I am trying to implement Template method pattern, but I need a slight variation that I don't think is best practice.
I have the following structure of classes
abstract class AbsClass {
public void algorithm(){
step1();
step2();
}
private void step1() {
//implementation
}
protected abstract void step2();
}
class A extends AbsClass {
protected void step2() {
// With implementation
}
}
class B extends AbsClass {
protected void step2() {
// No implementation needed
}
}
In the real case I have like 4 classes, and one of them doesn't need to have implementation for the second step. I don't think to leave the method empty would be good practice. I was thinking to put a comment(saying there is no need for implementation) in it, but I don't this would be the right solution.
Is there another approach I am not seeing?
We should not Force a design pattern. Here if we prefer Composition over inheritance then its better.
The code present in the question we have a method defined in a class but actually method has no behavior. Forcing a method in a class where it should not belomg to is not a good idea.
Below is one such possible implementation where you would not force a method to a class if it does not belong to it. Below is based on Strategy patter, but still I would say follow design principles and let the pattern itself suit your problem and do not force pattern to fit your solution.
public class AlgorithmClass {
private Strategy strategy;
public void setStrategy(Strategy strategy){
this.strategy = strategy;
}
public void algorithm(){
step1();
step2();
}
private void step1() {
//implementation
}
private void step2(){
if(this.strategy != null){
this.strategy.execute();
}
}
}
public interface Strategy{
public void execute();
}
public class Strategy1 implements Strategy{
public void execute(){
//implement your one of the stategies for step 2
}
}
public class Strategy2 implements Strategy{
public void execute(){
//implement your another stategy for step 2
}
}
I agree with #Vinay Avasthi's answer but I want to reinforce it.
Hook Method
Hook methods are defined in the base class and are a default implementation. And these can be overridden - they don't have to.
From Template-Method-Pattern Wikipedia page:
Template method's abstract class may also define hook methods that may be overridden by subclasses. These have a no-op implementation in the abstract class, but provide a "hook" on which to "hang" implementations.
Improvement
What you should do is leave a comment in the method body like // empty method body so that someone reading your code (and maybe your self) knows that this method has not been forgotten
Java's Default Methods
There is a second way to implement the Template-Method-Pattern in Java. Since Java 8 it is possible to have default method implementations in an interface.
If your methods do not depend on state it could look like:
interface AbsClass {
default void algorithm(){
step1();
step2();
}
default void step1() {
// implementation or empty
}
default void step2() {
// empty body in default case
}
}
class B implements AbsClass { }
I think it is absolutely fine. If the default behavior of step2 is to do nothing, then you can have an empty method in base class and override in child classes.
Related
I do have a service which needs to handle two types of meal.
#Service
class MealService {
private final List<MealStrategy> strategies;
MealService(…) {
this.strategies = strategies;
}
void handle() {
var foo = …;
var bar = …;
strategies.forEach(s -> s.remove(foo, bar));
}
}
There are two strategies, ‘BurgerStrategy’ and ‘PastaStrategy’. Both implements Strategy interface with one method called remove which takes two parameters.
BurgerStrategy class retrieves meals of enum type burger from the database and iterate over them and perform some operations. Similar stuff does the PastaStrategy.
The question is, does it make sense to call it Strategy and implement it this way or not?
Also, how to handle duplications of the code in those two services, let’s say both share the same private methods. Does it make sense to create a Helper class or something?
does it make sense to call it Strategy and implement it this way or not
I think these classes ‘BurgerStrategy’ and ‘PastaStrategy’ have common behaviour. Strategy pattern is used when you want to inject one strategy and use it. However, you are iterating through all behaviors. You did not set behaviour by getting one strategy and stick with it. So, in my honour opinion, I think it is better to avoid Strategy word here.
So strategy pattern would look like this. I am sorry, I am not Java guy. Let me show via C#. But I've provided comments of how code could look in Java.
This is our abstraction of strategy:
public interface ISoundBehaviour
{
void Make();
}
and its concrete implementation:
public class DogSound : ISoundBehaviour // implements in Java
{
public void Make()
{
Console.WriteLine("Woof");
}
}
public class CatSound : ISoundBehaviour
{
public void Make()
{
Console.WriteLine("Meow");
}
}
And then we stick with one behaviour that can also be replaced:
public class Dog
{
ISoundBehaviour _soundBehaviour;
public Dog(ISoundBehaviour soundBehaviour)
{
_soundBehaviour = soundBehaviour;
}
public void Bark()
{
_soundBehaviour.Make();
}
public void SetAnotherSound(ISoundBehaviour anotherSoundBehaviour)
{
_soundBehaviour = anotherSoundBehaviour;
}
}
how to handle duplications of the code in those two services, let’s say both share the same private methods.
You can create one base, abstract class. So basic idea is to put common logic into some base common class. Then we should create abstract method in abstract class. Why? By doing this, subclasses will have particular logic for concrete case. Let me show an example.
An abstract class which has common behaviour:
public abstract class BaseMeal
{
// I am not Java guy, but if I am not mistaken, in Java,
// if you do not want method to be overriden, you shoud use `final` keyword
public void CommonBehaviourHere()
{
// put here code that can be shared among subclasses to avoid code duplication
}
public abstract void UnCommonBehaviourShouldBeImplementedBySubclass();
}
And its concrete implementations:
public class BurgerSubclass : BaseMeal // extends in Java
{
public override void UnCommonBehaviourShouldBeImplementedBySubclass()
{
throw new NotImplementedException();
}
}
public class PastaSubclass : BaseMeal // extends in Java
{
public override void UnCommonBehaviourShouldBeImplementedBySubclass()
{
throw new NotImplementedException();
}
}
public boolean sendRequest(final Object... params) {
if (!super.sendRequest(params)) {
return false;
}
...
// Some Log code or tracing code here
...
}
Why not implement a new method to call sendRequest rather than overwrite?
public boolean Send(final Object... params){
if (!super.sendRequest(params)) {
return false;
}
...
// Some Log code or tracing code here
...
}
Do you want your class with the override to be able to be used in the same way as members of the original class? i.e.:
...
class MyClass extends TheirClass {
#Override
void doIt() {
super.doIt();
// also do my stuff
}
}
...
// the doSomething function is part of the library where TheirClass lives.
// I can pass instances of MyClass to it, and doIt will be called, because MyClass IS-A TheirClass
theirFunction.doSomething(new MyClass(...));
...
But perhaps you just want to use the functionality of doIt, but don't need to use and code which expects a TheirClass.
In that case it is probably better to use composition rather than inheritance:
class MyClass {
private final TheirClass theirClass;
public MyClass(TheirClass theirClass) {
this.theirClass = theirClass;
}
public void doMyStuff() {
theirClass.doIt();
// and do some other things
}
}
This is better than inheritance with a new method name, because then you would have two methods on the class which do about the same thing (except the original doIt doesn't do your stuff), and it may not be clear which should be called.
Even inheritance where you override the method may have problems. We don't know what code in TheirClass calls doIt, so perhaps the code we've added will be called when we don't expect it to be.
Overall, composition should be preferred to inheritance whenever possible.
I was wondering if it's frowned upon that when designing an framework to be used by others, a class has some function as default behavior and expects its customers to override it if necessary. An example would be something like the following:
public class RecordProcessor<T extends Record> {
// ...
public void process() {
// process record logic
}
}
Consumers of this library creates their concrete classes to process their own records of type T.
Now I want to add a function called preProcess() to offer the ability for the consumers to preprocess their records. It would then look something like this:
public class RecordProcessor<T extends Record> {
// ...
public void process() {
preprocess();
// process record logic
}
public void preProcess() {
// By default no preprocessing
}
}
I know I can make preProcess an abstract function, but I dont want to due to a couple reasons:
Not all customers need to preprocess their records
We have a pipeline structure that autodeploys pushed code, so making RecordProcessor an abstract class would immediately break our customers' applications.
Is making preProcess do nothing in the parent class and let child classes override it considered bad practice? If not, what should the best way be to let customers know that they now have the power to preprocess the records? Through java docs?
One approach is to mark the public method as final (but this might also break existing apps) and allow protected hook methods to be overridden. For example:
public class RecordProcessor<T extends Record> {
// ...
public final void process() {
doPreProcess();
doProcess();
doPostProcess();
}
protected void doPreProcess() {
// By default no preprocessing
return;
}
protected void doProcess() {
// some default implementation
}
protected void doPostProcess() {
// By default no postprocessing
return;
}
}
Having some documentation should make it natural for other developers to recognize the optional extension methods.
I don't see anything wrong with having a hook method which does nothing. However, it should contain a return statement so static analysis tools won't complain.
UPDATE: in order to avoid breaking existing apps, if possible mark the existing method as deprecated and introduce a new method. For example:
public class RecordProcessor<T extends Record> {
// ...
public final void execute() {
doPreProcess();
doProcess();
doPostProcess();
}
#Deprecated - use execute() method instead.
public void process() {
doProcess();
}
protected void doPreProcess() {
// By default no preprocessing
return;
}
protected void doProcess() {
// some default implementation
}
protected void doPostProcess() {
// By default no postprocessing
return;
}
}
Prefer composition over inheritance. If you want your clients to add custom pre processing then do it by delegating to a separate objects.
public interface RecordPreProcessor<T extends Record>{
public void process(T record);
}
public class RecordProcessor<T extends Record> {
private RecordPreProcessor<T> recordPreProcessor = null;
public void setRecordPreProcessor(RecordPreProcessor<T> recordPreProcessor) {
this.recordPreProcessor = recordPreProcessor;
}
public void process() {
if (recordPreProcessor != null) recordPreProcessor.process(record);
// process record logic
}
}
No, overriding is not discouraged in Java.
The language allows overriding.
The language makes all methods overridable by default.
The Java class library includes examples of the same pattern.
Your approach is one reasonable way to allow subclasses to extend the behavior of their parent class. There are alternatives, such as passing a behavior as an object. However, there is no one true way.
One way you could improve your code is to mark preProcess() as protected. It's an implementation detail of the class. You don't want just anyone holding a RecordProcessor to decide they can call preProcess() by itself, right?
public class RecordProcessor<T extends Record> {
...
protected void preProcess() {
^^^^^^^^^
// By default no preprocessing
}
}
Another way to improve this is to consider whether you intend anyone to create an instance of the superclass RecordProcessor. If you don't, make the class abstract, to prevent that. The class name can express that, if you like, or your coding guidelines call for it.
public abstract class AbstractRecordProcessor<T extends Record> {
^^^^^^^^ ^^^^^^^^
...
protected void preProcess() {
// By default no preprocessing
}
}
One common way to document such methods is with the phrase "The default implementation does nothing. Subclasses may override this method ...". For example, below is the documentation for java.util.concurrent.FutureTask.done(). You can find more examples by searching for the first sentence of that phrase online.
public class FutureTask<V> implements RunnableFuture<V> {
...
/**
* Protected method invoked when this task transitions to state
* {#code isDone} (whether normally or via cancellation). The
* default implementation does nothing. Subclasses may override
* this method to invoke completion callbacks or perform
* bookkeeping. Note that you can query status inside the
* implementation of this method to determine whether this task
* has been cancelled.
*/
protected void done() { }
}
What I ended up doing- which I also thought was pretty good, inspired by #tsolakp, was simply creating a child class to RecordProcessor, called something like PreprocessRecordProcessor. This has no way of interfering existing code because nothing existing was touched. The class would something like this:
public class PreprocessRecordProcessor<T extends Record> extends RecordProcessor<T> {
// ...
public void process() {
preProcess();
super.process();
}
protected abstract void preProcess();
}
And if customers of this library would like to add their own logic they can simply extend this class and they'd be forced to provide pre-processing logic (as supposed to having the option to provide, which may result in unexpected results if they forgot to.)
Given the following Class and Service layer signatures:
public class PersonActionRequest {
PersonVO person
// ... other fields
}
public class MyServiceLayerClass {
public void requestAction(PersonActionRequest request)
{
PersonVO abstractPerson = request.getPerson();
// call appropriate executeAction method based on subclass of PersonVO
}
private void executeAction(PersonVO person) {}
private void executeAction(EmployeeVO employee) {}
private void executeAction(ManagerVO manager) {}
private void executeAction(UnicornWranglerVO unicornWrangler) {}
}
As discussed here, java will select the best method based on type info at compile time. (Ie., it will always select executeAction(PersonVO person) ).
What's the most appropriate way to select the correct method?
The internet tells me that using instanceof gets me slapped. However, I don't see the appropraite way to select the method without explictly casting abstractPerson to one of the other concrete types.
EDIT: To Clarify - The VO passed in is a simple ValueObject exposed for web clients to instantiate and pass in. By convention it doesn't have methods on it, it's simply a data structure with fields.
For this reason, calling personVO.executeAction() is not an option.
Thanks
Marty
If executeAction was a method in a base class or interface that was common to PersonVO, EmployeeVO, ManagerVO and UnicornWranglerVO, you could just call abstractPerson.executeAction() instead of having multiple overridden methods.
Your principle obstacle to polymorphism here seems to be a 'dumb-struct' data object + 'manager class' service non-pattern. The "more polymorphic' approach would be for execute() to be a method that the various person implementations override.
Assuming that can't change, the way you do multiple dispatch in Java is with visitor-looking callbacks.
public interface PersonVisitor {
void executeAction(EmployeeVO employee);
void executeAction(ManagerVO manager);
void executeAction(UnicornWranglerVO unicornWrangler);
}
public abstract class PersonVO {
public abstract void accept(PersonVisitor visitor);
}
public class EmployeeVO extends PersonVO {
#Override
public void accept(PersonVisitor visitor) {
visitor.executeAction(this);
}
}
public class MyServiceLayerClass implements PersonVisitor {
public void requestAction(PersonActionRequest request)
{
PersonVO abstractPerson = request.getPerson();
abstractPerson.accept(this);
}
public void executeAction(EmployeeVO employee) {}
public void executeAction(ManagerVO manager) {}
public void executeAction(UnicornWranglerVO unicornWrangler) {}
}
You could change the way you are approaching the design and use a Visitor, passing the executor into the Person and have the person type determine which to call.
The Visitor pattern is often used to overcome Java lacking double-dispatch.
I would explicitly cast the abstractPerson. Not only does it ensure the JVM gets the right method, it makes it a hell of a lot easier to read and ensure you know what's going on.
In Java i have abstract class named Operation and three its subclasses called OperationActivation, OperationPayment and OperationSendEmail.
ADDED FROM COMMENT: Operation* objects are EJB Entity Beans so I can't have business logic inside them.
No I want to create processor class like this:
public class ProcessOperationService {
public void processOperation(Operation operation) {
out.println("process Operation");
process(operation);
}
public void process(OperationActivation operationActivation) {
out.println("process Activation");
}
public void process(OperationPayment operationPayment) {
out.println("process Payment");
}
public void process(OperationSendEmail operationSendEmail) {
out.println("process OperationSendEmail");
}
}
Processing each operation requires different logic so I want to have three different methods , one for each operation.
Of course this code doesn't compile. Am I missing something or it can't be done that way?
You are mixing up overloading and polymorphic method handling. When you overload methods based on the parameter type, that is static polymorphism. Those methods should be called from code that knows at compile-time what the type is. You could possibly do the following, but it wouldn't be clean object-oriented code:
public class ProcessOperationService {
public void processOperation(Operation operation) {
out.println("process Operation");
if (operation instanceof OperationActivation)
process((OperationActivation)operation);
else if (operation instanceof OperationPayment)
process((OperationPayment)operation);
...
}
public void process(OperationActivation operationActivation) {
out.println("process Activation");
}
...
}
It would be much better to let the automatic run-time polymorphism work, by doing as Brian Agnew suggested, and making process be a method of each Operation subtype itself.
Shouldn't your Operation* objects be doing the work themselves ? So you can write (say)
for (Operation op : ops) {
op.process();
}
You can encapsulate the logic for each particular operation in its own class, and that way everything related to OperationPayment remains in the OperationPayment class. You don't need a Processor class (and so you don't need to modify a Processor class everytime you add an Operation)
There are more complex patterns to enable objects to mediate wrt. what they need to execute, but I'm not sure you need something that complex at this stage.
Assumption: Operation* objects are subclasses of Operation
Unless the processOperation(Operation) method is performing some common functionality, you could just remove it and expose the process(Operation) methods.
The Command Pattern (JavaWorld Explanation) might be useful, but it's tricky to tell exactly what properties you want from your question.
The problem with the code is that any object that matches one of the process(Operation*) methods will also match the process(Operation) method. As there are 2 methods that can be used, the compiler is warning you of an ambiguous situation.
If you really want/need the code above, I would suggest implementing the process(Operation*) methods, and modify the process(Operation) method so it is called processCommon(Operation). Then, the first thing each process(Operation*) does is call processCommon.
Alternatively, you can code exactly as Avi said, using instanceof comparisons.
Neither is ideal, but it will accomplish what you want.
So you have an abstract class called 'Operation' and it has 3 classes extending it. Not sure if this is what you are after but I'd imagine it be designed something like this:
Operation.java
public abstract class Operation {
public abstract void process();
}
OperationActivation.java
public class OperationActivation extends Operation {
public void process() {
//Implement OperationActivation specific logic here
}
}
OperationPayment.java
public class OperationPayment extends Operation {
public void process() {
//Implement OperationPayment specific logic here
}
}
OperationSendEmail.java
public class OperationSendEmail extends Operation {
public void process() {
//Implement OperationSendEmail spepcific logic here
}
}
ProcessOperationService.java
public class ProcessOperationService {
public void processOperation(Operation operation) {
out.println("process Operation");
operation.process();
}
}
Won't the Visitor pattern be of use here ?
The class Operation can declare an "accept" method that takes a Visitor object and the subclasses can have provide the implementation :
public interface IOperationVisitor {
public void visit (OperationActivation visited);
public void visit (OperationPayment visited);
public void visit (OperationSendEmail visited);
}
abstract class Operation {
public void accept(IOperationVisitor visitor)();
}
class OperationActivation extends Operation {
public void accept(IOperationvisitor visitor) {
visitor.visit(this);
}
}
Similarly define "accept" method for classes OperationPayment and OperationSendEmail ..
Now your class can implement the visitor :
public class ProcessOperationService implements IOperationVisitor {
public void processOperation(Operation operation) {
operation.accept(this);
}
public void visit (OperationActivation visited) {
// Operation Activation specific implementation
}
public void visit (OperationPayment visited) {
// OperationPayment specific implementation
}
public void visit ((OperationSendEmail visited) {
// (Operation SendEmail specific implementation
}
}