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
}
}
android project has multiple modules. module A has some base class in kotlin
package xxx.module_a
open class InModule_A {
protected function action() {...}
}
class Runner() {
fun doSomething() {
InModule_A().action(). // it is NOT compile, but if the action() is internal it is ok since they are in same module
}
}
in module A the Runner class need to access the InModule_A() class member function action().
And the InModule_A.action() should only be visible inside the module A and to be overridden in its derived classes in other module.
In module B, it has class InModule_B derived from InModule_A.
package xxx.module_b
class InModule_B {
protected override function action() {// if InModule_A().action() were a internal it would not be able to override here
super.action()
... ...
}
}
how to make function has internal visibility and also to able to override in the derived class?
I'm not 100% sure I understand, but maybe creating an alternate function that calls through to the protected function would fit your situation.
open class InModule_A {
protected open fun action() {}
internal fun internalAction() = action()
}
class Runner() {
fun doSomething() {
InModule_A().internalAction()
}
}
Hi I would like know why the following example doesn't work
abstract class BaseClass {
}
class ConcretClasOne : BaseCalculator {
}
class ConcretClasTwo : BaseCalculator {
}
abstract class BaseRun {
abstract fun run(param: BaseClass): Int
}
class ConcretRun : BaseRun {
override fun run(param: ConcretClasOne): Int {
return 0
}
}
this shows me a message run overrides nothing.
I suppose that kotlin isn't able to match the abstract class and the concrete implementation, but what other alternative is there to emulate this behavior, that the run method in the concrete class ConcretRun should receive a concrete param ConcretClasOne?
Generics
Using generics, you can make the base class have a type extending the base class, so that the run method can take that type in.
abstract class BaseClass {
}
class ConcretClasOne: BaseCalculator {
}
class ConcretClasTwo: BaseCalculator {
}
abstract class BaseRun<T: BaseClass> {
abstract fun run(param: T): Int
}
class ConcretRun: BaseRun<ConcretClasOne> {
override fun run(param: ConcretClasOne): Int {
return 0
}
}
Why your code doesn't work
At the moment you are trying to override a method with a more specific type, but as the more general base method can accept more types the more specific method cannot override 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 a class with a nested, private class. I have a Builder, standard Java builder pattern, that constructs instances of this class. I don't want anyone outside of my class to be able to see my hidden class.
In Java I could do this:
public class Example {
private SneakyType doNotExposeThis;
private Example(Builder builder) {
// OK 'cause in Java you can access the private
// members of a nested class
doNotExposeThis = builder.doNotExposeThis;
}
private static class SneakyType {
SneakyType(String x) {
// stuff
}
}
public static class Builder {
private SneakyType doNotExposeThis;
public void addFoo(String something) {
doNotExposeThis = new SneakyType(something);
}
public Example build() { return new Example(this); }
}
}
But I can't figure out how to do the same in Kotlin:
class Example(builder: Builder) {
private lateinit var doNotExposeThis: SneakyType
init {
doNotExposeThis = builder.doNotExposeThis
}
class Builder {
// If private or internal I can't access it in Example.init
// and if public it gets exposed.
val doNotExposeThis: SneakyType
fun addFoo(something: String) {
// actual construction is quite a bit more complex
doNotExposeThis = SneakyType(something)
}
}
}
Note that for the sake of Java interop I want to keep my builder. I also want it because my object is complicated to construct and I want it to be immutable so I have a builder with lots of setters, adders, vals, etc. and then in init I construct a single immutable Example.
The only alternatives I see are:
Instead of have a SneakyType in my builder save all the info necessary to construct one and then construct it in Example. Works but adds a ton of complexity.
Give up on Example being immutable and allow the builder to call into it to set up a Sneaky
Expose the Sneaky
Is there no way to mimic the Java version?
I see two viable options:
Use the internal visibility modifier:
class Example private constructor(builder: Builder) {
private val doNotExposeThis: SneakyType
init {
doNotExposeThis = builder.doNotExposeThis
}
internal class SneakyType(x: String)
class Builder {
internal lateinit var doNotExposeThis: SneakyType
fun addFoo(something: String) {
doNotExposeThis = SneakyType(something)
}
fun build(): Example {
return Example(this)
}
}
}
This will make SneakyType only visible within your Kotlin compilation module.
Make Example independent of its builder (this is what I recommend):
class Example private constructor(private val doNotExposeThis: SneakyType) {
private class SneakyType(x: String)
class Builder {
private lateinit var doNotExposeThis: SneakyType
fun addFoo(something: String) {
doNotExposeThis = SneakyType(something)
}
fun build(): Example {
return Example(doNotExposeThis)
}
}
}