I'm going to preface this by saying that this is by no means a major issue, more of something I haven't really heard talked about in terms of programming language design, and I was wondering if anyone had any interesting solutions.
The crux of the problem is this. Sometimes in an object-oriented language, I want to be able to modify an object via one of its methods, but return the object itself instead of what that method returns.
to give a java example:
class MyClass{
public MyClass(List<Integer> list){
//do constructor stuff
}
public MyClass(Integer i){
//what I would like to be able to do
this((new LinkedList<Integer>).add(i));
}
}
I can't create a temporary list in the second constructor, because this() must be the first line. Obviously there are a lot of ways to do this by changing the implementation, like creating an add() method that returns the object, making it the responsibility of the function constructing the object to make the list, etc.
But, considering a lot of the time you can't/don't want to modify or create a subclass (for LinkedList) and you might not want to muddy up the calling code, being able to modify and return an object in the style of ++x could be really useful. Something like
this(#(new LinkedList).add(i) to signify you want to object, not the method return. Does anyone know of a language that allows this is some concise syntactic way? If not, would this be useful at all or am I missing something fundamental here?
Wouldn't this work?
public class MyClass{
public MyClass(List<Integer> list){
//do constructor stuff
}
public static MyClass create(Integer i) {
List<Integer> list = new new LinkedList<Integer>();
list.add(i);
MyClass myClass = new MyClass(list);
return myClass;
}
}
This is somewhat common design pattern called Factory Pattern.
I think the cleanest way to solve this is to have an initialize method called from the constructors.
class MyClass
{
public MyClass(List list){
init(list);
}
public MyClass(Integer i){
LinkedList<Integer> list = new LinkedList<Integer>();
list.add(i);
init(list);
}
protected init(List<Integer> list)
{
// do init stuff here
}
}
Related
I want to explain my question with an example. Lets say that i have an interface:
interface IActionPerformer
{
bool IsReadyToExecuteAction();
void Action();
IActionImplementor GetImplementor();
}
And an implementor for Action() method. I don't know if it is the right or wrong way to do so, but anyways, keep reading i will explain my purpose. Implementor:
interface IActionImplementor
{
void Action();
}
And an abstract class that implements IActionPerformer:
abstract class ActionPerformerBase: IActionPerformer
{
private IActionImplementor _implementor;
public abstract bool IsReadyToExecuteAction();
public IActionImplementor GetImplementor()
{
return _implementor;
}
public void Action()
{
if (IsReadyToExecuteAction())
{
GetImplementor().Action();
}
}
protected ActionPerformerBase(IActionImplementor implementor)
{
this._implementor = implementor;
}
}
Now sub classes which inherit from this abstract class, execute the actual action only if it is ready to execute.
But let's say that i have an object in my software, that inherits from a different super class. But at the same time, this object must behave like an IActionPerformer. I mean this object must implement IActionPerformer interface, like:
class SomeOtherSubClass : SomeOtherSuperClass, IActionPerformer
At this point, i want to execute Action() method with controlling if it is ready to execute.
I thought invoking method with another object might be a solution. I mean, a controller or handler object gets interface as a parameter and invokes method the way i want. Like:
IActionInvoker.Invoke(IActionPerformer performer)
{
if (performer.IsReadyToExecuteAction())
{
performer.Action();
}
}
Or every IActionPerformer implementor has a IActionPerformer or ActionPerformerBase(it feels better) object which handles the real control like:
class SomeOtherSubClass : SomeOtherSuperClass, IActionPerformer
{
ActionPerformerBase _realHandler;
public bool IsReadyToExecuteAction()
{
return _realHandler.IsReadyToExecuteAction();
}
public void Action()
{
_realHandler.Action();
}
.
.
.
}
//This is the one get the job done actually.
class RealHandlerOfSomething : ActionPerformerBase
I might not be that clear trying to explain my question. I'm new to concepts like abstraction, design patterns and sort of stuff like that. And trying to figure out them. This one looks like a decorator, it is a IActionPerformerand it has a IActionPerformer. But when i study decorator pattern, i saw it is like going from shell to the core, i mean every object executes its method and the wrapped objects method. It is a bit different in my example, i mean question. Is this what we call as "encapsulation"? Or do i have big issues understanding the concepts?
I hope i explained myself clearly. Thanks for everyone reading, trying to help.
Have a nice day/night.
As Design Patterns states in chapter one:
Favor object composition over class inheritance
This was in 1994. Inheritance makes things complicated. The OP is another example.
In the following, I'll keep IActionPerformer and ActionPerformerBase as is. Since inheritance is isomorphic to composition, everything you can do with inheritance, you can also do with composition - and more, such as emulating multiple inheritance.
Here's how you can implement the IActionPerformer interface from another subclass, and still reuse ActionPerformerBase:
public class SomeOtherSubClass : SomeOtherSuperClass, IActionPerformer
{
private readonly ActionPerformerBase #base;
public SomeOtherSubClass(ActionPerformerBase #base)
{
this.#base = #base;
}
public void Action()
{
// Perhaps do something before calling #base...
#base.Action();
// Perhaps do something else after calling #base...
}
// Other methods of IActionPerformer go here, possibly following the same pattern...
}
SomeOtherSubClass composes with any ActionPerformerBase, and since ActionPerformerBase has the desired functionality, that functionality is effectively reused.
Once you've figured out how to use composition for reuse instead of inheritance, do yourself a favour and eliminate inheritance from your code base. Trust me, you don't need it. I've been designing and writing production code for more than a decade without inheritance.
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.
I have a base class where all common functions are written. I many classes which override this functions by virtual keyword. Like,
public class Base
{
public virtual void sample()
{
..............
}
}
public class a : Base
{
public override sample()
{
}
}
public class implement
{
public void ToSample()
{
Base baseclass = new Base();
Switch(test)
{
case a: baseclass = a();
break;
case b: baseclass = b();
break;
}
baseclass.sample();
}
}
This perfect code for current situation but now I have more class to be assign in switch case. It is not good practice for adding huge amount of cases so I want something that automatically assign child class.
Is anybody know something to be implement ?
As stated in the comment, you can decouple the implementation by using dependency injection. Note however, that in some cases you have no choice but doing that kind of switch (e.g. when you need to create a class based on a text received in a socket). In such cases the important thing is to always keep the switch statement encapsulated in one method and make your objects rely on it (or, in other words, don't copy-and-paste it everywhere :)). The idea here is too keep your system isolated from a potentially harmful code. Of course that if you add a new class you will have to go and modify that method, however you will only have to do it in one time and in one specific place.
Another approach that I have seen (and sometimes used) is to build a mapping between values an classes. So, if your class-creation switch depends on an integer code, you basically create a mapping between codes and classes. What you are doing here is turning a "static" switch into a dynamic behavior, since you can change the mappings contents at any time and thus alter the way your program behaves. A typical implementation would be something like (sorry for the pseudocode, I'm not familiar with C#):
public class implement
{
public void ToSample()
{
class = this.mapping.valueForKey(test);
Base baseclass = new class();
baseclass.sample();
}
}
Note however that for this example to work you need reflection support, which varies according to the language you are using (again, sorry but I don't know the C# specifics).
Finally, you can also check the creational family of patterns for inspiration regarding object creation issues and some well known forms of solving them.
HTH
To show an example what is this question about:
I have currently a dilemma in PHP project I'm working on. I have in mind a method that will be used by multiple classes (UIs in this case - MVC model), but I'm not sure how to represent such methods in OO design. The first thing that came into my mind was to create a class with static functions that I'd call whenever I need them. However I'm not sure if it's the right thing to do.
To be more precise, I want to work, for example, with time. So I'll need several methods that handle time. I was thinking about creating a Time class where I'd be functions that check whether the time is in correct format etc.
Some might say that I shouldn't use class for this at all, since in PHP I can still use procedural code. But I'm more interested in answer that would enlighten me how to approach such situations in OOP / OOD.
So the actual questions are: How to represent such methods? Is static function approach good enough or should I reconsider anything else?
I would recommend creating a normal class the contains this behavior, and then let that class implement an interface extracted from the class' members.
Whenever you need to call those methods, you inject the interface (not the concrete class) into the consumer. This lets you vary the two independently of each other.
This may sound like more work, but is simply the Strategy design pattern applied.
This will also make it much easier to unit test the code, because the code is more loosely coupled.
Here's an example in C#.
Interface:
public interface ITimeMachine
{
IStopwatch CreateStopwatch();
DateTimeOffset GetNow();
}
Production implementation:
public class RealTimeMachine : ITimeMachine
{
#region ITimeMachine Members
public IStopwatch CreateStopwatch()
{
return new StopwatchAdapter();
}
public DateTimeOffset GetNow()
{
return DateTimeOffset.Now;
}
#endregion
}
and here's a consumer of the interface:
public abstract class PerformanceRecordingSession : IDisposable
{
private readonly IStopwatch watch;
protected PerformanceRecordingSession(ITimeMachine timeMachine)
{
if (timeMachine == null)
{
throw new ArgumentNullException("timeMachine");
}
this.watch = timeMachine.CreateStopwatch();
this.watch.Start();
}
public abstract void Record(long elapsedTicks);
public virtual void StopRecording()
{
this.watch.Stop();
this.Record(this.watch.ElapsedTicks);
}
}
Although you say you want a structure for arbitrary, unrelated functions, you have given an example of a Time class, which has many related functions. So from an OO point of view you would create a Time class and have a static function getCurrentTime(), for example, which returns an instance of this class. Or you could define that the constuctors default behaviour is to return the current time, whichever you like more. Or both.
class DateTime {
public static function getNow() {
return new self();
}
public function __construct() {
$this->setDateTime('now');
}
public function setDateTime($value) {
#...
}
}
But apart from that, there is already a builtin DateTime class in PHP.
Use a class as a namespace. So yes, have a static class.
class Time {
public static function getCurrentTime() {
return time() + 42;
}
}
I don't do PHP, but from an OO point of view, placing these sorts of utility methods as static methods is fine. If they are completely reusable in nature, consider placing them in a utils class.
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.