Is polymorphism possible without inheritance?

Question

In an interview I was asked if polymorphism can be achieved without inheritance. Is this possible?

Solution 1

The best explanation on the subject that I've ever read is an article by Luca Cardelli, a renown type theorist. The article is named On Understanding Types, Data Abstraction, and Polymorphism.

Types of Polymorphism

Cardelli defines several types of polymorphism in this article:

• Universal
  • parametric
  • inclusion
• Ad-hoc
  • overloading
  • coercion

The kind of polymorphism related to inheritance is classified as inclusion polymorphism or subtype polymorphism.

Wikipedia provides a good definition:

In object-oriented programming, subtype polymorphism or inclusion polymorphism is a concept in type theory wherein a name may denote instances of many different classes as long as they are related by some common super class. Inclusion polymorphism is generally supported through subtyping, i.e., objects of different types are entirely substitutable for objects of another type (their base type(s)) and thus can be handled via a common interface. Alternatively, inclusion polymorphism may be achieved through type coercion, also known as type casting.

Another Wikipedia article called Polymorphism in object-oriented programming seems to answer your questions as well.

In Java

This subtyping feature in Java is achieved, among other means, through the inheritance of classes and interfaces. Although the subtyping features of Java may not be evident in terms of inheritance all the time. Take for example the cases of covariance and contravariance with generics. Also, arrays are Serializable and Cloneable although this is not evident anywhere in the type hierarchy. It can also be said that through primitive widening conversion the numeric operators in Java are polymorphic, in certain cases even accepting totally unrelated operands (i.e. concatenation of strings and numbers or of a string plus some other object). Consider also the cases of boxing and unboxing of primitives. These latter cases of polymorphism (coercion and overloading) are not at all related to inheritance.

Examples

Inclusion

List<Integer> myInts = new ArrayList<Integer>();

This is the case to which your question seems to refer i.e. when there is an inheritance or implementation relationship between the types, as in this case where ArrayList implements List.

As I mentioned, though, when you introduce Java generics, some time the rules of subtyping get fuzzy:

List<? super Number> myObjs = new ArrayList<Object>();
List<? extends Number> myNumbers = new LinkedList<Integer>();

And in other cases, the relationships are not even evident in the API

Cloneable clone = new int[10];
Serializable obj = new Object[10]

Even so, all these, according to Cardelli, are forms of universal polymorphism.

Parametric

public <T> List<T> filter(Predicate<T> predicate, List<T> source) {
  List<T> result = new ArrayList<>();
  for(T item : source) {
    if(predicate.evaluate(item)){
         result.add(item);
    }
   return result;
  }
}

The same algorithm can be used to filter all kinds of lists with all kinds of predicates without having to repeat a single line of code for every possible type of list. The type of the actual list and the type of predicate are parametric. See this example with lambda expressions available in JDK 8 Preview (for the brevity of predicate implementation).

filter(x -> x % 2 == 0, asList(1,2,3,4,5,6)); //filters even integers
filter(x -> x % 2 != 0, asList(1L,2L,3L,4L,5L,6L)); //filters odd longs
filter(x -> x >= 0.0, asList(-1.0, 1.0)); //filters positive doubles

According to Cardelli, this is a form of universal polymorphism.

Coercion

double sum = 1 + 2.0;

Integer and floating-point arithmetic are totally different. Applying the plus operator to two operands of different types here is impossible without some form of coercion.

In this example, the types integer and double, are automatically coerced (converted) to type double without an explicit cast. The entire expression is promoted to double. This is so because in Java we have primitive widening conversions.

According to Cardelli, this form of automatic coercion is a form of ad-hoc polymorphism provided for the plus operator.

There are languages in which you could not even sum an integer and a floating-point number without an explicit cast (i.e. AFAIK, SML, in which, by the way, parametric polymorphism is key to overcome this kind of problems).

Overloading

double sum = 2.0 + 3.0;
String text = "The sum is" + sum;

The plus operator here means two different things depending on the arguments used. Evidently, the operator has been overloaded. This implies it has different implementations depending on the types of operands. According to Cardelli, this is a form of ad-hoc polymorphism provided for the plus operator.

This, of course, also applies to forms of method overloading in classes (i.e java.lang.Math methods min and max are overloaded to support different primitive types).

In Other Languages

Even when inheritance plays an important role in the implementation of some of these forms of polymorphism, certainly it is not the only way. Other languages that are not object-oriented provide other forms of polymorphism. Take, for example, the cases of duck typing in dynamic languages like Python or even in statically-typed languages like Go, or algebraic data types in languages like SML, Ocaml and Scala, or type classes in languages like Haskell, multi methods in Clojure, prototypal inheritance in JavaScript, etc.

Solution 2

Ad-hoc Polymorphism > Operator Overloading > Without Inheritence

Ad-hoc Polymorphism > Method Overloading > Without Inheritence

Ad-hoc Polymorphism > Method Overriding > With Inheritence

Parametric Polymorphism > Generics > Without Inheritence

Subtype Polymorphism or Inclusion Polymorphism > Polymorphic Assignment > With Inheritence

Subtype Polymorphism or Inclusion Polymorphism > Polymorphic Return Type > With Inheritence

Subtype Polymorphism or Inclusion Polymorphism > Polymorphic Argument Type > With Inheritence

Coercion Polymorphism > Widening > With or without Inheritence

Coercion Polymorphism > Auto boxing and unboxing > Without Inheritence

Coercion Polymorphism > Var args > Without Inheritence

Coercion Polymorphism > Type Casting > Without Inheritence

Solution 3

Sure. In Java, you can have two classes implement the same interface, and their results are polymorphic. No functionality is inherited.

public interface Foo {
  public int a();
}

public class A implements Foo {
  public int a() {
    return 5;
  }
}


public class B implements Foo {
  public int a() {
    return 6;
  }
}

Then elsewhere:

Foo x = new A();
System.out.println(x.a())
Foo y = new B();
System.out.println(y.a())

Both x and y are Foos, but they have different results when you call a().

Comments

[Novikov]

Copied Max's answer down to the mistake where B implements goo instead of foo.

[Richard Sitze]

The interface is inherited. The semantic contract (never inforceble) is implicitly inherited. See Edwin's response.

[Richard Sitze]

Great response. It'd be even better is you were to start it off with a definite and clear summary to answer the question.

[Edwin Dalorzo]

@RichardSitze I gave it a shot to a more detailed explanation. I hope I have not obscured the meaning of the original post.

[Max]

Fine, that's fair. I assumed the questioner meant 'implementation inheritance'. Without further clarification from the interviewer, and especially in a Java-oriented interview, this answer is probably what they're looking for. There are, of course, much more detailed answers.

[Richard Sitze]

@Edwin Delorzo Wow.. well actually I was trying to suggest was a one-liner at the top to state: Yes or No. The reader then has some idea where they're going as they read the rest. Also, as Stephen C has pointed out, the answer may be different Java (which was the original question), then for other languages. Nice job!

[Edwin Dalorzo]

Languages that are not object-oriented provide forms of polymorphism which do not rely on inheritance (i.e parametric polymorphism). Some of this is possible in Java through generics.

[Dhananjay]

@EdwinDalorzo : and what is definition of polymorphism in those languages? it may not be same as in oop.

[scott_fakename]

Yeah I agree. I tend to think of interfaces (rightly or wrongly) as a way to guarantee to the compiler that an object will have a certain method with a certain signature, which puts me in the mind of duck typing, which doesn't really imply inheritance. Technically it IS considered inheritance, sure.

[Faisal Naseer]

yup there are other languages like Ruby, Python, JavaScript, Visual Basic, and Smalltalk take advantage of powerful CPUs to use interpreters to defer type checking to run time (whether the source code is compiled to byte code or purely interpreted). By deferring type checking we break the link between inheritance and polymorphism,

[weberc2]

@Dhananjay No it doesn't. Inheritance (extension) copies behavior; implementing an interface does not.