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---
layout: post
title: "Implementing java interfaces"
keywords: using, module
permalink: modules/interface.html
---
Java interfaces are mapped to modules in JRuby. This means that you can also reopen the corresponding module and add further methods on the JRuby side. JRuby classes can now implement more than one Java interface. Since Java interfaces are mapped to modules in JRuby, you implement them not by subclassing, but by mixing them in.
```ruby
class SomeJRubyObject
include java.lang.Runnable
include java.lang.Comparable
end
```
Another example is implementing an interface from the `jbox2d` library, here is the java interface (_sans comments_):-
```java
package org.jbox2d.callbacks;
import org.jbox2d.collision.Manifold;
import org.jbox2d.dynamics.contacts.Contact;
public interface ContactListener {
public void beginContact(Contact contact);
public void endContact(Contact contact);
public void preSolve(Contact contact, Manifold oldManifold);
public void postSolve(Contact contact, ContactImpulse impulse);
}
```
And this is how we implement that interface in propane see [full example here][collision_listening], note empty methods are just fine.
```ruby
# A custom listener allows us to get the physics engine to
# to call our code, on say contact (collisions)
class CustomListener
include ContactListener
def begin_contact(cp)
# Get both fixtures
f1 = cp.getFixtureA
f2 = cp.getFixtureB
# Get both bodies
b1 = f1.getBody
b2 = f2.getBody
# Get our objects that reference these bodies
o1 = b1.getUserData
o2 = b2.getUserData
return unless [o1, o2].all? { |obj| obj.respond_to?(:change) }
o1.change
o2.change
end
def end_contact(_cp)
end
def pre_solve(_cp, _m)
end
def post_solve(_cp, _ci)
end
end
```
[collision_listening]:https://github.com/ruby-processing/picrate-examples/blob/master/external_library/gem/pbox2d/collision_listening.rb
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