I have a simple inner class variable, how do i access it in scala?
class Outer {
class Inner {
var x = 1
}}
object Main {
def main(args: Array[String]): Unit = {
val o = new Outer
val i = new o.Inner
println(i.x)
}
}
The problem is that IntelliJ complains that it cannot resolve x, but when i run the program it works fine.
you can simply use .member_name to access variables in scala.
scala> class Outer {
class Inner {
var x = 1 //it can be val which is immutable
}}
defined class Outer
scala> val o = new Outer
o: Outer = Outer#358b0b42
scala> val i = new o.Inner
i: o.Inner = Outer$Inner#512f2c7d
scala> i.x
res13: Int = 1
since your example has x defined as mutable, you can change the value of x,
scala> i.x = 100
i.x: Int = 100
scala> i.x
res14: Int = 100
See working example - https://scastie.scala-lang.org/prayagupd/C9k9an4ASdaISnohbYQBmA
If you don't really need Outer to be a class, you can define it as singleton,
scala> object Outer {
| class Inner {
| var x = 1 //it can be val which is immutable
| }}
defined object Outer
then, simple instantiate Inner and access variables,
scala> val inner = new Outer.Inner
inner: Outer.Inner = Outer$Inner#4bcdd11
scala> inner.x
res2: Int = 1
Regarding not working on intellij, File | Invalidate Caches/Restart... should work
Related
I know that one can define an operator in Scala like this :
class value(var valu:Int) {
def +(i:Int) = { this.valu + i }
def ==>(i:Int ) = { this.valu = i }
}
But I cannot seem to overload the = operator like this :
class value(var valu:Int) {
def =(i:Int) = { this.valu = i }
}
Do you know if there is any way to do this?
The syntax for making mutable objects isn't obvious and isn't encountered often because mutability is generally undesirable.
class Value(private var valu:Int) {
def update(i:Int) :Unit = valu = i
}
val v = new Value(19)
v() = 52
= is a reserved word like yield, so to use it as an identifier, you put it in backticks, though I suspect no one does that:
scala> class C(var i: Int) { def `=`(n: Int) = i = n }
defined class C
scala> val c = new C(42)
c: C = C#9efcd90
scala> c.`=`(27)
scala> c.i
res1: Int = 27
scala> c `=` 5
scala> c.i
res3: Int = 5
Compare:
scala> val yield = 2
^
error: illegal start of simple pattern
scala> val `yield` = 2
yield: Int = 2
The scope of a name introduced by a declaration or definition is the
whole statement sequence containing the binding. However, there is a
restriction on forward references in blocks: In a statement sequence
s[1]...s[n] making up a block, if a simple name in s[i] refers to
an entity defined by s[j] where j >= i, then for all s[k]
between and including s[i] and s[j],
s[k] cannot be a variable definition.
If s[k] is a value definition, it must be lazy.
Edit: I am not sure Mikaël Mayer's answer actually explained everything. Consider:
object Test {
def main(args: Array[String]) {
println(x)
lazy val x: Int = 6
}
}
Here, the lazy value x definitely has to be read/evaluated before it is actually defined in the code! Which would contradict Mikaël's claim that lazy evaluation does away with the need to evaluate things before they are defined.
Normally you cannot have this:
val e: Int = 2
val a: Int = b+c
val b: Int = c
val c: Int = 1
val d: Int = 0
because value c is not yet defined at the time of the definition of a. Because a references c, all values between a and c should be lazy so that the dependency is avoided
val e: Int = 2
lazy val a: Int = b+c
lazy val b: Int = c
lazy val c: Int = 1
val d: Int = 0
This in fact translates a, b and c as objects whose value is initialized when it is read, which would be after the declaration, i.e. this would be equivalent to:
val e: Int = 2
var a: LazyEval[Int] = null
var b: LazyEval[Int] = null
var c: LazyEval[Int] = null
a = new LazyEval[Int] {
def evalInternal() = b.eval() + c.eval()
}
b = new LazyEval[Int] {
def evalInternal() = c.eval()
}
c = new LazyEval[Int] {
def evalInternal() = 1
}
val d = 0
where LazyEval would be something like the following (implemented by the compiler itself)
class LazyEval[T] {
var value: T = _
var computed: Boolean = false
def evalInternal(): T // Abstract method to be overriden
def eval(): T = {
if(computed) value else {
value = evalInternal()
computed = true
value
}
}
}
Edit
vals don't really exist in java. They are local variables or do not exist in computation. Therefore, the declaration of lazy val exists before anything is done. And remember that closures are implemented in Scala.
Your block would be rewritten as it:
object Test {
def main(args: Array[String]) {
// Declare all variables, val, vars.
var x: Lazy[Int] = null
// No more variables to declare. Lazy/or not variable definitions
x = new LazyEval[Int] {
def evalInternal() = 6
}
// Now the code starts
println(x)
}
}
You're trying to avoid references to entities which are provably uninitialized (or which are maybe uninitialized).
In a block, assignments occur in source order, but in a class template, members can be overridden and initialized early.
For instance,
{ val a = b ; val b = 1 } // if allowed, value of a is undefined
but in a template
class X { val a = b ; val b = 1 } // warning only
val x = new { override val b = 2 } with X
x.a // this is 2
class Y(override val b: Int) extends X // similarly
You also want to avoid this:
locally {
def a = c
val b = 2 // everything in-between must be lazy, too
def c = b + 1
}
Local objects are explicitly the same as lazy vals:
{ object p { val x = o.y } ; object o { val y = 1 } }
Other kinds of forward reference:
{ val x: X = 3 ; type X = Int }
The spec talks about forward references to "entities" -- a "name refers to an entity" -- which elsewhere means both terms and types, but obviously it really means only terms here.
It will let you harm yourself:
{ def a: Int = b ; def b: Int = a; a }
Maybe your mode of self-destruction must be well-defined. Then it's OK.
I'm using Scala 2.10.1 and I'm trying the define a method which will retrieve all the vals (including the inherited ones) from an object.
I have the following:
import scala.reflect.runtime.{universe => ru}
object Reflection {
val mirror = ru.runtimeMirror(this.getClass.getClassLoader)
def findVals(x: Any): Iterable[String] = {
val theType = mirror.classSymbol(x.getClass).toType
theType.members.collect({case x if x.isTerm => x.asTerm}).filter(_.isVal).map(_.name.toString)
}
}
I am testing on these two classes:
class Base {
val x = 10
}
class Child extends Base {
val y = 20
}
When calling the following code:
val x = new Child
val vs = Reflection.findVals(x)
println(vs)
The result is List(y)
For some reason, the isVal method returns false for the term corresponding to the x field from the Base class.
Can someone tell me what's the problem here? Am I doing something wrong?
Per Why don't Scala case class fields reflect as public? you should use isAccessor instead of isVal.
I'm actually using isGetter and setter to properly filter vars per your comment:
def findVals(x: Any): Iterable[String] = {
val theType = mirror.classSymbol(x.getClass).toType
val xtm = theType.members.collect({case x if x.isTerm => x.asTerm})
xtm.filter(m => m.isGetter && !xtm.exists(m.setter == _)).map(_.name.toString)
}
Results:
scala> class Base {
| var x = 10
| val xx = 2
| }
defined class Base
scala> class Child extends Base {
| val y = 3
| }
defined class Child
scala> val x = new Child
x: Child = Child#1c0026e
scala> val vs = Reflection.findVals(x)
vs: Iterable[String] = List(y, xx)
scala> println(vs)
List(y, xx)
Using SMirror:
scala> implicit val mirror = scala.reflect.runtime.currentMirror
mirror: reflect.runtime.universe.Mirror = JavaMirror with scala.tool…
scala> import net.fwbrasil.smirror._
import net.fwbrasil.smirror._
scala> class Base {
val x = 10
}
defined class Base
scala> class Child extends Base {
val y = 20
}
defined class Child
scala> val x = new Child
x: Child = Child#448593d0
scala> x.reflect.vals
res5: List[net.fwbrasil.smirror.SInstanceVal[Child]] = List(val x: scala.Int (bound to Child#448593d0), val y: scala.Int (bound to Child#448593d0))
scala> x.reflect.vals.head.get
res7: Any = 10
So, this is terribly inelegant, but it seems to work:
import scala.reflect.runtime.{universe => ru}
object Reflection {
val mirror = ru.runtimeMirror(this.getClass.getClassLoader)
val ObjectClass = classOf[java.lang.Object];
def findVals(x: Any) : Iterable[String] = findVals( x.getClass, List.empty );
def findVals(clz: Class[_], accum : Iterable[String]): Iterable[String] = {
clz match {
case ObjectClass => accum;
case _ => {
val theType = mirror.classSymbol(clz).toType
val newVals = theType.members.collect({case x if x.isTerm => x.asTerm}).filter(_.isVal).map(_.name.toString)
findVals( clz.getSuperclass, accum ++ newVals )
}
}
}
}
Then...
scala> class Base {
| val x = 10
| var z = 20
| }
defined class Base
scala> class Child extends Base {
| val y = 20
| var a = 9
| }
defined class Child
scala> val x = new Child
x: Child = Child#3093266d
scala> val vs = Reflection.findVals(x)
vs: Iterable[String] = List("y ", "x ")
scala> println(vs)
List(y , x )
It seems that, at least for now, Scala reflection looks at the Java field to determine the presence of a val, so I guess you just have to climb the class hierarchy... I'm guessing it looks for the presence of a setter to distinguish val from var. Again, not so lovely, but functional.
I want to get a list of inner objects of a scala object. Sample code:
object Outer {
val v = "-"
def d = "-"
object O1
object O2
}
object Main {
def main(args: Array[String]) {
Outer.getClass.getDeclaredMethods.map(_.getName) foreach println // prints d and v
// Outer.getClass.get ... Objects???
}
}
I can find v and d, but how can I find O1 and O2 ?
With the new reflection library in Scala 2.10 (since Milestone 4) it is possible to get the inner objects:
scala> import scala.reflect.runtime.{universe => u}
import scala.reflect.runtime.{universe=>u}
scala> val outer = u.typeOf[Outer.type]
outer: reflect.runtime.universe.Type = Outer.type
scala> val objects = outer.declarations.filter(_.isModule).toList
objects: List[reflect.runtime.universe.Symbol] = List(object O1, object O2)
Object O1 and O2 are nested classes and are not a part of Outer Object.
println(Outer.O1.getClass.getName) //Outer$O1$
println(Outer.getClass.getName) //Outer$
println(Outer.O2.getClass.getName) //Outer$O2$
Why in this example, no error is thrown and b ends up holding default value?
scala> val b = a; val a = 5
b: Int = 0
a: Int = 5
When you do this in the REPL, you are effectively doing:
class Foobar { val b = a; val a = 5 }
b and a are assigned to in order, so at the time when you're assigning b, there is a field a, but it hasn't yet been assigned to, so it has the default value of 0. In Java, you can't do this because you can't reference a field before it is defined. I believe you can do this in Scala to allow lazy initialisation.
You can see this more clearly if you use the following code:
scala> class Foobar {
println("a=" + a)
val b = a
println("a=" + a)
val a = 5
println("a=" + a)
}
defined class Foobar
scala> new Foobar().b
a=0
a=0
a=5
res6: Int = 0
You can have the correct values assigned if you make a a method:
class Foobar { val b = a; def a = 5 }
defined class Foobar
scala> new Foobar().b
res2: Int = 5
or you can make b a lazy val:
scala> class Foobar { lazy val b = a; val a = 5 }
defined class Foobar
scala> new Foobar().b
res5: Int = 5