I have a sample(incomplete) class like
class ABC{
public:
void login();
void query_users();
//other methods
private:
//member data
}
This class should be used in a way that login needs to be called first and then only other methods like query_users, etc., can be called. Login sets some private member data for the other methods to use. Is there any simpler way to achieve this other than calling a function that checks if the member data is set at the start of every other method in the class?
There are two general approach I know of, and they differ a good bit. You'll have to pick the appropriate mechanism for the task - in standard class-based OO languages (e.g. Java/C++/C#/Python), they are the only two approaches I know of. (There may be other approaches in different paradigms that I am unfamiliar with.)
1. Check the state.
This is done in many classes already that have to track the state of the system/backing resource. Two common examples are (file) stream and database connections.
A "template" might look like:
void Logon(credentials) { ..; loggedOn = true }
void DieUnlessLoggedIn { if (!loggedOn) { throw .. } }
void DoStuff () { DieUnlessLoggedIn(); .. }
While the above approach is pretty generic, some languages may support invariants (Eiffel), decorations (Python), annotations, AOP, or other assertion mechanisms.
This approach is useful for dynamic state in a mutable world: e.g. what happens after "Logout"? The state for DoStuff is invalid again until a re-logon (if it's allowed). However, this approach cannot be used for compile-time checks in general in mainstream OOP languages because the run-time state simply is not available at compile-time.
2. Use multiple types to represent state.
Create two separate types, such that type ServiceLogon (method Logon) creates ServiceAccess (method DoStuff). Thus DoStuff can only be called (on type ServiceAccess) after created from Logon (on ServiceLogon). This works well to enforce calling order semantics in static languages with member hiding - because programs won't compile if it's wrong.
login = new ServiceLogon(credentials)
access = login.Logon();
access.DoStuff(); // can't be called before obtained via Logon
Using the type to encode additional state can be overly complex as it can fracture a class-based type system, but is useful in "builder" and "repository" patterns and such; basically, ask if the type warrants being split to maintain SRP, then considering this approach.
This approach cannot handle things like "logout" entirely without incorporating state checking as type ServiceAccess would (in the clean sense) always represent the same state due to it being encoded in the type.
1. & 2. Use state checking and state/role-specific types.
A hybrid is totally acceptable, of course, and the above two approaches are not mutually exclusive. It may make sense to separate the roles making one type (and thus methods invoked upon it) dependent upon another method while still checking runtime state as appropriate. As per above, #1 is really good for runtime guards (which can be highly dynamic) while #2 can enforce certain rules at compile-time.
What you can do is to create instances of ABC form a static factory method that returns the instance you can use. In pseudo-code:
abc = ABC.login(); //sets all the state
users = abc.query_users();
I am not sure this is the best way but you can make login() private and call it as part of the constructor, which would ensure that login() is called at time of object creation itself and after that only any other functions can be called (unless you have static functions)
class ABC{
public ABC(credentials){
login(credentails);
}
public:
void query_users();
//other methods
private:
void login();
//member data
}
It will already work first when it goes from the top down. If you want to make sure that login is successful then call the other methods from inside the login() method.
like:
public void login(){
//do login code
if(//code for login check){
//run other methods
}
else{
login(); //re-run login workings
}
}
If you really want to follow good patterns you might try making as many of your classes immutable as possible.
This would imply that your constructor sets the total state (does the entire login) and then the order of the method calls is totally irrelevant.
Related
I've the following method:
attemptToGetToken(TokenRequestAttempt attempt);
I use a third party component to get the token but it may fail in some case (in the attempt is not valid or missing parameters).
There is no reason to throw an Exception for this case because it is actually expected to fail.
However, depending on the result, the end-user may acts accordingly.
It means I need to return an Object which allows the user to decide what it can/should do.
The probably most obvious answer would be to simply have such class:
class AttemptResult {
constructor(status, token, errorMessage);
getStatus();
getToken();
getErrorMessage();
isSuccessful();
}
And simply return this object properly populated (I may add specific factory method for success/fail state, but it would be just a tool).
The problem I have with this approach is that the contract is not clear.
In the case of failure, the token would be null but the developper would then have to use the isSuccessful() method to check wether or not the attempt succeed.
But nothing will prevent him to call getToken() and get a null result which may bubble an error somewhere.
So I'm wondering if it wouldn't be better to actually return two kind of object.
interface TokenAttemptResult {}
class FailedTokenAttempt implements TokenAttemptResult {
getError();
}
class SuccessfulTokenAttempt implements TokenAttemptResult {
getToken();
}
Because I implement an empty interface to define a relationship between the both result, I may now use this interface as a return type in my service:
TokenAttemptResult attemptToGetToken(TokenRequestAttempt attempt);
When an user calls the method above, he is in charge to check the return type and act accordingly, it prevents him to call method which should not be supposed to be called and may generate side effects.
What do you think about this kind of design?
Any of you has experimented with it and has any feedbacks?
Please avoid unconstructive answer involving personal preferences without actual arguments, I'm really looking for serious design theories.
Thanks!
The subclassing approach will work, however since interfaces are open, the client has no way of being sure that they have exhaustively checked all possible implementing types of the interface. The OO approach to closing the possible cases is the visitor pattern e.g.
public interface TokenResultVisitor<TResult> {
TResult Visit(FailedTokenAttempt failure);
TResult Visit(SuccessfulTokenAttempt success);
}
and then
interface TokenAttemptResult {
T Accept<T>(TokenResultVisitor visitor);
}
This is a fairly heavyweight approach if you only have two possible cases, so if your language supports first-class functions you can simply define a method which takes two handlers e.g.
interface TokenAttemptResult {
T Handle<T>(success: Result => T, failure: TokenError => T);
}
Let's define a simple state machine:
public enum State {
A, B, C, D;
private List<State> validChange;
static {
A.validChange = Arrays.asList(B);
B.validChange = Arrays.asList(C);
C.validChange = Arrays.asList(A, D);
D.validChange = Arrays.asList(D);
}
public boolean couldChange(State newState) {
return validChange.contains(newState);
}
}
and a simple state object
public class StateObject {
private State currentState;
public State getCurrentState() {
return currentState;
}
public void setCurrentState(State currentState) {
if (this.currentState != null && !this.currentState.couldChange(currentState)) {
throw new IllegalStateException(String.format("Can not change from %s to %s", this.currentState, currentState));
}
this.currentState = currentState;
}
}
As we see in the setter we check that the state change is valid. My questions:
is it a good solution to add some logic to setter method (we are not interest how does it work but only fact logic inside setter)?
when should we add logic, when should we not?
if not why?
The state pattern has its reference implementation for years. The implementation is free of your concerns. States are not represented as enums but rather as classes inheriting from the same base class. This way the implementation of transition between states is much simpler and cleaner as each state is only responsible for its own transitions.
I wonder why you decide to try your own approach instead of following a good, reliable practice.
Answering your question - there is nothing fundamentally wrong in having a code in a setter. But the way you approach the implementation of the state pattern raises unnecessary issues.
In general, yes, it's OK to have logic in setters.
If you think SRP is a good idea, then you're OK if you add logic that is related to setter functionality, which primarily means maintaining the integrity of the object w.r.t to the change of the attribute's value.
Like I said, one of the cases when this is a bad idea is an SRP violation.
In fact, I would say the code posted shows an example of such a violation, since the setter has two responsibilities:
initialize the State of a new StateObject instance,
process any transition into a new State.
The second responsibility would become even more apparent if you'd implement the remaining part of a canonical state machine, i.e. the full transition function, including transition actions.
A primary example of where this separation of concerns would become important is in code where you only have setter injection - how do you differentiate between initialization and transition in this case? With this code, you can't.
Additionally, I believe the case would become more apparent if you'd refactor validChange to StateObject, which I think is necessary, since the current code breaks encapsulation - State, a state machine label, contains "guard specification" for the state transition function. Let's say you would like to have the same labels in a different state machine - as the code currently stands, you can't have that (there's even a nice code smell that should point you towards the problem, as instead of using an enum constructor, you're forced to create a static initialization block).
Does it affect the time in loading the application?
or any other issues in doing so?
The question is vague on what "long" means. Here are some possible interpretations:
Interpretation #1: The constructor has many parameters
Constructors with many parameters can lead to poor readability, and better alternatives exist.
Here's a quote from Effective Java 2nd Edition, Item 2: Consider a builder pattern when faced with many constructor parameters:
Traditionally, programmers have used the telescoping constructor pattern, in which you provide a constructor with only the required parameters, another with a single optional parameters, a third with two optional parameters, and so on...
The telescoping constructor pattern is essentially something like this:
public class Telescope {
final String name;
final int levels;
final boolean isAdjustable;
public Telescope(String name) {
this(name, 5);
}
public Telescope(String name, int levels) {
this(name, levels, false);
}
public Telescope(String name, int levels, boolean isAdjustable) {
this.name = name;
this.levels = levels;
this.isAdjustable = isAdjustable;
}
}
And now you can do any of the following:
new Telescope("X/1999");
new Telescope("X/1999", 13);
new Telescope("X/1999", 13, true);
You can't, however, currently set only the name and isAdjustable, and leaving levels at default. You can provide more constructor overloads, but obviously the number would explode as the number of parameters grow, and you may even have multiple boolean and int arguments, which would really make a mess out of things.
As you can see, this isn't a pleasant pattern to write, and even less pleasant to use (What does "true" mean here? What's 13?).
Bloch recommends using a builder pattern, which would allow you to write something like this instead:
Telescope telly = new Telescope.Builder("X/1999").setAdjustable(true).build();
Note that now the parameters are named, and you can set them in any order you want, and you can skip the ones that you want to keep at default values. This is certainly much better than telescoping constructors, especially when there's a huge number of parameters that belong to many of the same types.
See also
Wikipedia/Builder pattern
Effective Java 2nd Edition, Item 2: Consider a builder pattern when faced with many constructor parameters (excerpt online)
Related questions
When would you use the Builder Pattern?
Is this a well known design pattern? What is its name?
Interpretation #2: The constructor does a lot of work that costs time
If the work must be done at construction time, then doing it in the constructor or in a helper method doesn't really make too much of a difference. When a constructor delegates work to a helper method, however, make sure that it's not overridable, because that could lead to a lot of problems.
Here's some quote from Effective Java 2nd Edition, Item 17: Design and document for inheritance, or else prohibit it:
There are a few more restrictions that a class must obey to allow inheritance. Constructors must not invoke overridable methods, directly or indirectly. If you violate this rule, program failure will result. The superclass constructor runs before the subclass constructor, so the overriding method in the subclass will be invoked before the subclass constructor has run. If the overriding method depends on any initialization performed by the subclass constructor, the method will not behave as expected.
Here's an example to illustrate:
public class ConstructorCallsOverride {
public static void main(String[] args) {
abstract class Base {
Base() { overrideMe(); }
abstract void overrideMe();
}
class Child extends Base {
final int x;
Child(int x) { this.x = x; }
#Override void overrideMe() {
System.out.println(x);
}
}
new Child(42); // prints "0"
}
}
Here, when Base constructor calls overrideMe, Child has not finished initializing the final int x, and the method gets the wrong value. This will almost certainly lead to bugs and errors.
Interpretation #3: The constructor does a lot of work that can be deferred
The construction of an object can be made faster when some work is deferred to when it's actually needed; this is called lazy initialization. As an example, when a String is constructed, it does not actually compute its hash code. It only does it when the hash code is first required, and then it will cache it (since strings are immutable, this value will not change).
However, consider Effective Java 2nd Edition, Item 71: Use lazy initialization judiciously. Lazy initialization can lead to subtle bugs, and don't always yield improved performance that justifies the added complexity. Do not prematurely optimize.
Constructors are a little special in that an unhandled exception in a constructor may have weird side effects. Without seeing your code I would assume that a long constructor increases the risk of exceptions. I would make the constructor as simple as needed and utilize other methods to do the rest in order to provide better error handling.
The biggest disadvantage is probably the same as writing any other long function -- that it can get complex and difficult to understand.
The rest is going to vary. First of all, length and execution time don't necessarily correlate -- you could have a single line (e.g., function call) that took several seconds to complete (e.g., connect to a server) or lots of code that executed entirely within the CPU and finished quickly.
Startup time would (obviously) only be affected by constructors that were/are invoked during startup. I haven't had an issue with this in any code I've written (at all recently anyway), but I've seen code that did. On some types of embedded systems (for one example) you really want to avoid creating and destroying objects during normal use, so you create almost everything statically during bootup. Once it's running, you can devote all the processor time to getting the real work done.
Constructor is yet another function. You need very long functions called many times to make the program work slow. So if it's only called once it usually won't matter how much code is inside.
It affects the time it takes to construct that object, naturally, but no more than having an empty constructor and calling methods to do that work instead. It has no effect on the application load time
In case of copy constructor if we use donot use reference in that case
it will create an object and call the copy constructor and passing the
value to the copy constructor and each time a new object is created and
each time it will call the copy constructor it goes to infinite and
fill the memory then it display the error message .
if we pass the reference it will not create the new object for storing
the value. and no recursion will take place
I would avoid doing anything in your constructor that isn't absolutely necessary. Initialize your variables in there, and try not to do much else. Additional functionality should reside in separate functions that you call only if you need to.
I have an object called Parameters that gets tossed from method to method down and up the call tree, across package boundaries. It has about fifty state variables. Each method might use one or two variables to control its output.
I think this is a bad idea, beacuse I can't easily see what a method needs to function, or even what might happen if with a certain combination of parameters for module Y which is totally unrelated to my current module.
What are some good techniques for decreasing coupling to this god object, or ideally eliminating it ?
public void ExporterExcelParFonds(ParametresExecution parametres)
{
ApplicationExcel appExcel = null;
LogTool.Instance.ExceptionSoulevee = false;
bool inclureReferences = parametres.inclureReferences;
bool inclureBornes = parametres.inclureBornes;
DateTime dateDebut = parametres.date;
DateTime dateFin = parametres.dateFin;
try
{
LogTool.Instance.AfficherMessage(Variables.msg_GenerationRapportPortefeuilleReference);
bool fichiersPreparesAvecSucces = PreparerFichiers(parametres, Sections.exportExcelParFonds);
if (!fichiersPreparesAvecSucces)
{
parametres.afficherRapportApresGeneration = false;
LogTool.Instance.ExceptionSoulevee = true;
}
else
{
The caller would do :
PortefeuillesReference pr = new PortefeuillesReference();
pr.ExporterExcelParFonds(parametres);
First, at the risk of stating the obvious: pass the parameters which are used by the methods, rather than the god object.
This, however, might lead to some methods needing huge amounts of parameters because they call other methods, which call other methods in turn, etcetera. That was probably the inspiration for putting everything in a god object. I'll give a simplified example of such a method with too many parameters; you'll have to imagine that "too many" == 3 here :-)
public void PrintFilteredReport(
Data data, FilterCriteria criteria, ReportFormat format)
{
var filteredData = Filter(data, criteria);
PrintReport(filteredData, format);
}
So the question is, how can we reduce the amount of parameters without resorting to a god object? The answer is to get rid of procedural programming and make good use of object oriented design. Objects can use each other without needing to know the parameters that were used to initialize their collaborators:
// dataFilter service object only needs to know the criteria
var dataFilter = new DataFilter(criteria);
// report printer service object only needs to know the format
var reportPrinter = new ReportPrinter(format);
// filteredReportPrinter service object is initialized with a
// dataFilter and a reportPrinter service, but it doesn't need
// to know which parameters those are using to do their job
var filteredReportPrinter = new FilteredReportPrinter(dataFilter, reportPrinter);
Now the FilteredReportPrinter.Print method can be implemented with only one parameter:
public void Print(data)
{
var filteredData = this.dataFilter.Filter(data);
this.reportPrinter.Print(filteredData);
}
Incidentally, this sort of separation of concerns and dependency injection is good for more than just eliminating parameters. If you access collaborator objects through interfaces, then that makes your class
very flexible: you can set up FilteredReportPrinter with any filter/printer implementation you can imagine
very testable: you can pass in mock collaborators with canned responses and verify that they were used correctly in a unit test
If all your methods are using the same Parameters class then maybe it should be a member variable of a class with the relevant methods in it, then you can pass Parameters into the constructor of this class, assign it to a member variable and all your methods can use it with having to pass it as a parameter.
A good way to start refactoring this god class is by splitting it up into smaller pieces. Find groups of properties that are related and break them out into their own class.
You can then revisit the methods that depend on Parameters and see if you can replace it with one of the smaller classes you created.
Hard to give a good solution without some code samples and real world situations.
It sounds like you are not applying object-oriented (OO) principles in your design. Since you mention the word "object" I presume you are working within some sort of OO paradigm. I recommend you convert your "call tree" into objects that model the problem you are solving. A "god object" is definitely something to avoid. I think you may be missing something fundamental... If you post some code examples I may be able to answer in more detail.
Query each client for their required parameters and inject them?
Example: each "object" that requires "parameters" is a "Client". Each "Client" exposes an interface through which a "Configuration Agent" queries the Client for its required parameters. The Configuration Agent then "injects" the parameters (and only those required by a Client).
For the parameters that dictate behavior, one can instantiate an object that exhibits the configured behavior. Then client classes simply use the instantiated object - neither the client nor the service have to know what the value of the parameter is. For instance for a parameter that tells where to read data from, have the FlatFileReader, XMLFileReader and DatabaseReader all inherit the same base class (or implement the same interface). Instantiate one of them base on the value of the parameter, then clients of the reader class just ask for data to the instantiated reader object without knowing if the data come from a file or from the DB.
To start you can break your big ParametresExecution class into several classes, one per package, which only hold the parameters for the package.
Another direction could be to pass the ParametresExecution object at construction time. You won't have to pass it around at every function call.
(I am assuming this is within a Java or .NET environment) Convert the class into a singleton. Add a static method called "getInstance()" or something similar to call to get the name-value bundle (and stop "tramping" it around -- see Ch. 10 of "Code Complete" book).
Now the hard part. Presumably, this is within a web app or some other non batch/single-thread environment. So, to get access to the right instance when the object is not really a true singleton, you have to hide selection logic inside of the static accessor.
In java, you can set up a "thread local" reference, and initialize it when each request or sub-task starts. Then, code the accessor in terms of that thread-local. I don't know if something analogous exists in .NET, but you can always fake it with a Dictionary (Hash, Map) which uses the current thread instance as the key.
It's a start... (there's always decomposition of the blob itself, but I built a framework that has a very similar semi-global value-store within it)
I am working on a little pinball-game project for a hobby and am looking for a pattern to encapsulate constant variables.
I have a model, within which there are values which will be constant over the life of that model e.g. maximum speed/maximum gravity etc. Throughout the GUI and other areas these values are required in order to correctly validate input. Currently they are included either as references to a public static final, or just plain hard-coded. I'd like to encapsulate these "constant variables" in an object which can be injected into the model, and retrieved by the view/controller.
To clarify, the value of the "constant variables" may not necessarily be defined at compile-time, they could come from reading in a file; user input etc. What is known at compile time is which ones are needed. A way which may be easier to explain it is that whatever this encapsulation is, the values it provides are immutable.
I'm looking for a way to achieve this which:
has compile time type-safety (i.e. not mapping a string to variable at runtime)
avoids anything static (including enums, which can't be extended)
I know I could define an interface which has the methods such as:
public int getMaximumSpeed();
public int getMaximumGravity();
... and inject an instance of that into the model, and make it accessible in some way. However, this results in a lot of boilerplate code, which is pretty tedious to write/test etc (I am doing this for funsies :-)).
I am looking for a better way to do this, preferably something which has the benefits of being part of a shared vocabulary, as with design patterns.
Is there a better way to do this?
P.S. I've thought some more about this, and the best trade-off I could find would be to have something like:
public class Variables {
enum Variable {
MaxSpeed(100),
MaxGravity(10)
Variable(Object variableValue) {
// assign value to field, provide getter etc.
}
}
public Object getVariable(Variable v) { // look up enum and get member }
} // end of MyVariables
I could then do something like:
Model m = new Model(new Variables());
Advantages: the lookup of a variable is protected by having to be a member of the enum in order to compile, variables can be added with little extra code
Disadvantages: enums cannot be extended, brittleness (a recompile is needed to add a variable), variable values would have to be cast from Object (to Integer in this example), which again isn't type safe, though generics may be an option for that... somehow
Are you looking for the Singleton or, a variant, the Monostate? If not, how does that pattern fail your needs?
Of course, here's the mandatory disclaimer that Anything Global Is Evil.
UPDATE: I did some looking, because I've been having similar debates/issues. I stumbled across a list of "alternatives" to classic global/scope solutions. Thought I'd share.
Thanks for all the time spent by you guys trying to decipher what is a pretty weird question.
I think, in terms of design patterns, the closest that comes to what I'm describing is the factory pattern, where I have a factory of pseudo-constants. Technically it's not creating an instance each call, but rather always providing the same instance (in the sense of a Guice provider). But I can create several factories, which each can provide different psuedo-constants, and inject each into a different model, so the model's UI can validate input a lot more flexibly.
If anyone's interested I've came to the conclusion that an interface providing a method for each psuedo-constant is the way to go:
public interface IVariableProvider {
public int maxGravity();
public int maxSpeed();
// and everything else...
}
public class VariableProvider {
private final int maxGravity, maxSpeed...;
public VariableProvider(int maxGravity, int maxSpeed) {
// assign final fields
}
}
Then I can do:
Model firstModel = new Model(new VariableProvider(2, 10));
Model secondModel = new Model(new VariableProvider(10, 100));
I think as long as the interface doesn't provide a prohibitively large number of variable getters, it wins over some parameterised lookup (which will either be vulnerable at run-time, or will prohibit extension/polymorphism).
P.S. I realise some have been questioning what my problem is with static final values. I made the statement (with tongue in cheek) to a colleague that anything static is an inherently not object-oriented. So in my hobby I used that as the basis for a thought exercise where I try to remove anything static from the project (next I'll be trying to remove all 'if' statements ;-D). If I was on a deadline and I was satisfied public static final values wouldn't hamstring testing, I would have used them pretty quickly.
If you're just using java/IOC, why not just dependency-inject the values?
e.g. Spring inject the values via a map, specify the object as a singleton -
<property name="values">
<map>
<entry> <key><value>a1</value></key><value>b1</value></entry>
<entry> <key><value>a2</value></key><value>b3</value></entry>
</map>
</property>
your class is a singleton that holds an immutable copy of the map set in spring -
private Map<String, String> m;
public String getValue(String s)
{
return m.containsKey(s)?m.get(s):null;
}
public void setValues(Map m)
{
this.m=Collections.unmodifiableMap(m):
}
From what I can tell, you probably don't need to implement a pattern here -- you just need access to a set of constants, and it seems to me that's handled pretty well through the use of a publicly accessible static interface to them. Unless I'm missing something. :)
If you simply want to "objectify" the constants though, for some reason, than the Singleton pattern would probably be called for, if any; I know you mentioned in a comment that you don't mind creating multiple instances of this wrapper object, but in response I'd ask, then why even introduce the sort of confusion that could arise from having multiple instances at all? What practical benefit are you looking for that'd be satisfied with having the data in object form?
Now, if the values aren't constants, then that's different -- in that case, you probably do want a Singleton or Monostate. But if they really are constants, just wrap a set of enums or static constants in a class and be done! Keep-it-simple is as good a "pattern" as any.