Simple try-out for DCI programming style (basically mixins) using Java 8, virtual default methods and dynamic proxy.

MixinsExample.java

// Idea about simple DCI (data, context, interaction) using Java 8.
// Approach will force mixins to Java using virtual default methods
// and dynamic proxy. To keep it short there is no context object
// like DCI dictates.
//
// Run using
// javac MixinsExample.java; java MixinsExample
//
// Panu Wetterstrand <panu.wetterstrand@iki.fi>

import java.lang.reflect.*;
import java.lang.invoke.*;

public class MixinsExample {

    public static void main(String[] args) {
        // Lets have some accounts with initial balances
        Account a = new AccountImpl(100);
        Account b = new AccountImpl(50);

        // Assign roles
        TargetAccount target = assignRole(TargetAccount.class, a);
        SourceAccount source = assignRole(SourceAccount.class, b);

        // Print situation before
        System.out.println("Source: "+source.balance());
        System.out.println("Target: "+target.balance());

        // Make the transfer
        source.transferTo(target, 25);

        // Print the results
        System.out.println("Source: "+source.balance());
        System.out.println("Target: "+target.balance());
    }

    // Account is basic data object containint the balance.
    // Reason for this comes later...
    static public interface Account {
        public double balance();
        public void deposit(double amount);
        public void withdraw(double amount);
    }    

    // Then we have the implementation
    public static class AccountImpl implements Account {
        double balance = 0;

        public AccountImpl(double balance){
            this.balance = balance;
        }

        public double balance(){
            return balance;
        }

        public void deposit(double amount){
            balance += amount;
        }

        public void withdraw(double amount){
            balance -= amount;
        }

    }

    // We have a role that acts as a source account.
    // When this extends the Account interface we can use
    // this.withdraw...
    static public interface SourceAccount extends Account {
    default public void transferTo(TargetAccount target, double amount) {
        this.withdraw(amount);
        target.deposit(amount);
    }
    }


    // Just to mark the role of target account.
    static public interface TargetAccount extends Account {
    }

    
    // Creates proxy that makes it possible to "cast" data objects
    // to act in some role. This is where the magic happens...
    static <T> T assignRole(Class<T> roleClass, final Object o){
        return (T) Proxy.newProxyInstance(
                                          roleClass.getClassLoader(),
                                          new Class<?>[] { roleClass },

                                          (Object proxy, Method method, Object[] args) -> {

                                              if (method.isDefault()) {
                                                  // See https://rmannibucau.wordpress.com/2014/03/27/java-8-default-interface-methods-and-jdk-dynamic-proxies/#comment-1332
                                                  final Class<?> declaringClass = method.getDeclaringClass();
                                                  final MethodHandles.Lookup lookup = MethodHandles.publicLookup()
                                                      .in(declaringClass);

                                                  // ensure allowed mode will not check visibility
                                                  final Field f = MethodHandles.Lookup.class.getDeclaredField("allowedModes");
                                                  final int modifiers = f.getModifiers();
                                                  if (Modifier.isFinal(modifiers)) { // should be done a single time
                                                      final Field modifiersField = Field.class.getDeclaredField("modifiers");
                                                      modifiersField.setAccessible(true);
                                                      modifiersField.setInt(f, modifiers & ~Modifier.FINAL);
                                                      f.setAccessible(true);
                                                      f.set(lookup, MethodHandles.Lookup.PRIVATE);
                                                  }

                                                  return lookup
                                                      .unreflectSpecial(method, declaringClass)
                                                      .bindTo(proxy)
                                                      .invokeWithArguments(args);
                                              }

                                              return method.invoke(o,args);
                                          }
                                          );

    }

}

Hi! This is very interesting. Is it based on the DCI by James Coplien?
I'm starting to study this, and still a bit puzzled.

Hi! This is very interesting. Is it based on the DCI by James Coplien?
I'm starting to study this, and still a bit puzzled.

I think Data Context Interaction predates James Coplien, but yes this is simple Data Context Interaction implementation idea.