What is java interface equivalent in Ruby?

Question

Ruby has Interfaces just like any other language.

Note that you have to be careful not to conflate the concept of the Interface, which is an abstract specification of the responsibilities, guarantees and protocols of a unit with the concept of the interface which is a keyword in the Java, C# and VB.NET programming languages. In Ruby, we use the former all the time, but the latter simply doesn't exist.

It is very important to distinguish the two. What's important is the Interface, not the interface. The interface tells you pretty much nothing useful. Nothing demonstrates this better than the marker interfaces in Java, which are interfaces that have no members at all: just take a look at java.io.Serializable and java.lang.Cloneable; those two interfaces mean very different things, yet they have the exact same signature.

So, if two interfaces that mean different things, have the same signature, what exactly is the interface even guaranteeing you?

Another good example:

package java.util;

interface List<E> implements Collection<E>, Iterable<E> {
    void add(int index, E element)
        throws UnsupportedOperationException, ClassCastException,
            NullPointerException, IllegalArgumentException,
            IndexOutOfBoundsException;
}

What is the Interface of java.util.List<E>.add?

• that the length of the collection does not decrease
• that all the items that were in the collection before are still there
• that element is in the collection

And which of those actually shows up in the interface? None! There is nothing in the interface that says that the Add method must even add at all, it might just as well remove an element from the collection.

This is a perfectly valid implementation of that interface:

class MyCollection<E> implements java.util.List<E> {
    void add(int index, E element)
        throws UnsupportedOperationException, ClassCastException,
            NullPointerException, IllegalArgumentException,
            IndexOutOfBoundsException {
        remove(element);
    }
}

Another example: where in java.util.Set<E> does it actually say that it is, you know, a set? Nowhere! Or more precisely, in the documentation. In English.

In pretty much all cases of interfaces, both from Java and .NET, all the relevant information is actually in the docs, not in the types. So, if the types don't tell you anything interesting anyway, why keep them at all? Why not stick just to documentation? And that's exactly what Ruby does.

Note that there are other languages in which the Interface can actually be described in a meaningful way. However, those languages typically don't call the construct which describes the Interface "interface", they call it type. In a dependently-typed programming language, you can, for example, express the properties that a sort function returns a collection of the same length as the original, that every element which is in the original is also in the sorted collection and that no bigger element appears before a smaller element.

So, in short: Ruby does not have an equivalent to a Java interface. It does, however, have an equivalent to a Java Interface, and it's exactly the same as in Java: documentation.

Also, just like in Java, Acceptance Tests can be used to specify Interfaces as well.

In particular, in Ruby, the Interface of an object is determined by what it can do, not what class is is, or what module it mixes in. Any object that has a << method can be appended to. This is very useful in unit tests, where you can simply pass in an Array or a String instead of a more complicated Logger, even though Array and Logger do not share an explicit interface apart from the fact that they both have a method called <<.

Another example is StringIO, which implements the same Interface as IO and thus a large portion of the Interface of File, but without sharing any common ancestor besides Object.

Try rspec's "shared examples":

https://www.relishapp.com/rspec/rspec-core/v/3-5/docs/example-groups/shared-examples

You write a spec for your interface and then put one line in each implementer's spec, eg.

it_behaves_like "my interface"

Complete example:

RSpec.shared_examples "a collection" do
  describe "#size" do
    it "returns number of elements" do
      collection = described_class.new([7, 2, 4])
      expect(collection.size).to eq(3)
    end
  end
end

RSpec.describe Array do
  it_behaves_like "a collection"
end

RSpec.describe Set do
  it_behaves_like "a collection"
end

Update: Eight years later (2020) ruby now has support for statically-typed interfaces via sorbet. See Abstract Classes and Interfaces in the sorbet docs.

Can we expose interfaces in Ruby like we do in java and enforce the Ruby modules or classes to implement the methods defined by interface.

Ruby does not have that functionality. In principle, it does not need them as Ruby uses what is called duck typing.

There are few approaches you can take.

Write implementations that raise exceptions; if a subclass attempts to use the unimplemented method, it will fail

class CollectionInterface
  def add(something)
    raise 'not implemented'
  end
end

Along with above, you should write testing code that enforces your contracts (what other post here incorrectly call Interface)

If you find yourself writing void methods like above all the time, then write a helper module that captures that

module Interface
  def method(name)
    define_method(name) { |*args|
      raise "interface method #{name} not implemented"
    }
  end
end

class Collection
  extend Interface
  method :add
  method :remove
end

Now, combine the above with Ruby modules and you are close to what you want...

module Interface
  def method(name)
    define_method(name) { |*args|
      raise "interface method #{name} not implemented"
    }
  end
end

module Collection
  extend Interface
  method :add
  method :remove
end

col = Collection.new # <-- fails, as it should

And then you can do

class MyCollection
  include Collection

  def add(thing)
    puts "Adding #{thing}"
  end
end

c1 = MyCollection.new
c1.add(1)     # <-- output 'Adding 1'
c1.remove(1)  # <-- fails with not implemented

Let me emphasise once again: this is a rudimentary, as everything in Ruby happens at runtime; there is no compile time checking. If you couple this with testing, then you should be able to pick up errors. Even further, if you take the above further, you could probably be able to write an Interface that performs checking on the class first time an object of that class is created; making your tests as simple as calling MyCollection.new... yeah, over the top :)

As everyone here said, there is no interface system for ruby. But through introspection, you can implement it yourself quite easily. Here is a simple example that can be improved in many ways to help you get started:

class Object
  def interface(method_hash)
    obj = new
    method_hash.each do |k,v|
      if !obj.respond_to?(k) || !((instance_method(k).arity+1)*-1)
        raise NotImplementedError, "#{obj.class} must implement the method #{k} receiving #{v} parameters"
      end
    end
  end
end

class Person
  def work(one,two,three)
    one + two + three
  end

  def sleep
  end

  interface({:work => 3, :sleep => 0})
end

Removing one of the methods declared on Person or change it number of arguments will raise a NotImplementedError.

As many answers indicate, there is no way in Ruby to force a class to implement a specific method, by inheriting from a class, including a module or anything similar. The reason for that is probably the prevalence of TDD in Ruby community, which is a different way of defining the interface - the tests not only specify the signatures of the methods, but also the behavior. Thus if you want to implement a different class, that implements some already defined interface, you have to make sure that all the tests pass.

Usually the tests are defined in isolation using mocks and stubs. But there are also tools like Bogus, allowing for defining contract tests. Such tests not only define the behavior of the "primary" class, but also check that the stubbed methods exist in the cooperating classes.

If you are really concerned with interfaces in Ruby I would recommend using a testing framework that implements contract testing.

There are no such things as interfaces in the Java way. But there are other things you can enjoy in ruby.

If you want to implement some kind of types and interface - so that the objects can be checked whether they has some methods/messages you require from them -, you can then take a look at rubycontracts. It defines a mechanism similar to the PyProtocols. A blog about type checking in ruby is here.

The mentioned approached are not living projects, although the goal seems to be nice at first, it seems that most of the ruby developers can live without strict type checking. But the flexibility of ruby enables to implement type checking.

If you want to extend objects or classes (the same thing in ruby) by certain behaviors or somewhat have the ruby way of multiple inheritance, use the include or extend mechanism. With include you can include methods from another class or module into an object. With extend you can add behavior to a class, so that its instances will have the added methods. That was a very short explanation though.

I my opinion the best way to resolve the Java interface need is to understand the ruby object model (see Dave Thomas lectures for instance). Probably you will forget about Java interfaces. Or you have an exceptional application on your schedule.