Question

I’m new to scala and need clarification on the following code snippet involving constructors for classes.

class sample (a: Int, b: Int) {
/* define some member functions here */
}

Can I take that the variables a and b are private to the class sample?

class sample (val a: Int, val b: Int) {
/* define some member functions here */
}

And in this case, are a and b publicly accessible? What is the exact effect of adding the val keyword in the parameter list for the constructor? And if I use the def keyword instead of val, does it have the same effect as well?

Answer 1

class sample (a: Int, b: Int)

a and b are private in this case. Disassembling with javap shows a and b are not part of the class(Scala calls these class fields):

public class Sample extends java.lang.Object implements scala.ScalaObject{
    public Sample(int, int);
}

a and b preceded with val. Disassembling with javap shows a and b are now public fields in Sample.

class sample (val a: Int, val b: Int)

public class Sample extends java.lang.Object implements scala.ScalaObject{
    public int a();
    public int b();
    public Sample(int, int);
}

With def instead of val in the constructor it won’t compile. def is for defining functions. Not sure if you can use def in constructor as a parameter.

Also, note that private and protected behave as you would expect. Given this:

class Sample(private val a: Int, protected val b: Int, val c: Int)

Disassembles to the following with javap:

public class Sample extends java.lang.Object implements scala.ScalaObject{
    public int b();
    public int c();
    public Sample(int, int, int);
}