Using scala.Function1 from Java

function.scala

package lambda

object JFunction {
  def fun[T1, R](g: JFunction1[T1, R]): T1 => R = new (T1 => R) {
    override def apply(t: T1) = g.apply(t)
  }
}

JFunction1.java

package lambda;

public interface JFunction1<T1, R> {
  R apply(T1 t);
}

Lambda.java

package lambda;

import scala.Option;
import scala.Some;

public class Lambda {
  
  public static void main(String[] args) {
    
    Option<Integer> opt = new Some<Integer>(42);
    
    // Java 7
    System.out.println(opt.map(JFunction.fun(new JFunction1<Integer, String>() {
      @Override
      public String apply(Integer n) {
        return n.toString();
      }
    })));
    
    // Java 8 lambda syntax
    System.out.println(opt.map(JFunction.<Integer, String>fun(n -> n.toString())));
  }
}

Lambda2.java

package lambda;

import scala.Option;
import scala.Some;
import scala.runtime.AbstractFunction1;

public class Lambda {
  
  public static void main(String[] args) {
    
    Option<Integer> opt = new Some<Integer>(42);
    
    // Java 7
    System.out.println(opt.map(new AbstractFunction1<Integer, String>() {
      @Override
      public String apply(Integer n) {
        return n.toString();
      }
    }));
  }
}