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:

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

• java
• scala
• abstract-class
• extends

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]

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.