Imagine the following: I have a bunch of DTO's that inherit from Foo class
class Foo { }
class FooA : Foo { }
class FooB : Foo { }
class FooX : Foo { }
Than I have one class that have encapsulated all the related logic and orchestration related with Foo data types. I provide a method DoSomethingWithData(Foo data) that do all the logic related to data provided by argument
The method implementation is something like this:
void DoSomething(Foo data)
{
if (data is FooA)
DoSomethingWithFooA((FooA) data);
if (data is FooB)
DoSomethingWithFooB((FooA)data);
if (data is FooX)
DoSomethingWithFooC((FooA)data);
}
This is a very simplified example. The advantage of this approach is:
The "Client" invoke always the DoSomething method independently of
the Foo data type
If I add a new type I only have to change the method DoSomething
What i dont like is the downcasting
The alternative is instead of exposing only DoSomething method I expose a method by each Foo data type. The advantage is that we dont have downcast but increases the boilerplate/forwarding code.
What do you prefer? Or do you have other approaches?
In this case, I would approach the problem like this (I will use Java for this example).
In your approach, for every subclass of Foo you have to provide a specific processing logic - as you have shown, and cast the Foo object to its sub-type. Moreover, for every new class that you add, you have to change the DoSomething(Foo f) method.
You can make the Foo class an interface:
public interface Foo{
public void doSomething();
}
Then have your classes implement this interface:
public class FooA iplements Foo {
public void doSomething(){
//Whatever FooA needs to do.
}
}
public class FooB implements Foo {
public void doSomething(){
//Whatever FooB needs to do.
}
}
And so on. Then, the client can call the doSomething() method:
...
Foo fooA = new FooA();
Foo fooB = new FooB();
fooA.doSomething();
fooB.doSomething();
...
This way, you don't have to cast the object at run-time and if you add more classes, you don't have to change your existing code, except the client that has to call the method of a newly added object.
Related
https://dart.dev/guides/language/language-tour#extending-a-class
Argument types must be the same type as (or a supertype of) the
overridden method’s argument types. In the preceding example, the
contrast setter of SmartTelevision changes the argument type from int
to a supertype, num.
I was looking at the above explanation and wondering why the arguments of subtype member methods need to be defined more "widely"(generally) than the original class's one.
https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)#Function_types
class AnimalShelter {
Animal getAnimalForAdoption() {
// ...
}
void putAnimal(Animal animal) {
//...
}
}
class CatShelter extends AnimalShelter {
//↓ Definitions that are desirable in the commentary
void putAnimal(Object animal) {
// ...
}
//↓Definitions that are not desirable in the commentary
void putAnimal(Cat animal) {
// ...
}
//I can't understand why this definition is risky.
//What specific problems can occur?
}
I think this wikipedia sample code is very easy to understand, so what kind of specific problem (fault) can occur if the argument of the member method of the subtype is defined as a more "narrower"(specific) type?
Even if it is explained in natural language, it will be abstract after all, so it would be very helpful if you could give me a complete working code and an explanation using it.
Let's consider an example where you have a class hierarchy:
Animal
/ \
Mammal Reptile
/ \
Dog Cat
with superclasses (wider types) above subclasses (narrower types).
Now suppose you have classes:
class Base {
void takeObject(Mammal mammal) {
// ...
}
Mammal returnObject() {
// ...
}
}
class Derived extends Base {
// ...
}
The public members of a class specify an interface: a contract to the callers. In this case, the Base class advertises a takeObject method that accepts any Mammal argument. Every instance of a Base class thus is expected to conform to this interface.
Following the Liskov substitution principle, because Derived extends Base, a Derived instance is a Base, and therefore it too must conform to that same Base class interface: its takeObject method also must accept any Mammal argument.
If Derived overrode takeObject to accept only Dog arguments:
class Derived extends Base {
#override
void takeObject(Dog mammal) { // ERROR
// ...
}
}
that would violate the contract from the Base class's interface. Derived's override of takeObject could be invoked with a Cat argument, which should be allowed according to the interface declared by Base. Since this is unsafe, Dart's static type system normally prevents you from doing that. (An exception is if you add the covariant keyword to disable type-safety and indicate that you personally guarantee that Derived.takeObject will never be called with any Mammals that aren't Dogs. If that claim is wrong, you will end up with a runtime error.)
Note that it'd be okay if Derived overrode takeObject to accept an Animal argument instead:
class Derived extends Base {
#override
void takeObject(Animal mammal) { // OK
// ...
}
}
because that would still conform to the contract of Base.takeObject: it's safe to call Derived.takeObject with any Mammal since all Mammals are also Animals.
Note that the behavior for return values is the opposite: it's okay for an overridden method to return a narrower type, but returning a wider type would violate the contract of the Base interface. For example:
class Derived extends Base {
#override
Dog returnObject() { // OK, a `Dog` is a `Mammal`, as required by `Base`
// ...
}
}
but:
class Derived extends Base {
#override
Animal returnObject() { // ERROR: Could return a `Reptile`, which is not a `Mammal`
// ...
}
}
void main(){
Animal a1 = Animal();
Cat c1 = Cat();
Dog d1 = Dog();
AnimalCage ac1 = AnimalCage();
CatCage cc1 = CatCage();
AnimalCage ac2 = CatCage();
ac2.setAnimal(d1);
//cc1.setAnimal(d1);
}
class AnimalCage{
Animal? _animal;
void setAnimal(Animal animal){
print('animals setter');
_animal = animal;
}
}
class CatCage extends AnimalCage{
Cat? _cat;
#override
void setAnimal(covariant Cat animal){
print('cats setter');
_cat = animal;
/*
if(animal is Cat){
_cat = animal;
}else{
print('$animal is not Cat!');
}
*/
}
}
class Animal {}
class Cat extends Animal{}
class Dog extends Animal{}
Unhandled Exception: type 'Dog' is not a subtype of type 'Cat' of 'animal'
In the above code, even if the setAnimal method receives a Dog instance, a compile error does not occur and a runtime error occurs, so making the parameter the same type as the superclass's one and checking the type inside the method is necessary.
I am writing an object-oriented code in which I am trying to use Decorator pattern to implement a variety of optimizations to be applied on a family of core classes at runtime. The main behaviour of core classes is a complex behaviour that is fully implemented in those classes, which indeed calls other internal methods to fulfill pieces of the task.
The decorators will only customize the internal methods which are called by the complex behaviour in core class.
Here is a pseudo-code of what I'm trying to reach:
interface I{
complex();
step1();
step2();
}
class C implements I{
complex(){
...
this.step1();
...
this.step2();
}
step1(){
...
}
step2(){
...
}
}
abstract class Decorator implements I{
I wrapped;
constructor(I obj){
this.wrapped = obj;
}
complex(){
this.wrapped.complex();
}
step1(){
this.wrapped.step1();
}
step2(){
this.wrapped.step2();
}
}
class ConcreteDecorator extends Decorator{
constructor(I obj){
super(obj);
}
step2(){
... // customizing step2()
}
}
There are a variety of customizations possible which could be combined together, and that is the main reason I'm using decorator pattern. otherwise I'll get to create dozens to hundred subtypes for each possible combination of customizations.
Now if I try to create object of the decorated class:
x = new C();
y = new ConcreteDecorator(x);
y.complex();
I expect the complex() method to be executed form the wrapped core object, while using the overridden step2() method from decorator. But it does not work this way as the complex() method in abstract decorator directly calls the method on core object which indeed skips the overridden step2() in decorator.
My overall goal is to enable the decorators only overriding one or few of the stepx() methods and that would be invoked by the complex() method which is already implemented in the core object and invokes all the steps.
Could this functionality be implemented using Decorator design pattern at all? If yes how, and if not what is the appropriate design pattern for tackling this problem.
Thanks.
I guess you could resolve that problem with Strategy pattern, where the Strategy interface includes the methods that are vary from class to class. Strategy interface may include as only one method as well as several depending on their nature.
interface IStrategy {
step1(IData data);
step2(IData data);
}
interface I {
complex();
}
class C implements I {
IData data
constructor(IStrategy strategy) {}
complex() {
...
this.strategy.step1(this.data);
...
this.strategy.step2(this.data);
}
}
class S1 implements IStrategy {
constructor(IStrategy strategy)
step1(IData data) {
}
step2(IData data) {
}
}
strategy1 = new S1();
c = new C(strategy1)
The issue you are facing is that in your application of the Decorator design pattern, because you are not decorating complex(), the call to complex() on a decorator object will be delegated to the decorated object, which has "normal" version of step2.
I think a more appropriate design pattern to solve your problem would be the Template Method design pattern.
In your case complex() would play the role of the template method, whose steps can be customized by subclasses. Instead of using composition, you use inheritance, and the rest stays more or less the same.
Here is a sample application of the Template Method design pattern to your context:
public interface I {
void complex();
void step1(); // Better to remove from the interface if possible
void step2(); // Better to remove from the interface if possible
}
// Does not need to be abstract, but can be
class DefaultBehavior implements I {
// Note how this is final to avoid having subclass
// change the algorithm.
public final void complex() {
this.step1();
this.step2();
}
public void step1() { // Default step 1
System.out.println("Default step 1");
}
public void step2() { // Default step 2
System.out.println("Default step 1");
}
}
class CustomizedStep2 extends DefaultBehavior {
public void step2() { // Customized step 2
System.out.println("Customized step 2");
}
}
I have two classes Class A and Class SRD (Sample classes for understanding the problem. Real classes are different). Both classes have same Function(method1) with same arguments. Both are not derived from different Classes.
Class SRD is the member of Class A. a function in Class A creates a new object for SRD and calls method1(). It should call the mock function. but it calls the actual implementation
I have Written mock classes for both the classes and defined the mock method in both the classes and Wrote EXPECT_CALL in TEST function
class A{
private:
SRD* srd;
public :
bool Method1();
bool MethodX();
SRD* getsrd() {return srd;}
};
bool A :: MethodX()
{
srd.Method1(); // Not Calling Mock Method - Calling Actual
//Implementation
}
bool A::Method1()
{
....
}
class SRD{
public:
bool Method1();
};
class MockSRD : public SRD{
MOCK_METHOD0(Method1, bool())
};
class MockA : public MockA{
MOCK_METHOD0(Method1, bool())
};
bool SRD::Method1()
{
....
}
class TestA : public A {};
TEST_F(TestA, test1)
{
MockSRD srd;
EXPECT_CALL(srd, Method1())
.Times(testing::AnyNumber())
.WillRepeatedly(Return(true));
srd.Method1() //Working fine - Caling mock Method;
MethodX()
}
When i call s1.Method1(), It should call the mock method. how should i do that ?
I don't want to change the production code.
Thanks for taking time to respond the Question . #Chris Oslen & #sklott
I forgot to make the base class method to Virtual. Its worked fine when i change the base class methods
In the following example, interface IFoo declares a function signature requiring two number arguments. Abstract class BaseFoo implements this interface, but declares the function with a different signature. Finally, concrete class Foo extends BaseFoo and implements BaseFoo's version of the function declaration.
interface IFoo {
func(x: number ): number
}
abstract class BaseFoo implements IFoo {
abstract func(x: number): number
}
class Foo extends BaseFoo {
func() { return -1 } // Does not match interface func declaration
}
let foo: IFoo = new Foo() // Should not be able to instantiate a Foo as an IFoo
let y = foo.func() // Should not be able to call without an argument
console.log(y)
This contrived example illustrates something that happened in real life: I had an existing interface in a codebase. I updated one of it's function's signatures, with the expectation that the compiler would help me find all the classes who would need to be updated. But, no errors.
Why am I allowed to instantiate an abstract class with a function signature that doesn't match the interface?
Let's say I have the following abstractProductA class with a public method called methodA :
class abstractProductA = {
pub methodA => "name";
};
I would like to create an interface that says function methodA should always return a string. Something similar to
interface abstractProductA {
abstractProductA(): string
}
only in reason, and then have class implement it. Any suggestions are more than welcome. Thank you
What you're really asking for it seems is how to define and use an abstract class, which is called a virtual class in OCaml/Reason:
class virtual virtualProductA = {
pub virtual methodA: string;
};
class abstractProductA = {
inherit virtualProductA;
pub methodA = "name";
};
An interface is more for consumers to abstract away an implementation, and while a virtual class can be used as an interface by itself, since OCaml/Reason objects are structurally typed you can also just specify the object type you need. And of course you can bind it to a name if you like:
type interfaceA = {.
methodA : string
};
let f (p: interfaceA) => Js.log p#methodA;
f (new abstractProductA);