I want the protected fields of a class to be made publicly visible in a subclass:
class MyClass(protected val someDao: SomeDao)
class TestMyClass extends MyClass(mock[SomeDao])
Now I want to access someDao like so:
val testClass = new TestMyClass
testClass.someDao
I've read that in Java and Scala access modifiers can be broadened but not restricted in subclasses (although these are just constructor parameters, not private fields per se). How can I achieve that in this example?
EDIT:
The answers provided suggest making someDao public in the base class. This is not what I want. I want it to remain private, but just change its visibility to public in the TestMyClass class.
As far as I know, an overriden val cannot access the super implementation.
If you can modify the base class, one solution is to explicitely define an accessor method, which can be overriden; something like this:
class MyClass(_someDao: SomeDao) {
protected def someDao = _someDao
}
class TestMyClass extends MyClass(mock[SomeDao]) {
override def someDao = super.someDao
}
If you cannot change the base class, one workaround would be to define a public accessor method with a different name:
class MyClass(protected val someDao: SomeDao)
class TestMyClass extends MyClass(mock[SomeDao]) {
def someDao2 = someDao
}
Add var or val to parameters declaration:
class MyClass(val someDao: SomeDao)
In this case only getter function will be generate for someDao field. So when you write testClass.someDao you retrieve not the someDao as field you use their getter function.
Good example about visibility of class parameters in Scala demostrated at this article.
Regarding to your last updates
Set package scope for parameter declaration:
class MyClass(private[lastPackageOfSomeDao] val someDao: SomeDao)
I'm not sure why this won't compile - I'm trying to use an inner class (or trait) such that other objects can work with the resulting "RequestReturn", but only Trait Request and it's descendants can construct the object in the first place. I may be taking the wrong approach but shouldn't this code logically work? I've flagged the RequestReturn constructor as protected[Request] so it stands to reason that class RequestContinue would have the ability to call the constructor also.
trait Request {
class RequestReturn protected[Request](val x:Any)
def fulfill(item:Boolean):RequestReturn = new RequestReturn(item) //this line compiles
}
trait RequestContinue extends Request{
override def fulfill(item:Boolean):RequestReturn = new RequestReturn(item) //this won't compile
}
Error:(19, 54) constructor RequestReturn in class RequestReturn cannot be accessed in trait RequestContinue
Access to protected constructor RequestReturn not permitted because
enclosing trait RequestContinue in package .... is not a subclass of
class RequestReturn in trait Request where target is defined
override def fulfill(item:Boolean):RequestReturn = new RequestReturn(item)
^
The rules are here:
Access from RequestReturn, descendants and their companion modules; and from Request and its companion module.
You could define a protected factory method in Request for your use case.
i needed two instances that has access to each other privates. i naturaly thought of a companion object that grants access to a one and only instance of it's companion class. the class itself i made private, so users cannot just create instances using new.
object A {
def apply = dual
lazy val dual = new A
}
private class A {
//some irrelevant logic...
}
this code does not compile. i get: class A escapes its defining scope as part of type A error, which i don't really understand. my current workaround was to define a trait with every method declaration the class should have and make class A extend that trait, while dual is of the trait type, and not class A type.
what's the theoretic problem i'm missing here? why is this forbiden?
Paolo's solution is good (+1), but he didn't explain the error message, so let me try that. The problem stems from the fact that every method needs a return type. Your original definition of apply and dual returned an object of class A, thus the implicit return type of both was A. That implies that A must be visible to clients - how else could they call the function or access the val? Moreover, as both - and their parent object too - are public, they are globally visible. However, you declared A private which means it must not be visible outside its package. So there is a conflict which can't be resolved by the compiler.
The general rule is that all parameter and return type of functions / members must have (at least) the same scope of visibility as the referring member itself*. Thus one trivial way to solve this problem would be to reduce the visibility of apply and dual to private. This would satisfy the compiler, but not you :-)
Your solution gets around the problem by changing the static return type to a public trait, which thus has the same visibility as the members referring to it. The dynamic type of the returned object is still class A, however, this need not be visible to clients. This is a classic example of the principle "program to interfaces, not implementations".
Note that to apply this principle to the full extent, one could turn class A into a private inner class of object A, thus making it innaccessible even for other classes within the same package:
trait A {
//...
}
object A {
def apply: A = dual
lazy val dual: A = new AImpl
private class AImpl extends A {
//some irrelevant logic...
}
}
* To be pedantic, the enclosing class / object may reduce the visibility of its members, like here:
private class Holder {
def member = new Hidden
}
private class Hidden
where member is public but its enclosing class is private, effectively hiding its members from the external world. So the compiler emits no complaints here.
I think you don't want a private class, but a class with a private constructor.
class A private()
object A {
def apply = dual
lazy val dual = new A
}
Now your class is "visible" to outside code, but only your companion object can create instances of it.
In the Scaladoc of class Enumeration#Val, I can read: "A class implementing the Value type. This class can be overridden to change the enumeration's naming and integer identification behaviour." I am puzzled: how do I override a class? Things like override class Val extends super.Val are not permitted.
There are no virtual classes in Scala (yet), so you can't write override class Val ..., and then be sure that invoking new Val will dynamically choose the right class for the new instance. What would happen instead is that the class would be chosen based on the type of the reference to the instance of the enclosing class (in this case, Enumeration).
The general trick to emulate virtual classes is to write class Val extends super.Val, and then override a protected method which serves as a factory for instances of the class. In this case, you would also have to override the method:
protected def Value(i: Int, name: String): Value = new Val(i, name)
Enumeration will create instances of Val only using this factory method. In general, this pattern requires discipline on the programmer's part, but can be ensured by declaring the constructors private, forcing the programmer to use the factory method.
I'm just going over some Scala tutorials on the Internet and have noticed in some examples an object is declared at the start of the example.
What is the difference between class and object in Scala?
tl;dr
class C defines a class, just as in Java or C++.
object O creates a singleton object O as instance of some anonymous class; it can be used to hold static members that are not associated with instances of some class.
object O extends T makes the object O an instance of trait T; you can then pass O anywhere, a T is expected.
if there is a class C, then object C is the companion object of class C; note that the companion object is not automatically an instance of C.
Also see Scala documentation for object and class.
object as host of static members
Most often, you need an object to hold methods and values/variables that shall be available without having to first instantiate an instance of some class.
This use is closely related to static members in Java.
object A {
def twice(i: Int): Int = 2*i
}
You can then call above method using A.twice(2).
If twice were a member of some class A, then you would need to make an instance first:
class A() {
def twice(i: Int): Int = 2 * i
}
val a = new A()
a.twice(2)
You can see how redundant this is, as twice does not require any instance-specific data.
object as a special named instance
You can also use the object itself as some special instance of a class or trait.
When you do this, your object needs to extend some trait in order to become an instance of a subclass of it.
Consider the following code:
object A extends B with C {
...
}
This declaration first declares an anonymous (inaccessible) class that extends both B and C, and instantiates a single instance of this class named A.
This means A can be passed to functions expecting objects of type B or C, or B with C.
Additional Features of object
There also exist some special features of objects in Scala.
I recommend to read the official documentation.
def apply(...) enables the usual method name-less syntax of A(...)
def unapply(...) allows to create custom pattern matching extractors
if accompanying a class of the same name, the object assumes a special role when resolving implicit parameters
A class is a definition, a description. It defines a type in terms of methods and composition of other types.
An object is a singleton -- an instance of a class which is guaranteed to be unique. For every object in the code, an anonymous class is created, which inherits from whatever classes you declared object to implement. This class cannot be seen from Scala source code -- though you can get at it through reflection.
There is a relationship between object and class. An object is said to be the companion-object of a class if they share the same name. When this happens, each has access to methods of private visibility in the other. These methods are not automatically imported, though. You either have to import them explicitly, or prefix them with the class/object name.
For example:
class X {
// class X can see private members of object X
// Prefix to call
def m(x: Int) = X.f(x)
// Import and use
import X._
def n(x: Int) = f(x)
private def o = 2
}
object X {
private def f(x: Int) = x * x
// object X can see private members of class X
def g(x: X) = {
import x._
x.o * o // fully specified and imported
}
}
An object has exactly one instance (you can not call new MyObject). You can have multiple instances of a class.
Object serves the same (and some additional) purposes as the static methods and fields in Java.
As has been explained by many, object defines a singleton instance. The one thing in the answers here that I believe is left out is that object serves several purposes.
It can be the companion object to a class/trait, containing what might be considered static methods or convenience methods.
It can act much like a module, containing related/subsidiary types and definitions, etc.
It can implement an interface by extending a class or one or more traits.
It can represent a case of a sealed trait that contains no data. In this respect, it's often considered more correct than a case class with no parameters. The special case of a sealed trait with only case object implementors is more or less the Scala version of an enum.
It can act as evidence for implicit-driven logic.
It introduces a singleton type.
It's a very powerful and general construct. What can be very confusing to Scala beginners is that the same construct can have vastly different uses. And an object can serve many of these different uses all at once, which can be even more confusing.
Defining an object in Scala is like defining a class in Java that has only static methods. However, in Scala an object can extend another superclass, implement interfaces, and be passed around as though it were an instance of a class. (So it's like the static methods on a class but better).
The formal difference -
you can not provide constructor parameters for Objects
Object is not a type - you may not create an instance with new operator. But it can have fields, methods, extend a superclass and mix in traits.
The difference in usage:
Scala doesn't have static methods or fields. Instead you should use object. You can use it with or without related class. In 1st case it's called a companion object. You have to:
use the same name for both class and object
put them in the same source file.
To create a program you should use main method in object, not in class.
object Hello {
def main(args: Array[String]) {
println("Hello, World!")
}
}
You also may use it as you use singleton object in java.
In scala, there is no static concept. So scala creates a singleton object to provide entry point for your program execution.
If you don't create singleton object, your code will compile successfully but will not produce any output. Methods declared inside Singleton Object are accessible globally. A singleton object can extend classes and traits.
Scala Singleton Object Example
object Singleton{
def main(args:Array[String]){
SingletonObject.hello() // No need to create object.
}
}
object SingletonObject{
def hello(){
println("Hello, This is Singleton Object")
}
}
Output:
Hello, This is Singleton Object
In scala, when you have a class with same name as singleton object, it is called companion class and the singleton object is called companion object.
The companion class and its companion object both must be defined in the same source file.
Scala Companion Object Example
class ComapanionClass{
def hello(){
println("Hello, this is Companion Class.")
}
}
object CompanoinObject{
def main(args:Array[String]){
new ComapanionClass().hello()
println("And this is Companion Object.")
}
}
Output:
Hello, this is Companion Class.
And this is Companion Object.
In scala, a class can contain:
1. Data member
2. Member method
3. Constructor Block
4. Nested class
5. Super class information etc.
You must initialize all instance variables in the class. There is no default scope. If you don't specify access scope, it is public. There must be an object in which main method is defined. It provides starting point for your program. Here, we have created an example of class.
Scala Sample Example of Class
class Student{
var id:Int = 0; // All fields must be initialized
var name:String = null;
}
object MainObject{
def main(args:Array[String]){
var s = new Student() // Creating an object
println(s.id+" "+s.name);
}
}
I am sorry, I am too late but I hope it will help you.
The object keyword creates a new singleton type, which is like a class that only has a single named instance. If you’re familiar with Java, declaring an object in Scala is a lot like creating a new instance of an anonymous class.
Scala has no equivalent to Java’s static keyword, and an object is often used in Scala where you might use a class with static members in Java.
Object is a class but it already has(is) an instance, so you can not call new ObjectName. On the other hand, Class is just type and it can be an instance by calling new ClassName().
A class is just like any other class in other languages. You define class just like any other language with some syntax difference.
class Person(val name: String)
val me = new Person("My name")
However, object is a class with single object only. This makes it interesting as it can be used to create static members of a class using companion object. This companion object has access to private members of the class definition and it has the same name as the class you're defining.
class Person(var name: String) {
import Person._
def hi(): String = sayHello(name)
}
object Person {
private def sayHello(name: String): String = "Hello " + name
}
val me = new Person("My name")
me.hi()
Also, noteworthy point is that object class is lazily created which is another important point. So, these are not instantiated unless they are needed in our code.
If you're defining connection creation for JDBC, you can create them inside object to avoid duplication just like we do in Java with singleton objects.
Scala class same as Java Class but scala not gives you any entry method in class, like main method in java. The main method associated with object keyword. You can think of the object keyword as creating a singleton object of a class that is defined implicitly.
more information check this article
class and object keyword in scala programming
The object is similar to the static class in Java to some extend, the static characteristic means the static class need not to create an object when putting to the JVM, it can be used by it's class name directly and the same instance(same data state) is shared wherever it is used.
If you are coming from java background the concept of class in scala is kind of similar to Java, but class in scala cant contain static members.
Objects in scala are singleton type you call methods inside it using object name, in scala object is a keyword and in java object is a instance of class