What is java interface equivalent in Ruby?

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,

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 {

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.

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