I encountered the situation mentioned in the topic now more than once and now I want to ask in here for
other opinions, hints, explanations, why someone should/would/ do things like this:
There is an object of class A, which implements the interface I_1o
This object has a static member, a collection, typed by interface I_1.
The class A has an interface-implemented method, which is called get_instance ( key-params ).
It looks inside the collection for a specified object fitting the key params and returns the
relevant object.
Is there a name for this (design pattern, whatever), a reason, a "best practice" explanation, why this seems to be a singleton but on the other hand it is not, just recursive object holding?
If no one understands, what I mean, just let me know, I will try to clarify it then.
This sounds an awful lot like an Object Pool design pattern. Documentation here.
This looks something like this:
public class Pool
{
private static int MAX_ELEMS = 10;
private static List<Object> instances;
private static void initialise()
{
if(instances == null) {
instances = new ArrayList<Object>();
// Initialise all the objects in the list.
}
}
public static Object getInstance(String key)
{
for(Object instance : instances) {
if(instance.equals(key)) { // Just an example
return instance;
}
}
}
}
The reason for this design pattern is to avoid the expensive re-instanciation of objects. If you have a load of, for example, Server connection objects, and you want to limit the amount of connections to the server, then you implement a pattern like this. It will mean that no more than MAX_ELEMS objects exist at one time, and it also means that they are not created during use of the program; they are built during some loading period in the program.
This looks like a Registry or IdentityMap.
Related
I'm learning OOP and trying to write a simple program that will execute some method every time when a specific varible will change.
I have two classes:
public class SomeClass {
private OtherClass object;
public OtherClass getObject() {
return this.object;
}
public void setObject(OtherClass object) {
objectChanged();
this.object = object;
}
private void objectChanged() {
System.out.println("Object has changed");
}
}
public class OtherClass {
private int value = 5;
public int getValue() {
return this.value;
}
public void setValue(int value) {
this.value = value;
}
}
The variable objectChanged should be called every time when variable "object" is changed. My first naive idea was to put the method call inside of set function. But what if you change the object after you set it? Like this:
SomeClass someObject = new SomeClass();
OtherClass otherObject = new OtherClass();
someObject.setObject(otherObject); //"Object has changed"
otherObject.setValue(10); //nothing happens yet
I need someObject to realize that object stored inside of it changed its value to 10, but how do i do it? Is it even possible in OOP?
It looks to be reasonable, but one should consider many things. This is why there is no automatic way to do it in general. It is not part of the OOP paradigm as such. If this would be some automatic behavior, it would cause huge overhead as it is not often needed to observe changes this way. But you can, of course, implement your way depending on your concrete requirements.
There are at least two approaches.
In MVVM (like WPF) there is an INotifyPropertyChanged interface (let's call it a pattern) you can use to trigger a notification yourself, mutch like you did with SomeClass. However when you are nesting objects, you need to wire up that mechanism.to cascade: you should do the same with OtherClass and also connect the actual instances to bubble up changes.
See: https://rehansaeed.com/tag/design-patterns/
An other option is the Observable pattern. Each time the object changes state, you emit an instance - the current instance. However, you should care to emit unmutable objects. At least by using an interface that makes it read-only. But you still need to wire up the object tree to react to the changes of nested objects.
If your platform supports reflection, and you create a proper toolset, you could make this wiring up quite simple. But again: this is not strictly related to the paradigm.
So I'm working on a middleware layer.
I'm consuming a COM DLL that deals with the low level hardware interaction, and providing an interface for the UI to do IO with the hardware.
As part of the design of my layer, we put in a contextmanager that arranges the various pieces of hardware to produce contexts that our application can work with.
So I want to guarantee that a developer that is working with my code has a single context manager to work with and then inside that context manager I can guarantee that we only allocate 1 work queue per hardware device.
Just to complicate this there is some initialization that must be done before I can start adding in hardware devices. Something that would be simple if not for the fact that typically you only access a singleton via a readonly property.
I know the singleton pattern can make alot of things difficult because of it's global accessibility. I really do not want, nor need for this class to have the global availability of a singleton, I just want the guarantee that only one will be created inside the app.
For that would I be crazy to do something like this, to basically give my singleton a constructor:
public class MySingleton
{
private static MySingleton _MySingleton;
private static object singletonLock = new object();
private MySingleton(int foo1, string foo2)
{
//do init stuff
}
public static MySingleton StartSingleton(int foo1, string foo2)
{
try
{
Monitor.Enter(singletonLock);
if (_MySingleton == null)
{
_MySingleton = new MySingleton(foo1, foo2);
}
else
throw new Exception("Singleton already initialized");
}
finally
{
Monitor.Exit(singletonLock);
}
return _MySingleton;
}
public static MySingleton Instance
{
get
{
try
{
Monitor.Enter(singletonLock);
if (_MySingleton == null)
{
throw new Exception("Singleton must be Initialized");
}
}
finally
{
Monitor.Exit(singletonLock);
}
return _MySingleton;
}
}
}
It's not crazy code but it is a singleton anyway. If you remove Instance property then it won't be singleton anymore.
Global accessibility is not all that makes singletons nasty. What makes them nasty is that they are used all through the system directly without you being able to track all those usages. That's why it is such a nightmare in multi-threading code, that's why it is so hard to unit test anything with singletons inside.
So if it is your code I'd recommend creating just one object during application initialization and pass it around with dependency injection or as plain constructor argument. If it is a library, you can either check in constructor if it is first object being created or not and throw an exception or you can go with static constructor as you did but without Instance property, forcing developers to pass instance around.
As always, you can just create singleton, after all all it matters is that product works and customers enjoy using it, singletons or no singletons doesn't really matter.
You wouldn't be crazy. A singleton avoids the drawbacks of global variables by virtue of being namespaced. Even though it is globally accessible via a static function call, it is not a global variable. And further, it is accessed via a namespace, so noone is likely to put it in a global var called temp, then later assign something else to temp. They should always get a local reference to it by doing
MySingleton singletonRef = MySingleton.Instance();
when their scope closes, the reference dies, and so it's not a global variable.
So if I just need to garuntee that you can only create one version of my object then something like this would work:
public class MySingleton
{
private static int objectCount = 0;
private static object singletonLock = new object();
public MySingleton(int foo1, string foo2)
{
try{
Monitor.Enter(singletonLock);
if (objectCount != 0)
{
throw new Exception("MySingleton Already exsists");
}
else
{
objectCount++;
}
}
finally{
Monitor.Exit(singletonLock);
}
//do initialization stuff
}
}
obviously not a true singleton anymore.
Can a class return an object of itself.
In my example I have a class called "Change" which represents a change to the system, and I am wondering if it is in anyway against design principles to return an object of type Change or an ArrayList which is populated with all the recent Change objects.
Yes, a class can have a method that returns an instance of itself. This is quite a common scenario.
In C#, an example might be:
public class Change
{
public int ChangeID { get; set; }
private Change(int changeId)
{
ChangeID = changeId;
LoadFromDatabase();
}
private void LoadFromDatabase()
{
// TODO Perform Database load here.
}
public static Change GetChange(int changeId)
{
return new Change(changeId);
}
}
Yes it can. In fact, that's exactly what a singleton class does. The first time you call its class-level getInstance() method, it constructs an instance of itself and returns that. Then subsequent calls to getInstance() return the already-constructed instance.
Your particular case could use a similar method but you need some way of deciding the list of recent changes. As such it will need to maintain its own list of such changes. You could do this with a static array or list of the changes. Just be certain that the underlying information in the list doesn't disappear - this could happen in C++ (for example) if you maintained pointers to the objects and those objects were freed by your clients.
Less of an issue in an automatic garbage collection environment like Java since the object wouldn't disappear whilst there was still a reference to it.
However, you don't have to use this method. My preference with what you describe would be to have two clases, changelist and change. When you create an instance of the change class, pass a changelist object (null if you don't want it associated with a changelist) with the constructor and add the change to that list before returning it.
Alternatively, have a changelist method which creates a change itself and returns it, remembering the change for its own purposes.
Then you can query the changelist to get recent changes (however you define recent). That would be more flexible since it allows multiple lists.
You could even go overboard and allow a change to be associated with multiple changelists if so desired.
Another reason to return this is so that you can do function chaining:
class foo
{
private int x;
public foo()
{
this.x = 0;
}
public foo Add(int a)
{
this.x += a;
return this;
}
public foo Subtract(int a)
{
this.x -= a;
return this;
}
public int Value
{
get { return this.x; }
}
public static void Main()
{
foo f = new foo();
f.Add(10).Add(20).Subtract(1);
System.Console.WriteLine(f.Value);
}
}
$ ./foo.exe
29
There's a time and a place to do function chaining, and it's not "anytime and everywhere." But, LINQ is a good example of a place that hugely benefits from function chaining.
A class will often return an instance of itself from what is sometimes called a "factory" method. In Java or C++ (etc) this would usually be a public static method, e.g. you would call it directly on the class rather than on an instance of a class.
In your case, in Java, it might look something like this:
List<Change> changes = Change.getRecentChanges();
This assumes that the Change class itself knows how to track changes itself, rather than that job being the responsibility of some other object in the system.
A class can also return an instance of itself in the singleton pattern, where you want to ensure that only one instance of a class exists in the world:
Foo foo = Foo.getInstance();
The fluent interface methods work on the principal of returning an instance of itself, e.g.
StringBuilder sb = new StringBuilder("123");
sb.Append("456").Append("789");
You need to think about what you're trying to model. In your case, I would have a ChangeList class that contains one or more Change objects.
On the other hand, if you were modeling a hierarchical structure where a class can reference other instances of the class, then what you're doing makes sense. E.g. a tree node, which can contain other tree nodes.
Another common scenario is having the class implement a static method which returns an instance of it. That should be used when creating a new instance of the class.
I don't know of any design rule that says that's bad. So if in your model a single change can be composed of multiple changes go for it.
I am new to OOP. Though I understand what polymorphism is, but I can't get the real use of it. I can have functions with different name. Why should I try to implement polymorphism in my application.
Classic answer: Imagine a base class Shape. It exposes a GetArea method. Imagine a Square class and a Rectangle class, and a Circle class. Instead of creating separate GetSquareArea, GetRectangleArea and GetCircleArea methods, you get to implement just one method in each of the derived classes. You don't have to know which exact subclass of Shape you use, you just call GetArea and you get your result, independent of which concrete type is it.
Have a look at this code:
#include <iostream>
using namespace std;
class Shape
{
public:
virtual float GetArea() = 0;
};
class Rectangle : public Shape
{
public:
Rectangle(float a) { this->a = a; }
float GetArea() { return a * a; }
private:
float a;
};
class Circle : public Shape
{
public:
Circle(float r) { this->r = r; }
float GetArea() { return 3.14f * r * r; }
private:
float r;
};
int main()
{
Shape *a = new Circle(1.0f);
Shape *b = new Rectangle(1.0f);
cout << a->GetArea() << endl;
cout << b->GetArea() << endl;
}
An important thing to notice here is - you don't have to know the exact type of the class you're using, just the base type, and you will get the right result. This is very useful in more complex systems as well.
Have fun learning!
Have you ever added two integers with +, and then later added an integer to a floating-point number with +?
Have you ever logged x.toString() to help you debug something?
I think you probably already appreciate polymorphism, just without knowing the name.
In a strictly typed language, polymorphism is important in order to have a list/collection/array of objects of different types. This is because lists/arrays are themselves typed to contain only objects of the correct type.
Imagine for example we have the following:
// the following is pseudocode M'kay:
class apple;
class banana;
class kitchenKnife;
apple foo;
banana bar;
kitchenKnife bat;
apple *shoppingList = [foo, bar, bat]; // this is illegal because bar and bat is
// not of type apple.
To solve this:
class groceries;
class apple inherits groceries;
class banana inherits groceries;
class kitchenKnife inherits groceries;
apple foo;
banana bar;
kitchenKnife bat;
groceries *shoppingList = [foo, bar, bat]; // this is OK
Also it makes processing the list of items more straightforward. Say for example all groceries implements the method price(), processing this is easy:
int total = 0;
foreach (item in shoppingList) {
total += item.price();
}
These two features are the core of what polymorphism does.
Advantage of polymorphism is client code doesn't need to care about the actual implementation of a method.
Take look at the following example.
Here CarBuilder doesn't know anything about ProduceCar().Once it is given a list of cars (CarsToProduceList) it will produce all the necessary cars accordingly.
class CarBase
{
public virtual void ProduceCar()
{
Console.WriteLine("don't know how to produce");
}
}
class CarToyota : CarBase
{
public override void ProduceCar()
{
Console.WriteLine("Producing Toyota Car ");
}
}
class CarBmw : CarBase
{
public override void ProduceCar()
{
Console.WriteLine("Producing Bmw Car");
}
}
class CarUnknown : CarBase { }
class CarBuilder
{
public List<CarBase> CarsToProduceList { get; set; }
public void ProduceCars()
{
if (null != CarsToProduceList)
{
foreach (CarBase car in CarsToProduceList)
{
car.ProduceCar();// doesn't know how to produce
}
}
}
}
class Program
{
static void Main(string[] args)
{
CarBuilder carbuilder = new CarBuilder();
carbuilder.CarsToProduceList = new List<CarBase>() { new CarBmw(), new CarToyota(), new CarUnknown() };
carbuilder.ProduceCars();
}
}
Polymorphism is the foundation of Object Oriented Programming. It means that one object can be have as another project. So how does on object can become other, its possible through following
Inheritance
Overriding/Implementing parent Class behavior
Runtime Object binding
One of the main advantage of it is switch implementations. Lets say you are coding an application which needs to talk to a database. And you happen to define a class which does this database operation for you and its expected to do certain operations such as Add, Delete, Modify. You know that database can be implemented in many ways, it could be talking to file system or a RDBM server such as MySQL etc. So you as programmer, would define an interface that you could use, such as...
public interface DBOperation {
public void addEmployee(Employee newEmployee);
public void modifyEmployee(int id, Employee newInfo);
public void deleteEmployee(int id);
}
Now you may have multiple implementations, lets say we have one for RDBMS and other for direct file-system
public class DBOperation_RDBMS implements DBOperation
// implements DBOperation above stating that you intend to implement all
// methods in DBOperation
public void addEmployee(Employee newEmployee) {
// here I would get JDBC (Java's Interface to RDBMS) handle
// add an entry into database table.
}
public void modifyEmployee(int id, Employee newInfo) {
// here I use JDBC handle to modify employee, and id to index to employee
}
public void deleteEmployee(int id) {
// here I would use JDBC handle to delete an entry
}
}
Lets have File System database implementation
public class DBOperation_FileSystem implements DBOperation
public void addEmployee(Employee newEmployee) {
// here I would Create a file and add a Employee record in to it
}
public void modifyEmployee(int id, Employee newInfo) {
// here I would open file, search for record and change values
}
public void deleteEmployee(int id) {
// here I search entry by id, and delete the record
}
}
Lets see how main can switch between the two
public class Main {
public static void main(String[] args) throws Exception {
Employee emp = new Employee();
... set employee information
DBOperation dboper = null;
// declare your db operation object, not there is no instance
// associated with it
if(args[0].equals("use_rdbms")) {
dboper = new DBOperation_RDBMS();
// here conditionally, i.e when first argument to program is
// use_rdbms, we instantiate RDBM implementation and associate
// with variable dboper, which delcared as DBOperation.
// this is where runtime binding of polymorphism kicks in
// JVM is allowing this assignment because DBOperation_RDBMS
// has a "is a" relationship with DBOperation.
} else if(args[0].equals("use_fs")) {
dboper = new DBOperation_FileSystem();
// similarly here conditionally we assign a different instance.
} else {
throw new RuntimeException("Dont know which implemnation to use");
}
dboper.addEmployee(emp);
// now dboper is refering to one of the implementation
// based on the if conditions above
// by this point JVM knows dboper variable is associated with
// 'a' implemenation, and it will call appropriate method
}
}
You can use polymorphism concept in many places, one praticle example would be: lets you are writing image decorer, and you need to support the whole bunch of images such as jpg, tif, png etc. So your application will define an interface and work on it directly. And you would have some runtime binding of various implementations for each of jpg, tif, pgn etc.
One other important use is, if you are using java, most of the time you would work on List interface, so that you can use ArrayList today or some other interface as your application grows or its needs change.
Polymorphism allows you to write code that uses objects. You can then later create new classes that your existing code can use with no modification.
For example, suppose you have a function Lib2Groc(vehicle) that directs a vehicle from the library to the grocery store. It needs to tell vehicles to turn left, so it can call TurnLeft() on the vehicle object among other things. Then if someone later invents a new vehicle, like a hovercraft, it can be used by Lib2Groc with no modification.
I guess sometimes objects are dynamically called. You are not sure whether the object would be a triangle, square etc in a classic shape poly. example.
So, to leave all such things behind, we just call the function of derived class and assume the one of the dynamic class will be called.
You wouldn't care if its a sqaure, triangle or rectangle. You just care about the area. Hence the getArea method will be called depending upon the dynamic object passed.
One of the most significant benefit that you get from polymorphic operations is ability to expand.
You can use same operations and not changing existing interfaces and implementations only because you faced necessity for some new stuff.
All that we want from polymorphism - is simplify our design decision and make our design more extensible and elegant.
You should also draw attention to Open-Closed Principle (http://en.wikipedia.org/wiki/Open/closed_principle) and for SOLID (http://en.wikipedia.org/wiki/Solid_%28Object_Oriented_Design%29) that can help you to understand key OO principles.
P.S. I think you are talking about "Dynamic polymorphism" (http://en.wikipedia.org/wiki/Dynamic_polymorphism), because there are such thing like "Static polymorphism" (http://en.wikipedia.org/wiki/Template_metaprogramming#Static_polymorphism).
You don't need polymorphism.
Until you do.
Then its friggen awesome.
Simple answer that you'll deal with lots of times:
Somebody needs to go through a collection of stuff. Let's say they ask for a collection of type MySpecializedCollectionOfAwesome. But you've been dealing with your instances of Awesome as List. So, now, you're going to have to create an instance of MSCOA and fill it with every instance of Awesome you have in your List<T>. Big pain in the butt, right?
Well, if they asked for an IEnumerable<Awesome>, you could hand them one of MANY collections of Awesome. You could hand them an array (Awesome[]) or a List (List<Awesome>) or an observable collection of Awesome or ANYTHING ELSE you keep your Awesome in that implements IEnumerable<T>.
The power of polymorphism lets you be type safe, yet be flexible enough that you can use an instance many many different ways without creating tons of code that specifically handles this type or that type.
Tabbed Applications
A good application to me is generic buttons (for all tabs) within a tabbed-application - even the browser we are using it is implementing Polymorphism as it doesn't know the tab we are using at the compile-time (within the code in other words). Its always determined at the Run-time (right now! when we are using the browser.)
Suppose we have an object that represents the configuration of a piece of hardware. For the sake of argument, a temperature controller (TempController). It contains one property, the setpoint temperature.
I need to save this configuration to a file for use in some other device. The file format (FormatA) is set in stone. I don't want the TempController object to know about the file format... it's just not relevant to that object. So I make another object, "FormatAExporter", that transforms the TempController into the desired output.
A year later we make a new temperature controller, let's call it "AdvancedTempController", that not only has a setpoint but also has rate control, meaning one or two more properties. A new file format is also invented to store those properties... let's call it FormatB.
Both file formats are capable of representing both devices ( assume AdvancedTempController has reasonable defaults if it lacks settings ).
So here is the problem: Without using 'isa' or some other "cheating" way to figure out what type of object I have, how can FormatBExporter handle both cases?
My first instinct is to have a method in each temperature controller that can provide a customer exporter for that class, e.g., TempController.getExporter() and AdvancedTempController.getExporter(). This doesn't support multiple file formats well.
The only other approach that springs to mind is to have a method in each temperature controller that returns a list of properties and their values, and then the formatter can decide how to output those. It'd work, but that seems convoluted.
UPDATE: Upon further work, that latter approach doesn't really work well. If all your types are simple it might, but if your properties are Objects then you end up just pushing the problem down a level... you are forced to return a pair of String,Object values, and the exporter will have to know what the Objects actually are to make use of them. So it just pushes the problem to another level.
Are there any suggestions for how I might keep this flexible?
What you can do is let the TempControllers be responsible for persisting itself using a generic archiver.
class TempController
{
private Temperature _setPoint;
public Temperature SetPoint { get; set;}
public ImportFrom(Archive archive)
{
SetPoint = archive.Read("SetPoint");
}
public ExportTo(Archive archive)
{
archive.Write("SetPoint", SetPoint);
}
}
class AdvancedTempController
{
private Temperature _setPoint;
private Rate _rateControl;
public Temperature SetPoint { get; set;}
public Rate RateControl { get; set;}
public ImportFrom(Archive archive)
{
SetPoint = archive.Read("SetPoint");
RateControl = archive.ReadWithDefault("RateControl", Rate.Zero);
}
public ExportTo(Archive archive)
{
archive.Write("SetPoint", SetPoint);
archive.Write("RateControl", RateControl);
}
}
By keeping it this way, the controllers do not care how the actual values are stored but you are still keeping the internals of the object well encapsulated.
Now you can define an abstract Archive class that all archive classes can implement.
abstract class Archive
{
public abstract object Read(string key);
public abstract object ReadWithDefault(string key, object defaultValue);
public abstract void Write(string key);
}
FormatA archiver can do it one way, and FormatB archive can do it another.
class FormatAArchive : Archive
{
public object Read(string key)
{
// read stuff
}
public object ReadWithDefault(string key, object defaultValue)
{
// if store contains key, read stuff
// else return default value
}
public void Write(string key)
{
// write stuff
}
}
class FormatBArchive : Archive
{
public object Read(string key)
{
// read stuff
}
public object ReadWithDefault(string key, object defaultValue)
{
// if store contains key, read stuff
// else return default value
}
public void Write(string key)
{
// write stuff
}
}
You can add in another Controller type and pass it whatever formatter. You can also create another formatter and pass it to whichever controller.
In C# or other languages that support this you can do this:
class TempController {
int SetPoint;
}
class AdvancedTempController : TempController {
int Rate;
}
class FormatAExporter {
void Export(TempController tc) {
Write(tc.SetPoint);
}
}
class FormatBExporter {
void Export(TempController tc) {
if (tc is AdvancedTempController) {
Write((tc as AdvancedTempController).Rate);
}
Write(tc.SetPoint);
}
}
I'd have the "temp controller", through a getState method, return a map (e.g. in Python a dict, in Javascript an object, in C++ a std::map or std::hashmap, etc, etc) of its properties and current values -- what's convoluted about it?! Could hardly be simpler, it's totally extensible, and totally decoupled from the use it's put to (displaying, serializing, &c).
Well, a lot of that depends on the file formats you're talking about.
If they're based on key/value combinations (including nested ones, like xml), then having some kind of intermediate memory object that's loosely typed that can be thrown at the appropriate file format writer is a good way to do it.
If not, then you've got a scenario where you've got four combinations of objects and file formats, with custom logic for each scenario. In that case, it may not be possible to have a single representation for each file format that can deal with either controller. In other words, if you can't generalize the file format writer, you can't generalize it.
I don't really like the idea of the controllers having exporters - I'm just not a fan of objects knowing about storage mechanisms and whatnot (they may know about the concept of storage, and have a specific instance given to them via some DI mechanism). But I think you're in agreement with that, and for pretty much the same reasons.
If FormatBExporter takes an AdvancedTempController, then you can make a bridge class that makes TempController conform to AdvancedTempController. You may need to add some sort of getFormat() function to AdvancedTempController though.
For example:
FormatBExporter exporterB;
TempController tempController;
AdvancedTempController bridged = TempToAdvancedTempBridge(tempController);
exporterB.export(bridged);
There is also the option of using a key-to-value mapping scheme. FormatAExporter exports/imports a value for key "setpoint". FormatBExporter exports/imports a values for keys "setpoint" and "ratecontrol". This way, old FormatAExporter can still read the new file format (it just ignores "ratecontrol") and FormatBExporter can read the old file format (if "ratecontrol" is missing, it uses a default).
In the OO model, the object methods as a collective is the controller. It's more useful to separate your program in to the M and V and not so much the C if you're programming using OO.
I guess this is the where the Factory method pattern would apply