Integrating Java with Scala: Extending from Scala classes

Question

I have created a project that uses scala that has some abstract classes definitions such as:

abstract class A(n: String) {
    require(n!= null)
    val name = n

    var inputs: List[Input[_]]
    var outputs: List[Output[_]]
}

abstract class B(n: String) extends A(n){  
   def act
   def isValid : Boolean
   def getCs : List[C[_]]
}

abstract class C[Type]{
   var value: Type 
}

Now I want to use that project in Java. I have imported scala lib in a new project and added scala project in the build path. I am using eclipse.

However, when I try to do this:

public class CoolClass extends B {  
    public CoolClass(){ }   
}

EDIT:

I have some problems using this in Java that raised me some doubts. Let me enumerate them:

Can I use Lists from Scala or that gives errors in Java?
Why vars from A aren't recognized in classes that extend B in Java?
How can I declare getCs so it provides an interface with List<C> instead of List<C<?>> ?

drexin · Accepted Answer · 2012-04-06 10:54:30Z

7

I'm just assuming, that you implemented all the methods you define in you class B also in your java class CoolClass. What is missing here is the call to B's constructor, as the error message says.

class CoolClass extends B {
  public CoolClass() {
    super("");
  }
  // methods implementations
}

There is no default constructor defined on your class B, so you have to explicitly call the one that is defined, which is B(n: String).

edit:

1: You can use anything you want on both sides. Though the method names may be different in some special cases like anything that includes special chars in scala.

2: They are recognized, though they are not fields, but methods in the java code. You can access them with this.inputs() and this.outputs()

3: Try def getCs[C]: List[C]

edited Apr 6, 2012 at 10:54

answered Apr 5, 2012 at 22:47

drexin

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8 Comments

Tiago Almeida Over a year ago

The constructer solved some problems but I keep getting problems and many of them don't make sense. I keep with one doubt: Can I use Lists (from Scala) ? Or Java doesn't like that?

Tiago Almeida Over a year ago

Class A has 2 vars that have a List as Type and Java can't recognize those vars as fields of the class A.

drexin Over a year ago

Is the code you posted the code you have problems with? Your def getCs : List[C[_]] will not compile, because C is not known. You are talking of 2 fields in the A class, could you please edit your original post accordingly?

Tiago Almeida Over a year ago

Sorry for my incomplete question. I thought that most of the problems would be solved with the first problems I posted here. Added one more problem at my question to prevent repeated information on StackOverflow.

drexin Over a year ago

Edited my post to answer you new questions.

|

Jed Wesley-Smith · Accepted Answer · 2012-04-06 02:52:30Z

1

Your B class is essentially as you describe:

public abstract class B extends A {  
  public B(String n) {
    super(n);
  }

  public abstract void act()
  public abstract boolean isValid();
  public abstract List<C<?>> getCs();
}

Your CoolClass is missing the explicit super call in the constructor:

public CoolClass extends B {
  public CoolClass() { // override ctor with no string parameter
    super("defaultStringValue");
  }

  public CoolClass(String n) { // override ctor with no string parameter
    super(n);
  }
  …

As you can see Scala makes it so trivial to declare and call constructors we forget how much boilerplate they add in Java.

answered Apr 6, 2012 at 2:52

Jed Wesley-Smith

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