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I want use generic interfaces that will be specified by their implementing classes, and all in all this has been working fine, like:
interface Remove <E> { fun remove(entity: E) }
class MyHandler : Remove <MyClass> {
override fun remove(entity: MyClass) { */do stuff/* }
}
However, I have a case (so far, expecting more to come) where I want the function itself to be generic. Writing the interface is no problem at all:
interface FindByID <E> { fun <K : Serializable> findByID(id: K): E }
I need K to be serializable because that's a requirement of some function I need to call.
The compiler doesn't seem to agree with my implementation attempt. When I do this:
override fun <String> findByID(id: String): User {
return someFunction(User::class.java, id) as User
}
I get two compiler errors:
overrides nothing
id is not Serializable
However, when I remove override and <String> from the signature, it works fine. This means String is serializable, which my research shows as well.
What seems to be the problem here?
Also, yes, I know that I could work around this issue in a couple of ways, like
making the interface function accept Serializable instead of <K : Serializable>
not specifying K on override, but on call (myHandler.findByID<String>("myID"))
"rerouting" the call, implementing (not overriding) in MyHandler a function that accepts Strings and then internally calls the overriden function
Although open to suggestions, I'm less interested in workarounds, but would rather like to understand and (if possible) solve the actual problem or at least know it can't be done so I can take that into account for planning
The current problem
With your current declaration of <String>, you're not specializing the type parameter as you might think. What you're actually doing is declaring a type parameter that happens to be named String (nothing to do with the well-known String type, just an unfortunate name collision). With syntax coloring, you should see that String here is in the color of a type parameter, not the same color as the String type would be. Change this name to anything else, and you'll realize the confusion.
So if we rename this to K, the problems become obvious: problem 1 "overrides nothing" is because your generic type parameter doesn't have the same : Serializable constraint as the findByID method defined in the interface. And problem 2 stems from it.
Solutions
Also, yes, I know that I could work around this issue in a couple of ways, like
not specifying K on override, but on call (myHandler.findByID("myID"))
This point is actually the essence of the issue, not a workaround: defining a generic function in your interface actually makes it part of the contract for this function to be generic.
Implementations must have a generic type parameter here.
What to do to fix it depends on what you expect to happen.
If you're ok with having generic functions in your implementations, then leave the interface as you declared it, but accept that the implementations have to deal with all possible values of K, which is most likely not what you want here.
If you want to define a specific type in your implementations, K should be part of the interface definition (not the interface method), and the method should not have a type parameter itself, but simply accept the existing K from the interface:
interface FindByID<E, K : Serializable> {
fun findByID(id: K): E
}
For example, suppose I this:
class Gundam00 extends Gundam implements MobileSuit {
...
public void fight(final List<MobileSuit> mobiruSuitso, final List<Gundam> theOtherDudes, final List<Person> casualities) {
....
}
}
Suppose theOtherDudes and casualities parameters are optional. How can I make this method as clean as possible? I thought about having booleans indicating if they're null, and then checking them as needed.
I could also have different versions of the method for each combination of parameters but there would be a lot of code duplication I think.
Any suggestions?
I find that past 2-3 arguments, the ability to remember what all the arguments to a function are suffers. And comprehensibility along with it.
Passing named arguments can help. Languages with a convenient hash-like literal syntax make this really easy. Take JavaScript:
g = new Gundam00();
g.fight({opponent: enemy, casualties: 'numerous'});
You can also take advantage of variable length argument features to work this in (treat odd arguments as names, even arguments as the actual parameters).
g.fight('opponent',enemy,'casualties', 'numerous');
And some languages actually support named arguments straight-out (see: http://en.wikipedia.org/wiki/Named_parameter#Use_in_programming_languages ).
Finally, you might want to consider adding other methods for this using what some call a Fluent Interface (http://en.wikipedia.org/wiki/Fluent_interface ). Basically, you've got method call which return the object itself, so you can chain calls together:
g.opponent(enemy).casualties('numerous').fight();
This might be the easiest option if you're working in a manifestly/statically-typed class-focused language.
Update
Responding to Setsuna's comment... in that last example, if you've got the luxury, you can make methods like opponent and casualties simple setters that don't affect any internal state or computation in any other way than setting a parameter for which they're named. They simply set internal properties up, and then all of the real work happens inside action methods like fight.
If you can't do that (or if you don't like writing methods whose operations are sub-atomic), you could stake out a half-way spot between this idea with the hash-like literal idea, and create your own collection class specifically for invoking named arguments:
n = new NArgs();
g.fight(n.arg('opponent',enemy).arg('casualties','numerous').arg('motion','slow'));
A little more unwieldy, but it separates out the named arguments problem and lets you keep your methods a bit more atomic, and NArgs is probably something you could implement pretty easily just wrapping some methods around one type of Collection (HashTable?) or another that's available in your language.
Add the methods. Overloading methods is generally an antipattern and a refactoring opportunity for someone else.
http://www.codinghorror.com/blog/2007/03/curlys-law-do-one-thing.html
I thought about having booleans indicating if they're null, and then checking them inside and reacting accordingly.
Or ... you could just check if they're null.
if(theOtherDudes == null)
...
If there is only one "main method" in your class, then you can implement the optional arguments as getter/setter functions. Example:
public void setOtherDudes(final List<Gundam> theOtherDudes) {} // for input arguments
public List<Person> getCasualities() {} // for output arguments
And then, in your documentation, mention that if the caller has any optional input arguments it has to be passed in before calling fight(), and the optional output values will be available when fight() has been called.
This is worthwhile if there are dozens of optional arguments. Otherwise, I suggest overloading the method as the simplest way.
I have question regarding the use of function parameters.
In the past I have always written my code such that all information needed by a function is passed in as a parameter. I.e. global parameters are not used.
However through looking over other peoples code, functions without parameters seem to be the norm. I should note that these are for private functions of a class and that the values that would have been passed in as paramaters are in fact private member variables for that class.
This leads to neater looking code and im starting to lean towards this for private functions but would like other peoples views.
E.g.
Start();
Process();
Stop();
is neater and more readable than:
ParamD = Start(paramA, ParamB, ParamC);
Process(ParamA, ParamD);
Stop(ParamC);
It does break encapsulation from a method point of view but not from a class point of view.
There's nothing wrong in principle with having functions access object fields, but the particular example you give scares me, because the price of simplifying your function calls is that you're obfuscating the life cycle of your data.
To translate your args example into fields, you'd have something like:
void Start() {
// read FieldA, FieldB, and FieldC
// set the value of FieldD
}
void Process() {
// read FieldA and do something
// read FieldD and do something
}
void Stop() {
// read the value of FieldC
}
Start() sets FieldD by side effect. This means that it's probably not valid to call Process() until after you've called Start(). But the code doesn't tell you that. You only find out by searching to see where FieldD is initialized. This is asking for bugs.
My rule of thumb is that functions should only access an object field if it's always safe to access that field. Best if it's a field that's initialized at construction time, but a field that stores a reference to a collaborator object or something, which could change over time, is okay too.
But if it's not valid to call one function except after another function has produced some output, that output should be passed in, not stored in the state. If you treat each function as independent, and avoid side effects, your code will be more maintainable and easier to understand.
As you mentioned, there's a trade-off between them. There's no hard rule for always preferring one to another. Minimizing the scope of variables will keep their side effect local, the code more modular and reusable and debugging easier. However, it can be an overkill in some cases. If you keep your classes small (which you should do) then the shared variable would generally make sense. However, there can be other issues such as thread safety that might affect your choice.
Not passing the object's own member attributes as parameters to its methods is the normal practice: effectively when you call myobject.someMethod() you are implicitly passing the whole object (with all its attributes) as a parameter to the method code.
I generally agree with both of Mehrdad and Mufasa's comments. There's no hard and fast rule for what is best. You should use the approach that suits the specific scenarios you work on bearing in mind:
readability of code
cleanliness of code (can get messy if you pass a million and one parameters into a method - especially if they are class level variables. Alternative is to encapsulate parameters into groups, and create e.g. a struct to whole multiple values, in one object)
testability of code. This is important in my opinion. I have occassionally refactored code to add parameters to a method purely for the purpose of improving testability as it can allow for better unit testing
This is something you need to measure on a case by case basis.
For example ask yourself if you were to use parameter in a private method is it ever going to be reasonable to pass a value that is anything other than that of a specific property in the object? If not then you may as well access the property/field directly in the method.
OTH you may ask yourself does this method mutate the state of the object? If not then perhaps it may be better as a Static and have all its required values passed as parameters.
There are all sorts of considerations, the upper most has to be "What is most understandable to other developers".
In an object-oriented language it is common to pass in dependencies (classes that this class will communicate with) and configuration values in the constructor and only the values to actually be operated on in the function call.
This can actually be more readable. Consider code where you have a service that generates and publishes an invoice. There can be a variety of ways to do the publication - via a web-service that sends it to some sort of centralized server, or via an email sent to someone in the warehouse, or maybe just by sending it to the default printer. However, it is usually simpler for the method calling Publish() to not know the specifics of how the publication is happening - it just needs to know that the publication went off without a hitch. This allows you to think of less things at a time and concentrate on the problem better. Then you are simply making use of an interface to a service (in C#):
// Notice the consuming class needs only know what it does, not how it does it
public interface IInvoicePublisher {
pubic void Publish(Invoice anInvoice);
}
This could be implemented in a variety of ways, for example:
public class DefaultPrinterInvoicePublisher
DefaultPrinterInvoicePublisher _printer;
public DefaultPrinterInvoicePublisher(DefaultPrinterFacade printer) {
_printer = printer
}
public void Publish(Invoice anInvoice) {
printableObject = //Generate crystal report, or something else that can be printed
_printer.Print(printableObject);
}
The code that uses it would then take an IInvoicePublisher as a constructor parameter too so that functionality is available to be used throughout.
Generally, it's better to use parameters. Greatly increases the ability to use patterns like dependency injection and test-driven design.
If it is an internal only method though, that's not as important.
I don't pass the object's state to the private methods because the method can access the state just like that.
I pass parameters to a private method when the private method is invoked from a public method and the public method gets a parameter which it then sends to the private method.
Public DoTask( string jobid, object T)
{
DoTask1(jobid, t);
DoTask2(jobid, t);
}
private DoTask1( string jobid, object T)
{
}
private DoTask2( string jobid, object T)
{
}
I've heard people saying that good design involves using inheritance instead of littering your code with if block. In what situation should we use inheritance, and when condition block is just fine?
One refactoring like this is called Replace Conditional With Polymorphism.
We actually favor composition over inheritance, but ... it's really too broad of a topic to discuss here, however the book is worth a read and can get you started.
Generally, it isn't good object-oriented programming practice to have code like the following:
var chioce = PAINT_SCREEN_BLUE
if (choice == PAINT_SCREEN_BLUE):
paintScreen(blue)
else if (choice == PAINT_SCREEN_RED):
paintScreen(red)
else if (choice == PAINT_SCREEN_GREEN):
paintScreen(green)
The above can utilize polymorphism to call some method that will perform the action that is abstracted away in related classes with a common ancestor:
interface ScreenPainter:
function perform()
class BlueScreenPainter implements ScreenPainter:
function perform():
paintScreen(blue)
...
// The conditional block can be replaced with the following call:
var choice = PAINT_SCREEN_BLUE
ScreenPainter p = Painters.getPainter(choice)
p.perform()
However, this practice definitely should not apply for all conditionals, however, when it comes to long switch blocks or if-elseif-else blocks, in many cases, use of inheritance will make your code more extensible and more robust.
If blocks (or switch blocks) that are used to decide what logic to do based on the type of an argument should be replaced with polymorphism.
If new features in a program will cause you to make additional case or else blocks use polymorphism.
If statements should be used for natural ordering type comparisons ( <, >, ==, etc.) or to check if something is null.
This is one of those questions that, while others can offer guidelines, the final answer for any particular situation is always, "it depends on the situation."
The best way to be able to develop the ability to answer it for any particular situation is to gain experience: i.e., experiment. Try both ways, where you can do so reasonably. (Obviously, you don't want to be flipping back and forth in situations where you have to change a thousand lines of code to do so.) Which worked better? Which felt better to you? Why?
Do this enough, and you'll soon reach the level of expertise where you no longer need the general advice, and people start asking you how you know what to do in any particular situation. (You won't be able to explain it; that's part of the nature of expertise. See The Dreyfus Model of Skill Acquisition for more details.
Here is something I ran into the other day
PHP is my language of choice
class Cache {
public function __construct($resource) {
// if statement to determine from type if it is a file, url or string. instead, change it to accept a string
}
}
or... it could of been extended
class cacheURL extends Cache {
public function __construct($url) {
$string = file_get_contents($url);
parent::__construct($string);
}
}
Rough example, but I hope you catch my drift....
In my opinion the two are not mutually exclusive. I typically use inheritance when I have 2 or more classes that have common functionality.
When to Use Inheritance from Microsoft
Composition vs. Inheritance in Java
Inheritance is better when some condition is checking something intrinsic to your object that you check constantly (like a state or type). In this example, it's an operator for a mathematical expression tree. An operator, like addition, has an expression before and after the plus sign (infix notation) so I call them left and right because it is represented as a tree. There are also unary operators like a hyphen that negates a value, but it only has one child expression.
Using inheritance means the type (simplified to just unary vs. binary) is factored into the inheritance:
class Operator
{
abstract string ToString();
}
class UnaryOperator : Operator
{
Expression Expression { get; }
override string ToString() {...}
}
class BinaryOperator : Operator
{
Expression LeftExpression { get; }
Expression RightExpression { get; }
}
class AdditionOperator : BinaryOperator
{
override string ToString()
{
return this.LeftExpression.ToString() + "+" + this.RightExpression.ToString();
}
}
As opposed to:
class Operator
{
Expression LeftExpression { get; }
Expression RightExpression { get; }
OperatorType Type { get; }
string ToString()
{
switch(this.Type)
{
case OperatorType.Addition:
return this.LeftExpression.ToString() + "+" + this.RightExpression.ToString();
case OperatorType.Negation:
return "-" + this.LeftExpression.ToString();
case ...:
}
}
}
enum OperatorType
{
// Unary operators
Negation,
// Binary operators
Addition,
Subtraction,
Multiplication
}
Here your Unary operator doesn't have a Left and a Right expression, just an expression. This means you have to introduce convention that Left is used and Right is null. ToString would have to check OperatorType; so would any other method that needs to act on what operator it is.
The latter approach is most often seen in XML DOM implementations where XmlNode is pretty much everything any XML node could contain...ever. It makes for some very confusing situations:
How can an attribute or text node have ChildNodes?
How can a document have ChildNodes when it can only have one? (It need not be a collection)
How can a document have a ParentNode?
How can a document have a Value?
Etc.
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How can polymorphism be described in an easy-to-understand way?
We can find a lot of information about the subject on the Internet and books, like in Type polymorphism. But let's try to make it as simple as we can.
Two objects respond to the same message with different behaviors; the sender doesn't have to care.
Every Can with a simple pop lid opens the same way.
As a human, you know that you can Open() any such can you find.
When opened, not all cans behave the same way. Some contain nuts, some contain fake snakes that pop out. The result depends on what TYPE of can, if the can was a "CanOfNuts" or a "CanOfSnakes", but this has no bearing on HOW you open it. You just know that you may open any Can, and will get some sort of result that is decided based on what type of Can it was that you opened.
pUnlabledCan->Open(); //might give nuts, might give snakes. We don't know till we call it
Open() has a generic return type of "Contents" (or we might decide no return type), so that open always has the same function signature.
You, the human, are the user/caller.
Open() is the virtual/polymorphic function.
"Can" is the abstract base class.
CanOfNuts and CanOfSnakes are the polymorphic children of the "Can" class.
Every Can may be opened, but what specifically it does and what specific tye of contents it returns are defined by what sort of can it is.
All that you know when you see pUnlabledCan is that you may Open() it, and it will return the contents. Any other behaviors (such as popping snakes in your face) are decided by the specific Can.
This is from my answer from a similiar question. Here's an example of polymorphism in pseudo-C#/Java:
class Animal
{
abstract string MakeNoise ();
}
class Cat : Animal {
string MakeNoise () {
return "Meow";
}
}
class Dog : Animal {
string MakeNoise () {
return "Bark";
}
}
Main () {
Animal animal = Zoo.GetAnimal ();
Console.WriteLine (animal.MakeNoise ());
}
The Main() method doesn't know the type of the animal and depends on a particular implementation's behavior of the MakeNoise() method.
The simplest description of polymorphism is that it is a way to reduce if/switch statements.
It also has the benefit of allowing you to extend your if/switch statements (or other people's ones) without modifying existing classes.
For example consider the Stream class in .NET. Without polymorphism it would be a single massive class where each method implements a switch statement something like:
public class Stream
{
public int Read(byte[] buffer, int offset, int count)
{
if (this.mode == "file")
{
// behave like a file stream
}
else if (this.mode == "network")
{
// behave like a network stream
}
else // etc.
}
}
Instead we allow the runtime to do the switching for us in a more efficient way, by automatically choosing the implementation based on the concrete type (FileStream, NetworkStream), e.g.
public class FileStream : Stream
{
public override int Read(byte[] buffer, int offset, int count)
{
// behave like a file stream
}
}
public class NetworkStream : Stream
{
public override int Read(byte[] buffer, int offset, int count)
{
// behave like a network stream
}
}
Poly: many
Morphism: forms / shapes
The Actor vs. the Character (or Role)
Apples and oranges are both fruit. Fruit can be eaten. Hence, both apples and oranges can be eaten.
The kicker? You eat them differently! You peel the oranges, but not the apples.
So the implementation differs, but the end result is the same, you eat the fruit.
If it walks like a duck and quacks like a duck, then you can treat it as a duck anywhere you need a duck.
This is a better article actually
Polymorphism allows Objects to "Look" the same, but behave in different ways. The usual example is to take an animal base class with a Speak() Method, A dog subclass would emit a Bark whereas a Pig subclass would emit an oink.
The 5 second short answer most people use so other developers can get their head around Polymorphism is overloading and overriding
Same syntax, different semantics.
Simplest way to describe it: a verb that can apply to more than one kind of object.
Everything else, as Hillel said, is just commentary.
Polymorphism is treating things abstractly by relying on knowledge of a common "parent" (think heirarchies like Animal as a parent of Dogs and Cats).
For example, all Animals can breathe oxygen, and while they may each do this differently you could design a facility that provides oxygen for Animals to breathe, supporting both Dogs and Cats.
As a little extra, you can do this even though Animal is an "abstract" identifier (there is no real "Animal" thing, just types of Animals).
Polymorphism is the storing of values of more than one type in a location of a single type.
Note that most of the other answers to this question, at the time of my writing, are actually describing dynamic dispatch, not polymorphism.
Dynamic dispatch requires polymorphism, but the reverse is not true. One could imagine a language very similar to Java or C# but whose System.Object had no members; typecasting would be necessary before doing anything with the value. In this notional language, there would be polymorphism, but not necessarily virtual methods, or any other dynamic dispatch mechanisms.
Dynamic dispatch is the related but distinct concept, well enough described in most of the other answers. However, the way it normally works in object-oriented languages (selecting a function based on the first ('this' or 'Self') argument type) is not the only way it can work. Multiple dispatch is also possible, where the selection is applied across the types of all the arguments.
Similarly, overload resolution and multiple dispatch are exact analogues of one another; overload resolution is multiple dispatch applied to static types, while multiple dispatch is overload resolution applied to runtime types stored in polymorphic locations.
Polymorphism is dividing the world into boxes based on common properties and treating the items in a given box as interchangeable when you only want to use these common properties.
Polymorphism is the ability to treat different things as if they were the same thing by establishing a shared identity between them then exploiting it.
Polymorphism is what you get when the same method applies to multiple classes. For example, both a String and a List might have "Reverse" methods. Both methods have the same name ("Reverse"). Both methods do something very similar (reverse all the characters or reverse the order of the elements in the list). But the implementation of each "Reverse" method is different and specific to its class. (In other words, the String reverses itself like a string, and the List reverses itself like a list.)
To use a metaphor, you could say "Make Dinner" to a French chef or to a Japanese chef. Each would perform "make dinner" in their own characteristic way.
The practical result is that you could create a "Reversing Engine" that accepts an object and calls "Reverse" on it. As long as the object has a Reverse method, your Reversing Engine will work.
To extend the chef analogy, you could build a "Waiterbot" that tells chefs to "Make Dinner". The Waiterbot doesn't have to know what type of dinner is going to be made. It doesn't even have to make sure it's talking to a chef. All that matters is that the "chef" (or fireman, or vending machine, or pet food dispenser) knows what to do when it's told to "Make Dinner".
What this buys you as a programmer is fewer lines of code and either type-safety or late binding. For example here's an example with type safety and early binding (in a c-like language that I'm making up as I go):
class BankAccount {
void SubtractMonthlyFee
}
class CheckingAccount : BankAccount {}
class SavingsAccount : BankAccount {}
AssessFee(BankAccount acct) {
// This will work for any class derived from
// BankAccount; even classes that don't exist yet
acct.SubtractMonthlyFee
}
main() {
CheckingAccount chkAcct;
SavingsAccount saveAcct;
// both lines will compile, because both accounts
// derive from "BankAccount". If you try to pass in
// an object that doesn't, it won't compile, EVEN
// if the object has a "SubtractMonthlyFee" method.
AssessFee(chkAcct);
AssessFee(saveAcct);
}
Here's an example with no type safety but with late binding:
class DatabaseConnection {
void ReleaseResources
}
class FileHandle {
void ReleaseResources
}
FreeMemory(Object obj) {
// This will work for any class that has a
// "ReleaseResources" method (assuming all
// classes are ultimately derived from Object.
obj.ReleaseResources
}
main() {
DatabaseConnection dbConn;
FileHandle fh;
// You can pass in anything at all and it will
// compile just fine. But if you pass in an
// object that doesn't have a "ReleaseResources"
// method you'll get a run-time error.
FreeMemory(dbConn);
FreeMemory(fh);
FreeMemory(acct); //FAIL! (but not until run-time)
}
For an excellent example, look at the .NET ToString() method. All classes have it because all classes are derived from the Object class. But each class can implement ToString() in a way that makes sense for itself.
EDIT: Simple != short, IMHO
Polymorphism is language functionality allowing high-level algorithmic code to operate unchanged on multiple types of data.
This is done by ensuring the operations invoke the right implementation for each data type. Even in an OOP context (as per this question's tag), this "right implementation" may be resolved at compile-time or run-time (if your language supports both). In some languages like C++, compiler-supplied support for run-time polymorphism (i.e. virtual dispatch) is specific to OOP, whereas other types of polymorphism can also operate on data types that aren't objects (i.e. not struct or class instances, but may be builtin types like int or double).
( The types of polymorphism C++ supports are listed and contrasted in my answer: Polymorphism in c++ - even if you program other languages, it's potentially instructive )
The way I try and think of it is something that looks the same but can have different functionality depending on the instance. So you can have a type
interface IJobLoader
but depending on how it is used can have different functionality while still looking the same. You may have instances for BatchJobLoader, NightlyJobLoader etc
Maybe I am way off.
The term polymorphism can also apply to overloading functions. For example,
string MyFunc(ClassA anA);
string MyFunc(ClassB aB);
is a non-object oriented example of polymorphism.
Is the ability that objects have to respond to the same message in different ways.
For instance , in languages such as smalltalk, Ruby, Objective-C, you just have to send the message and they will respond.
dao = XmlDao.createNewInstance() #obj 1
dao.save( data )
dao = RdbDao.createNewnewInstance() #obj 2
dao.save( data )
In this example two different objects, responded in different ways to the same messages: "createNewInstance() and save( obj )"
They act in different ways, to the same message. In the above languages, the classes might not even be in the same class hierarchy, it is enough that they respond to the message.
In languages such as Java, C++, C# etc. In order to assign the object to an object reference, they must share the same type hierarchy either by implementing the interface or by being subclass of a common class.
easy .. and simple.
Polymorphism is by far, the most important and relevant feature of object oriented programming.
It is a way to treat different things that can do something something similar in the same way without caring how they do it.
Let's say you have a game with a bunch of different types of Vehicles driving around such as Car, Truck, Skateboard, Airplane, etc... They all can Stop, but each Vehicle stops in a different way. Some Vehicles may need to shift down gears, and some may be able to come to a cold stop. Polymophism lets you do this
foreach (Vehicle v in Game.Vehicles)
{
v.Stop();
}
The way that stop is implemented is deferred to the different Vehicles so your program doesn't have to care about it.
It's just a way to get old cold to call new code. You write some application that accepts some "Shape" interface with methods that others must implement (example - getArea). If someone comes up with a new whiz-bang way to implement that interface your old code can call that new code via the the getArea method.
The ability of an object of some type (e.g. a car) to act (e.g. brake) like one of another type (e.g. a vehicle) which usually suggests common ancestry (e.g. car is a subtype of vehicle) at one point in the type hierarchy.
Polymorphism is the Object Oriented solution to problem of passing a function to another function. In C you can do
void h() { float x=3.0; printf("%f", x); }
void k() { int y=5; printf("%i", y); }
void g(void (*f)()) { f(); }
g(h); // output 3.0
g(k); // output 5
In C things get complicated if the function depends on additional parameters. If the functions h and k depend on different types of parameters you are in trouble and you must use casting. You have to store those parameters in a data structure, and pass a pointer to that data structure to g which passes it to h or k. h and k cast the pointer into a pointer to the proper structure and unpack the data. Very messy and very unsafe because of possible casting errors:
void h(void *a) { float* x=(float*)a; printf("%f",*x); }
void k(void *a) { int* y=(int*)a; printf("%i",*y); }
void g(void (*f)(void *a),void *a) { f(a); }
float x=3.0;
int y=5;
g(h,&x); // output x
g(k,&y); // output y
So they invented polymorphism. h and k are promoted to classes and the actual functions to methods, the parameters are member variables of the respective class, h or k. Instead of passing the function around, you pass an instance of the class that contains the function you want. The instance contains its own parameters.
class Base { virtual public void call()=0; }
class H : public Base { float x; public void call() { printf("%f",x);} } h;
class K : public Base { int y; public void call() { printf("%i",y);} } k;
void g(Base &f) { f.call(); };
h.x=3.0;
k.y=5;
g(h); // output h.x
g(k); // output k.x