void square(int a);
// default method
default void show()
{
System.out.println("Default Method Executed");
}
}
Things are not so simple.
If a class implements multiple interfaces that defines default methods with the same signature the compiler will force you to override this method for the class.
For example with these two interfaces :
public interface Foo {
default void doThat() {
// ...
}
}
public interface Bar {
default void doThat() {
// ...
}
}
It will not compile :
public class FooBar implements Foo, Bar{
}
You should define/override the method to remove the ambiguity.
You could for example delegate to the Bar implementation such as :
public class FooBar implements Foo, Bar{
#Override
public void doThat() {
Bar.super.doThat();
}
}
or delegate to the Foo implementation such as : :
public class FooBar implements Foo, Bar {
#Override
public void doThat() {
Foo.super.doThat();
}
}
or still define another behavior :
public class FooBar implements Foo, Bar {
#Override
public void doThat() {
// ...
}
}
That constraint shows that Java doesn't allow multiple inheritancy even for interface default methods.
I think that we cannot apply the same logic for multiple inheritances because multiples issues could occur which the main are :
overriding/removing the ambiguity for a method in both inherited classes could introduce side effects and change the overall behavior of the inherited classes if they rely on this method internally. With default interfaces this risk is also around but it should be much less rare since default methods are not designed to introduce complex processings such as multiple internal invocations inside the class or to be stateful (indeed interfaces cannot host instance field).
how to inherit multiple fields ? And even if the language allowed it you would have exactly the same issue as this previously quoted : side effect in the behavior of the inherited class : a int foo field defined in a A and B class that you want to subclass doesn't have the same meaning and intention.
The language designers already thought about that, so these things are enforced by the compiler. So if you define:
interface First {
default void go() {
}
}
interface Second {
default void go() {
}
}
And you implement a class for both interfaces:
static class Impl implements First, Second {
}
you will get a compilation error; and you would need to override go to not create the ambiguity around it.
But you could be thinking that you can trick the compiler here, by doing:
interface First {
public default void go() {
}
}
static abstract class Second {
abstract void go();
}
static class Impl extends Second implements First {
}
You could think that First::go already provides an implementation for Second::go and it should be fine. This is too taken care of, thus this does not compile either.
JLS 9.4.1.3 : Similarly, when an abstract and a default method with matching signatures are inherited, we produce an error. In this case, it would be possible to give priority to one or the other - perhaps we would assume that the default method provides a reasonable implementation for the abstract method, too. But this is risky, since other than the coincidental name and signature, we have no reason to believe that the default method behaves consistently with the abstract method's contract - the default method may not have even existed when the subinterface was originally developed. It is safer in this situation to ask the user to actively assert that the default implementation is appropriate (via an overriding declaration).
The last point I would bring in, to solidify that multiple inheritance is not allowed even with new additions in java, is that static methods from interfaces are not inherited. static methods are inherited by default:
static class Bug {
static void printIt() {
System.out.println("Bug...");
}
}
static class Spectre extends Bug {
static void test() {
printIt(); // this will work just fine
}
}
But if we change that for an interface (and you can implement multiple interfaces, unlike classes):
interface Bug {
static void printIt() {
System.out.println("Bug...");
}
}
static class Spectre implements Bug {
static void test() {
printIt(); // this will not compile
}
}
Now, this is prohibited by the compiler and JLS too:
JLS 8.4.8 : A class does not inherit static methods from its superinterfaces.
Java doesn't allow multiple inheritance for fields. This would be difficult to support in the JVM as you can only have references to the start of an object where the header is, not arbitrary memory locations.
In Oracle/Openjdk, objects have a header followed by the fields of the most super class, then the next most super class, etc. It would be a significant change to allow the fields of a class to appear at different offsets relative to the header of an object for different subclasses. Most likely object references would have to become a reference to the object header and a reference to the fields to support this.
default methods in interfaces pose a problem that :
If both of the implemented interfaces define a default method with
same method signature, then the implementation class does not know
which default method to use.
The implementation class should define explicitly specify which default method to use or define it's own one.
Thus default methods in Java-8 do not facilitate multiple inheritance. The main motivation behind default methods is that if at some point we need to add a method to an existing interface, we can add a method without changing the existing implementation classes. In this way, the interface is still compatible with older versions. However, we should remember the motivation of using Default Methods and should keep the separation of interface and implementation.
The main issues with multiple inheritance are ordering (for overriding and calls to super), fields and constructors; interfaces don't have fields or constructors, so they don't cause problems.
If you look at other languages they usually fall in two broad categories:
Languages with multiple inheritance plus a few features to disambiguate special cases: virtual inheritance [C++], direct calls to all superconstructors in the most-derived class [C++], linearization of superclasses [Python], complex rules for super [Python], etc.
Languages with a differente concept, usually called interfaces, traits, mixins, modules, etc. that impose some limitations such as: no constructors [Java] or no constructors with parameters [Scala until very recently], no mutable fields [Java], specific rules for overriding (e.g. mixins take precedence over base classes [Ruby] so you can include them when you need a bunch of utility methods), etc. Java has become a language like these.
Why just by disallowing fields and constructors you solve many issues related to multiple inheritance?
You can't have duplicated fields in duplicated base classes.
The main class hierarchy is still linear.
You can't construct your base objects the wrong way.
Imagine if Object had public/protected fields and all subclasses had constructors setting those fields. When you inherit from more than one class (all of them derived from Object), which one gets to set the fields? The last class? They become siblings in the hierarchy, so they know nothing about each other. Should you have multiple copies of Object to avoid this? Would all classes interoperate correctly?
Remember that fields in Java are not virtual (overridable), they are simply data storage.
You could make a language where fields behave like methods and could be overridden (the actual storage would be always private), but that would be a much bigger change and problably wouldn't be called Java anymore.
Interfaces can't be instantiated by themselves.
You should always combine them with a concrete class. That eliminates the need for constructors and makes the programmer's intent clearer too (that is, what is meant to be a concrete class and what's an accessory interface/mixin). This also provides a well-defined place to solve all ambiguities: the concrete class.
That is mostly related to "diamonds problem" i think. Right now if you implement multiple interfaces with the same method, compiler forces you to override method the one you want to implement, because it don't know which on to use. I guess Java creators wanted to remove this problem back when interfaces couldn't use default methods. Now they came up with idea, that is good to be able to have methods with implementation in interfaces, as you can still use those as functional interfaces in streams / lambda expressions and utilize their default methods in processing. You cannot do that with classes but diamond problem still exist there. That is my guess :)
class A{
void m1(){
System.out.println("m1-A");
}
}
class B{
void m1(){
System.out.println("m1-B");
}
}
class C extends A, B{ // this will give an error
// inheritance means making all variables and/or methods available to the child class, here child class will get confused as which m1() method to inherit, hence an error
}
JAVA DOES SUPPORT MULTIPLE INHERITANCE.
If you make a OVERALL COMPARISON OF THE PROGRAMMING LANGUAGE,JAVA,THEN YOU COME TO KNOW THAT I AM TRUE.
Java's topclass or the root class in the Ancestor Hierarchy is the Object class.
This class is a Superclass of all other classes. Hence, each class in Java that we declare or is predefined in the API itself inherits this Object class.
Moreover, Java provides us to inherit one more class of our choice.
Hence, we can say that we are performing INTERLOCKED BUT MULTIPLE INHERITANCE.
2ND Way
Java supports Multiple Inheritance of Interfaces. So you can use as many interface implementations you want. But note, implementing an interface does not define IS A relationship as in case of Inheritance of Classes is possible.

Implementing interface with many methods = huge class. Patterns come to rescue?

I have an interface with a lot of methods. (which i cannot split into different interfaces)
When i create a class that implements the interface i get a lot of methods in 1 class file.
Things get worst when the body of all these methods get larger -> the class file becomes huge and quite difficult to navigate.
Even with ide's like eclipse because the Outline window containing all classes get a vertical scrollbar because not all methods can fit in the outline.
Is there a pattern that prevents this from happening?
No there is no way to split the implementation in many classes.
But you can delegate from the implementing class to any other classes.
This will reduce the code in the implementation but the number of methods stay the same.
I am posting this reply after an answer is accepted, hoping that future comers might find it useful.
As simas_ch said:
No there is no way to split the implementation in many classes. But
you can delegate from the implementing class to any other classes.
This will reduce the code in the implementation but the number of
methods stay the same.
Once I worked on a rather huge application in which I had to define a Lifecycle interface, which contained many states and many functions that could create a hassle, so I came around with something like this:
You can create a class and make it abstract and implement most of the common functions.
public interface TheBigFunctional {
public void functionalA();
public void functionalB();
public void functionalC();
//....
public void functionalZ();
}
public abstract class FunctionalBase implements TheBigFunctional {
public void functionalA() {
aInternal();
}
protected abstract void aInternal();
// Rest of methods implementations.
// You may choose to skip those if you want child classes to implement them.
}
public class FunctionalTypeFoo extends FunctionalBase {
// Implementations.
}
public class FunctionalTypeBar extends FunctionalBase {
// Implementations.
}
There are many (good) ways to come around the sitation, but I am sharing what I did.
I do not quite understand why you cannot split the interface into multiple ones...
I surely would try to use inheritance, like:
First interface:
public interface FatherInterface {
String methodOne(String var);
String methodTwo(String var);
String methodThree(String var);
}
Second interface:
public interface SonInterface extends FatherInterface {
String methodFour(String var);
String methodFive(String var);
}
Third interface:
public interface SecondSonInterface extends SonInterface {
String methodSix(String var);
String methodSeven(String var);
}
And so on... Each interface inheriting from the preceding one.
As for class files becoming large, go with inheritance too.
Father class:
public class Father implements FatherInterface
Son class:
public class Son extends Father implements SonInterface
And so on...
Edit
If you cannot split the interface (as when given by third party), I would do the implementations of the methods by parts. That is, only some of them implemented in each class. Using abstract classes if needed (or leaving blank methods). Each class inheriting from the above and implementing some of the remaining methods.
Perhaps you could employ the strategy pattern on the big interface by implementing some of those methods from different classes. Then, when you wish to use any of those methods, simply call it from the class that implements your 'large' interface.
More on strategy pattern here: https://www.tutorialspoint.com/design_pattern/strategy_pattern.htm
In this case i would consider whether all methods in your class are well designed. Probably they do not have clear purpose and should be splitted per few.
You have to define clear purpose of some particular interface.
Besides, if you are using Java 8, consider ability to provide some default implementation for some methods.
Is there a pattern that prevents this from happening?
In Java:
Subtyping: If you can group sets of methods into different interfaces such that a clear hierarchical relation makes sense. This is what the accepted answer is assuming.
Interface Segregation: If you can group sets of methods into different sibling "domains" or "categories". Take a look at this example from jcabi-github. See how Github is this API's entrypoint, while exposing different sets of functionalities via successive "grouping" interfaces.

Separating the interface from an implementation in Java

How can you separate the interface from an implementation in Java?
In C/C++, this can be done by creating 2 files a .c and .h file with the same file name.
How can you do this in Java?
The closest analogy to .h and .c separation is interfaces.
MyInterface.java
interface MyInterface {
void doSomething();
}
MyImplementation.java
class MyImplementation implements MyInterface {
public void doSomething() {
System.out.println("Hello world");
}
}
You then use the interface type everywhere except for the actual constructor
MyInterface instance = new MyImplementation();
Of course, there are several differences.
A single class may implement multiple interfaces, and multiple classes may a single interface
Member values are not included in an interface, only methods.
Only public methods appears in a Java interface.
No implementation is ever allowed in an interface (as opposed to C++ templates, which depend on .h implementation).
But this is how "programming to interfaces" is accomplished.
There is no consistent convention for naming interfaces vs concrete classes in Java. C# (nearly identical in its treatment of interfaces) has a convention which I have come to use where the interface begins with I, e.g. IList. The Java standard libraries tend to use the "pure" name for the interface, and then modify it for the concrete implementation, such as List (interface) and ArrayList (implementation).
By creating an interface, and implementing it also (generally) in two files -
Say.java the interface -
public interface Say {
public void say(String msg);
}
and Hello.java, the concrete implementation (and in this case) also a main() method -
public class Hello implements Say {
#Override
public void say(String msg) {
System.out.printf("%s, World!%n", msg);
}
public static void main(String[] args) {
Say s = new Hello();
s.say("Hello");
}
}
Note: In Java you can have multiple classes with main() methods. It is also a good practice to use the #Override annotation.
You don't. You could do it via Java interfaces, but doing that for every class would be bad design. Interfaces should only be used when you expect multiple classes to implement them, or when used as a callback function description. Really the point of separating the code from the .h file in C++ is
1)To prevent bloating the binary as compilers would make multiple versions of each function for each file its included in
2)To make it easier to read for those who just want the implementation.
In java, 1 isn't an issue. And Java prefers to solve 2 via automated documentation tools (JavaDoc).
TLDR- you don't.
In Java, interfaces and classes (ie "implementation") are NOT distinguished at file level or by their names. They are both declarations. Specifically, a class declares which interface(s) it implements. For example:
public class MyClass implements Interface1, Interface2, .... {
// implementations
}
Java does not have the same "linking" concept as in C/C++. You can call your class or interface in any legal form. The "link" between an interface and its implementation classes at runtime are done through the class loader.
To have an interface and implementation separated, suggest below steps..
( 1).Identify methods which be part of definition of your class. This becomes your template for new classes with similar functionality.
(2 ). Create a new interface which contains methods which were identified to be part of interface definition (may be many interfaces)
( 3.) Modify your class to implement the interface(s) you have created

java - unique difference between abstract class and concrete class

I know few differences between abstract class and concrete class. I know that you can't create an instance with abstract class unlike concrete class, abstract class can have 'abstract' methods.
But i have an example like the following. A lot of times, we see the following examples at work. I will just skip some common methods that can be defined in the Parent class.
public abstract class Parent {
public void init() {
doInit();
}
public abstract void doInit();
}
public class Child extends Parent {
public void doInit() {
// implementation
}
}
I think that we can do the same thing with a concrete class like the following.
public class Parent {
public void init() {
doInit();
}
public void doInit() {
// Empty
}
}
I am curious to see if there is any unique situation that we have to use abstract class. Is there any significant difference during runtime with the above example?
Thank you.
The reason to use abstract class in this situation is to force everyone inheriting your base class to override the abstract doInit method. Without the class and the method being abstract, they may forget to do so, and the compiler would not catch them.
In addition to this pragmatic purpose, abstract classes provide a powerful way to communicate your design idea to the readers of your code. An abstract class tells the reader that the methods inside provide some common implementation for a group of related classes, rather than implementing a single concept that you are modeling. Very often communicating your intent to your readers is as important as it is to write correct code, because otherwise they might break something while maintaining your code.
It is customary in Java to call abstract classes Abstract...; in your example that would be AbstractParent.
Of course you can do it that way, but it all depends on the right business logic.There might arise a situation where you'd want to enforce a policy on people extending your code.
For example, I write an Employee class and you extend my class for writing a ProjectManager class. But suppose the business does not allow direct instantiation of Employee (like I said, just an example). So I declare my Employee class as abstract, thereby enforcing upon all extenders (read:you) of my class the rule that they can't instantiate Employee directly. (It will happen indirectly through the inheritance chain, of course, i.e. parent objects are created before child objects.)
Used properly, a person at place A controls how another person at place B will code.
A concrete class is one which has implementation (code inside) for all the methods. It does not matter whether it is derived from some other class.
public abstract class IAmAbstract{
public void writeMe(){
System.out.println("I am done with writing");
}
}
public class IAmConcrete extends IAmAbstract{
public void writeMe(){
System.out.println("I am still writing");
}
}
Abstract classes have a variety of useful properties in use with software design.
Other than the obvious differences, such as being unable to be instantiated and being able to hold abstract methods. They are useful for defining common, yet overridable, functions, holding static methods that deal with it's children in a logical manner.
My favorite is the abstract factory pattern though.
By making a factory that is the parent of all the classes it may create, it can force functionality required for creation, this actually causes an odd artefact where technically tighter-coupled code is actually easier to maintain.

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