I am confused how delegation works in Kotlin. Wikipedia says:
With language-level support for delegation, this is done implicitly by having self in the delegate refer to the original (sending) object, not the delegate (receiving object).
Given the following Code:
interface BaseInterface {
fun print()
}
open class Base() : BaseInterface {
override fun print() { println(this) }
}
class Forwarded() {
private val base = Base()
fun print() { base.print() }
}
class Inherited() : Base() {}
class Delegated(delegate: BaseInterface) : BaseInterface by delegate
fun main(args: Array<String>) {
print("Forwarded: ")
Forwarded().print();
print("Inherited: ")
Inherited().print();
print("Delegated: ")
Delegated(Base()).print();
}
I get this output:
Forwarded: Base#7440e464
Inherited: Inherited#49476842
Delegated: Base#78308db1
I'd expect Delegated to return Delegated because self/this should refer to the original object. Do I get it wrong or is Kotlins delegation different?
Kotlin delegation is very simple - it generates all interface methods and implicitly invokes it on delegated object, except for methods explicitly overriden by the user.
Your example is functionally the same as:
class Delegated(delegate: BaseInterface) : BaseInterface{
// when generating bytecode kotlin assigns delegate object to internal final variable
// that is not visible at compile time
private val d = delegate
override fun print(){
d.print()
}
}
So it's pretty clear why it prints Base.
I think this is easiest to understand if we look at the decompiled Java bytecode this gets compiled into:
You can do this by going to Tools > Kotlin > Show Kotlin Bytecode and then clicking Decompile
public final class Delegated implements BaseInterface {
// $FF: synthetic field
private final BaseInterface $$delegate_0;
public Delegated(#NotNull BaseInterface delegate) {
Intrinsics.checkParameterIsNotNull(delegate, "delegate");
super();
this.$$delegate_0 = delegate;
}
public void print() {
this.$$delegate_0.print();
}
}
So when you do interface delegation what happens is that Kotlin creates field for the delegate named $$delegate_0 and adds methods in your delegating class which will operate on $$delegate_0. You can have multiple delegates as well, they will get their own fields. There is one caveat though: you can't access $$delegate_0 directly, not even if you make it a var like this:
class Delegated(var delegate: BaseInterface) : BaseInterface by delegate
This will compile to:
public final class Delegated implements BaseInterface {
#NotNull
private BaseInterface delegate;
// $FF: synthetic field
private final BaseInterface $$delegate_0;
#NotNull
public final BaseInterface getDelegate() {
return this.delegate;
}
public final void setDelegate(#NotNull BaseInterface var1) {
Intrinsics.checkParameterIsNotNull(var1, "<set-?>");
this.delegate = var1;
}
public Delegated(#NotNull BaseInterface delegate) {
Intrinsics.checkParameterIsNotNull(delegate, "delegate");
super();
this.$$delegate_0 = delegate;
this.delegate = delegate;
}
public void print() {
this.$$delegate_0.print();
}
}
sadly. I've written about this topic here.
I'm new to OSGI framework and I'm trying to access the 'Derived' Class variable 'publicVariable' from another class 'Derived2' like "Derived.publicVariable" but publicVariable is always shows null. I really appreciate if someone can help me out with this.
Thanks
Manifest file - Derived2
Require-Bundle:com.xxxxxx.Derived1
Java code
abstract class Base {
protected Vector <String> supportedCommands = new Vector <String> ();
protected abstract void initialiseCommands();
}
class Derived extends Base {
private static Derived derivedPlugin = null;
public Derived()
{
derivedPlugin = this;
}
public static Derived getPlugin()
{
return derivedPlugin;
}
public String publicVariable = null;
protected void initialiseCommands()
{
publicVariable = "someData";
System.out.println("Derived" + publicVariable);
}
}
class Derived2 extends Base {
protected void initialiseCommands()
{
supportedCommands.add(Derived.getPlugin().publicVariable);
System.out.println("IMRSAUtilitiesPlugin" +supportedCommands);
}
Also referred below link, which is a similar issue but i'm not using any static variable, it is just a public variable.
how use Singleton object in different class loader....?
The code in the question will not compile. You are trying to access an instance field (publicVariable in class Derived) in a static way, i.e. Derived.publicVariable.
OSGi does not change the semantics of the Java language, and if you cannot even compile your code then OSGi will certainly not be able to run it.
I am new to Kotlin and I am confused between open and public keywords. Could anyone please tell me the difference between those keywords?
The open keyword means “open for extension“ - i.e. it's possible to create subclasses of an open class:
The open annotation on a class is the opposite of Java's final: it allows others to inherit from this class. By default, all classes in Kotlin are final, which corresponds to Effective Java, Item 17: Design and document for inheritance or else prohibit it.
You also need to be explicit about methods you want to make overridable, also marked with open:
open class Base {
open fun v() {}
fun nv() {}
}
The public keyword acts as a visibility modifier that can be applied on classes, functions, member functions, etc. If a top-level class or function is public, it means it can be used from other files, including from other modules. Note that public is the default if nothing else is specified explicitly:
If you do not specify any visibility modifier, public is used by default, which means that your declarations will be visible everywhere
class A { ... } in Java is equal to open class A { ... } in Kotlin.
final class B { ... } in Java is equal to class B { ...} in Kotlin.
It is not related with public.
In Kotlin, everything without access modifiers is public by default. You can explicitly say public in the definition, but it is not necessary in Kotlin.
So,
public class A { ... }
and
class A { ... }
are the same in Kotlin.
I put here just for my memo, maybe useful for someone else :
open class in kotlin means that a class can be inherited because by default they are not:
class Car{....}
class Supercar:Car{....} : // give an error
open Car2{....}
class Supercar:Car2{....} : // ok
public class in Java is about the visibility of class (nothing to do with inheritance : unless a class in java is final, it can be inherited by default).
In kotlin all the class are public by default.
open method in kotlin means that the method can be overridden, because by default they are not.
Instead in Java all the methods can be overridden by default
The method of an open class cannot be overridden by default as usual (doesn't matter if the class is open), they must be declared that they can be overridden :
open class Car{
fun steering{...}
}
class Supercar:Car{
override fun steering {...} // give an error
}
open class Car2{
open fun steering{...}
}
class Supercar:Car2{
override fun steering {...} // ok
}
for more details : https://kotlinlang.org/docs/reference/classes.html
public: public keyword in Kotlin is similar to java it is use to make the visibility of classes, methods, variables to access from anywhere.
open: In Kotlin all classes, functions, and variables are by defaults final, and by inheritance property, we cannot inherit the property of final classes, final functions, and data members. So we use the open keyword before the class or function or variable to make inheritable that.
open is opposite to Final in java.
If the class is not 'open', it can't be inherited.
class First{}
class Second:First(){} // Not allowed. Since 'First' is Final(as in Java) by default. Unless marked "open" it can't be inherited
Don't get confused with open and public. public is a visibility modifier
class Third{} // By default this is public
private class Fourth{}
class Fifth{
val third = Third() // No issues
val fourth = Fourth() // Can't access because Fourth is private
}
All classes, methods, and members are public by default BUT not open
Keyword open in kotlin means "Open for Extension"
means if you want any class to be inherited by any subclass or method to be overriden in subclasses you have to mark as open otherwise you will get compile time error
NOTE: abstract classes or methods are open by default you do not need to add explicitly.
OPEN VS FINAL VS PUBLIC
OPEN :
child class can access this because they are inherited by its parent.
In Kotlin you need to add 'open' keyword unlike java whose all classes are 'open' by default
Example :
Kotlin : open class A () {}
Java : class A () {}
FINAL :
child class can't access or inherit.
In JAVA you need to add 'final' keyword unlike kotlin whose all classes are 'final' by default
Example :
Kotlin : class A () {}
Java : final class A () {}
PUBLIC : Any class whether its inherited or not can access its data or methods.
Example in Kotlin :
//Final
class DemoA() {
protected fun Method() {
}
}
class DemoB() : DemoA {
Method() // can't access
}
//OPEN
open class DemoA() {
protected fun Method() {
}
}
class DemoB() : DemoA {
Method() // can access
}
//Public
class DemoA() {
fun Method() {
}
}
class DemoB() {
val a = DemoA()
a.Method() // can access
}
Example in Java :
//FINAL
final class DemoA() {
protected void name() {
}
}
class DemoB() extends DemoA {
name(); // Can't access
}
//Open
class DemoA() {
protected void name() {
}
}
class DemoB() extends DemoA {
name(); // Can access
}
//Public
class DemoA() {
void name() {
}
}
class DemoB(){
DemoA a = new DemoA()
a.name(); // Can access
}
Summarized answer (Kotlin)
The defaults of declarations of classes, methods, and properties are
(public + final). final prevents any inheritance attempts.
In order to be able to extend a class, you must mark the
parent class with the open keyword.
In order to be able to override the methods or properties, you must
mark them in the parent class with the open keyword, in addition to
marking the overriding method or parameter with the override keyword.
public is just encapsulation, it affects the visibility of classes/ methods. Public will make them visible everywhere.
Reference
I have wsimport-ed Java classes with standard bean conventions:
public class Request {
protected String vin;
public String getVin() {
return vin;
}
public void setVin(String value) {
this.vin = value;
}
}
I expected to use this class in Kotlin using nice property syntax:
override fun search(request: Request): Response {
log.info("search(vin={})", request.vin);
...
but this code does not compile:
Error:(59, 64) Kotlin: Cannot access 'vin': it is 'protected/*protected and package*/' in 'SmvSearchRequest'
request.getVin() works, of course, but that doesn't look better than Java. Is there some way to treat those classes as property holders?
This was missing pre-M13, it is now fixed in M13, see Youtrack
i have a question regarding design patterns.
suppose i want to design pig killing factory
so the ways will be
1) catch pig
2)clean pig
3) kill pig
now since these pigs are supplied to me by a truck driver
now if want to design an application how should i proceed
what i have done is
public class killer{
private Pig pig ;
public void catchPig(){ //do something };
public void cleanPig(){ };
public void killPig(){};
}
now iam thing since i know that the steps will be called in catchPig--->cleanPig---->KillPig manner so i should have an abstract class containing these methods and an execute method calling all these 3 methods.
but i can not have instance of abstract class so i am confused how to implement this.
remenber i have to execute this process for all the pigs that comes in truck.
so my question is what design should i select and which design pattern is best to solve such problems .
I would suggest a different approach than what was suggested here before.
I would do something like this:
public abstract class Killer {
protected Pig pig;
protected abstract void catchPig();
protected abstract void cleanPig();
protected abstract void killPig();
public void executeKillPig {
catchPig();
cleanPig();
killPig();
}
}
Each kill will extend Killer class and will have to implement the abstract methods. The executeKillPig() is the same for every sub-class and will always be performed in the order you wanted catch->clean->kill. The abstract methods are protected because they're the inner implementation of the public executeKillPig.
This extends Avi's answer and addresses the comments.
The points of the code:
abstract base class to emphasize IS A relationships
Template pattern to ensure the steps are in the right order
Strategy Pattern - an abstract class is as much a interface (little "i") as much as a Interface (capital "I") is.
Extend the base and not use an interface.
No coupling of concrete classes. Coupling is not an issue of abstract vs interface but rather good design.
public abstract Animal {
public abstract bool Escape(){}
public abstract string SaySomething(){}
}
public Wabbit : Animal {
public override bool Escape() {//wabbit hopping frantically }
public override string SaySomething() { return #"What's Up Doc?"; }
}
public abstract class Killer {
protected Animal food;
protected abstract void Catch(){}
protected abstract void Kill(){}
protected abstract void Clean(){}
protected abstract string Lure(){}
// this method defines the process: the methods and the order of
// those calls. Exactly how to do each individual step is left up to sub classes.
// Even if you define a "PigKiller" interface we need this method
// ** in the base class ** to make sure all Killer's do it right.
// This method is the template (pattern) for subclasses.
protected void FeedTheFamily(Animal somethingTasty) {
food = somethingTasty;
Catch();
Kill();
Clean();
}
}
public class WabbitHunter : Killer {
protected override Catch() { //wabbit catching technique }
protected override Kill() { //wabbit killing technique }
protected override Clean() { //wabbit cleaning technique }
protected override Lure() { return "Come here you wascuhwy wabbit!"; }
}
// client code ********************
public class AHuntingWeWillGo {
Killer hunter;
Animal prey;
public AHuntingWeWillGo (Killer aHunter, Animal aAnimal) {
hunter = aHunter;
prey = aAnimal;
}
public void Hunt() {
if ( !prey.Escape() ) hunter.FeedTheFamily(prey)
}
}
public static void main () {
// look, ma! no coupling. Because we pass in our objects vice
// new them up inside the using classes
Killer ElmerFudd = new WabbitHunter();
Animal BugsBunny = new Wabbit();
AHuntingWeWillGo safari = new AHuntingWeWillGo( ElmerFudd, BugsBunny );
safari.Hunt();
}
The problem you are facing refer to part of OOP called polymorphism
Instead of abstract class i will be using a interface, the difference between interface an abstract class is that interface have only method descriptors, a abstract class can have also method with implementation.
public interface InterfaceOfPigKiller {
void catchPig();
void cleanPig();
void killPig();
}
In the abstract class we implement two of three available methods, because we assume that those operation are common for every future type that will inherit form our class.
public abstract class AbstractPigKiller implements InterfaceOfPigKiller{
private Ping pig;
public void catchPig() {
//the logic of catching pigs.
}
public void cleanPig() {
// the logic of pig cleaning.
}
}
Now we will create two new classes:
AnimalKiller - The person responsible for pig death.
AnimalSaver - The person responsible for pig release.
public class AnimalKiller extends AbstractPigKiller {
public void killPig() {
// The killing operation
}
}
public class AnimalSaver extends AbstractPigKiller {
public void killPing() {
// The operation that will make pig free
}
}
As we have our structure lets see how it will work.
First the method that will execute the sequence:
public void doTheRequiredOperation(InterfaceOfPigKiller killer) {
killer.catchPig();
killer.cleanPig();
killer.killPig();
}
As we see in the parameter we do not use class AnimalKiller or AnimalSever. Instead of that we have the interface. Thank to this operation we can operate on any class that implement used interface.
Example 1:
public void test() {
AnimalKiller aKiller = new AnimalKiller();// We create new instance of class AnimalKiller and assign to variable aKiller with is type of `AnimalKilleraKiller `
AnimalSaver aSaver = new AnimalSaver(); //
doTheRequiredOperation(aKiller);
doTheRequiredOperation(aSaver);
}
Example 2:
public void test() {
InterfaceOfPigKiller aKiller = new AnimalKiller();// We create new instance of class AnimalKiller and assign to variable aKiller with is type of `InterfaceOfPigKiller `
InterfaceOfPigKiller aSaver = new AnimalSaver(); //
doTheRequiredOperation(aKiller);
doTheRequiredOperation(aSaver);
}
The code example 1 and 2 are equally in scope of method doTheRequiredOperation. The difference is that in we assign once type to type and in the second we assign type to interface.
Conclusion
We can not create new object of abstract class or interface but we can assign object to interface or class type.