It is code worked in java but after convert to kotlin it does not compile.
Having a base class which has some defines as static protected member in the companion object:
abstract class ParentClass {
companion object {
#JvmField
final protected val SERVICE_TYPE_A = "the_service_type_a"
}
}
and the child class:
class ChildClass: ParentClass {
public override fun getServiceType(): String {
return SERVICE_TYPE_A. //<== got compile error
}
}
it does not compile.
how to access a parent class static protected member from subclass?
You need to use #JvmStatic instead as follows:
abstract class ParentClass {
companion object {
#JvmStatic
protected val SERVICE_TYPE_A = "the_service_type_a"
}
abstract fun getServiceType(): String
}
The final keyword in SERVICE_TYPE_A is redundant since everything is final by default in Kotlin. This also mean that if you want ParentClass to be extended, then you need to explicitly define it as open.
Then your ChildClass would look as follows:
class ChildClass: ParentClass() {
override fun getServiceType(): String {
return SERVICE_TYPE_A
}
}
In Kotlin I have an abstract class that other classes can inherit from. I would like to have some functions that the class that inherits this class can optionally implement. In the code below, the function is protected abstract. This however requires that the class that is inheriting this class MUST implement these functions. Is there a way to make it so that the class that is inheriting can choose to implement the functions or not implement them?
abstract class BaseDialogFragment {
protected abstract fun getButton1Text(): String
protected abstract fun getButton2Text(): String
}
It is very simple, you just provide the default implementation like in the example below and your inheritors can override them:
abstract class BaseDialogFragment {
open fun getButton1Text(): String {
TODO("Your default implementation here")
}
open fun getButton2Text(): String {
TODO("Your default implementation here")
}
}
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 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