Is there an equivalent to C#'s new modifier in Java?
I'd like to use it for unit tests - every init() method should be marked final and annotated with #Before. Then, jUnit executes all of these init() methods.
I don't want to bother coming up with new names for each of these init() methods, and I definitely wants to mark them as final to make sure they don't override eachother (an alternative pattern is to override and call super.init() from every init() method).
A common pattern is to make your own 'before' methods final and create protected (abstract) methods for the subclasses.
In the superclass
#Before
public final void before() {
onBefore();
}
protected void onBefore() {
}
In the subclass
protected void onBefore() {
// prepare the test fixture
}
This gives you the best of both worlds:
a well-known method to override in sub-classes;
overriding is optional;
the superclass method is never overriden;
the client method is invoked when the super-class decides, i.e. either before or after the super-class decides.
It does have a single downside - it ties you to a single super-class. Still, that may not be an issue to your environment.
Unfortunately not. Heck, before #Override there wasn't even any way of protecting against typos when overriding.
You can't create a method with the same signature as a superclass method without it overriding that method. Admittedly I try not to do this even in C#...
Have you considered using initFoo and initBar for classes Foo and Bar respectively (etc)? It's a simple enough pattern to follow, and would avoid the name collisions. A bit ugly, admittedly.
Java does not have an equivalent to the C# new operator which is
Used to hide an inherited member from a base class member.
For what you'd like to do, why not create a base class that your other tests can extend, and create an abstract method named init() (marked with #Before) in the base class? This forces all subclasses to supply an init() method.
Related
Is there a design pattern (probably but not necessarily OOP) whereby you can implicitly (not explicitly) call one function/method A form all the other methods in an object each time those other methods are called?
for example:
//pseudocode
class Foo {
int count = 0;
void synchronized countUp(){
count++;
}
void doSomethingFirst(){
//call countUp() implicitly here
}
void doSomethingSecond(){
//call countUp() implicitly here
}
}
Would there be a way to use annotations in Java for instance? I could mark methods that need to call super methods or something. So it would be like an implicit call to super. I am not saying this is a great idea I am just wondering if it can be done somehow with a design pattern.
You could probably copy the system (I don't think it is a design pattern) used by the Spring MVC framework. It relies on making the inherited method final, and providing an overloadable method for descendants.
Using part of your example code (in Java):
final void doSomethingFirst(){
countUp();
doSomethingFirstInternal();
}
protected void doSomethingFirstInternal() {
// Empty implementation that descendants can override when necessary.
}
I'm not saying this is a great idea - you need to be 100% sure that your code should execute first. But is an option and careless programmers cannot introduce bugs because they forgot to call super().
You could extend the class and overwrite the countUp() method, after. Make all methods protected and call the superclass methods that you need from the child class.
class Boo extends Foo {
#Override
void countUp() {
setUp();
super.countUp();
tearDown();
}
}
To make this more generic, you could use the observer pattern:
make Foo class observable
get all its methods at init via reflection
Foo.class.getMethods()
wrap all methods in a class and register as observers
I am running into the trouble of extending/altering private methods in subclasses. For instance, I am creating a mock object that inherits from a super class in order to be used in testing.
Here is an example of the code from the parent class that I would like to alter:
private void build(int skill) {
// switch screen
state = Constants.STATE_GENERATING;
percentdone = 0;
notifyViewerRedraw() ;
// select generation method
switch(method){
case 1 : mazebuilder = new MazeBuilderPrim(); // generate with Prim's algorithm
break ;
case 0: // generate with Falstad's original algorithm (0 and default), note the missing break statement
default : mazebuilder = new MazeBuilder();
break ;
}
}
I know that private methods cannot be overwritten in subclasses. So therefore I should create a new method of the same signature. Would I also have to recreate all the private variables in this class too? But if I do that, I am unsure if that would change the behavior to be different from the parent class since the I know that space is actually reserved for private variables from the parent class in the subclass. Therefore, I would have duplicate private variables. I don't know what the best way is to approach this.
If you find the need to override a method in a subclass, perhaps the logic that method is responsible for is abstract enough to warrant a public or protected method.
Consider the design of the Strategy Pattern. Basically, there shouldn't be a need to override a private method, because those methods should be reserved for things outside your interface and only specific to that particular concrete class.
Something as integral and specific as build() to me sounds like it belongs as a protected method which your highest superclass may use at some point during construction but which shouldn't be called externally. Of course, if it's safe to call build() as many times as necessary (idempotent), like a render() method might be in a game character class, then it should be safe to make it public and document what your expectations are of its implementation.
Try using mocking API, for example Jmockit .
Using it, will save you a lot of trouble doing hand mocks, like in your case extending class with private methods! Good luck.
Im not sure how your program works overall but it might be worth you looking into abstract classes and methods if you want all of the base classes to share the same methods(?)
You can call super() in the subclasses method which will call the superclasses method and then make the extra changes you need in the subclasses method.
Edit: Read the original question wrong. If a method has functionality in the superclass and you want to extend or alter it, you shouldn't set it to private. Protected is probably the best bet in your case, as it allows subclasses to extend it using super() or just override it.
I have what amounts to a lightweight test framework written as a JUnit Abstract test. What I would like to do is have the implementing subclasses each define their custom test class setup. My plan was to have the abstract superclass define an #BeforeClass method that calls into an abstract setup method that each subclass would be forced to define, but this fails as the #BeforeClass methods must be static and static methods cannot be made abstract nor can they call instance methods.
I could just assume that subclasses will do the setup by including what's required in the documentation or by throwing an IllegalStateException, but I'd really like to be able to enforce this at an interface level for a number of reasons. Can anyone think of a work around for this?
By the way, I had the same issue with making these tests parameterized (subclasses define the parameters, but #Parameters annotated methods must be static). I got around this by running with the 3rd party JUnitParams runner which allows method level parameters. Check it out here: https://github.com/Pragmatists/JUnitParams
One option is to have subclasses implement a, say, static doSetupOnce() method, and find and invoke that method reflectively from the base class #BeforeClass method. Since this needs to be a static method, its existence can only be enforced at runtime.
Another approach would be to have an abstract doSetupOnce instance method in the base class which gets invoked the first time the parent's #Before method gets invoked. This enforces the issue at compile time, but then implementors will have to be careful not to access instance fields from this method (since this is probably not what they want).
In general (and without knowing the details of your situation), I'm not very fond of either of these approaches, and would rather leave it up to implementors to declare a #BeforeClass method if needed. Locking them up in a rigid base class scheme can cause more problems than it solves. Also consider the use of JUnit rules, which often are a better choice than base classes (e.g. because they are composable). Of course you can also combine the two approaches, relying mainly on JUnit rules and additionally offering some base classes with predefined rules for convenience.
For your main question, why not make your parent class abstract and use the #Before annotation instead of #BeforeClass ? For example:
public abstract class TestParent {
#Before
public void setup() {
doSetup();
}
protected abstract void doSetup();
// other stuff...
}
public class RealTest extends TestParent {
protected void doSetup() {
// custom setup
}
// custom tests...
}
This will force the subclasses to redefine the doSetup() method without using static methods.
This may be out of scope or overkill, but I think it's worth mentioning since they're not that different. So I'm taking a leap of faith and suggest that you could try TestNG because methods annotated with #BeforeClass do not have to be static, nor do those annotated with #Parameters.
You can read about the differences between the 2 frameworks here and it looks like they also have support for migrating JUnit tests to TestNG
I don't think it is possible to do this in a clean OO way. Not only is #BeforeClass a static method, but JUnit will call the parent's #BeforeClass before the child's #BeforeClass.
Anyway you try to do this must necessarily expose the parent class's internal static state so a child class can set the parent's fields, breaking encapsulation.
I think the best way is to to use #Before, but also have a static flag that sets if the method has been called before, that way at least you can short circuit and only do the initialization for the first call...
I have an abstract class A
I have about 10 classes that extend A
Class A has one or two static methods and it makes sense that these are static, because they belong to the 10 classes, NOT instances of them. One static method e.g. is called getAllFromX, which gets all all instances of the class from X, whatever that may be, it may be a server, well it actually is, but it doesn't matter. So you see it makes sense these methods are static and are not bound to an instance.
At the same time class A has a NON-static abstract method, each subclass overrides this method (just returns a string). I cannot make it static because static methods cannot be overridden (...).
To summarize: abstract class A has a static method and a abstract non-static method, that is overriden by the subclasses. I cannot make the second method static because it must be overriden. On the otherhand I could make the first method non-static, but it would be very ugly and bad programming style, so I'll leave it that way.
The catch? The static method in class A must get the value the non-static method returns (for the subclass the static method is inherited from, of course).
Is the "easiest" way to use reflection to get this done? I mean...really?
Like e.g., I get the class the static method is in:
Class<?> cl=new Object(){}.getClass().getEnclosingClass(); (a hack I found here, thank god...)
I then use getConstructor to construct an object of this subclass.
And then I use this object to call the non-static method.
Really?? Can it not be done easier? I mean that is if I want to design my program conceptually correct...
Coming from C# I don't like that (and the type erasure thing). It is just ugly. Doable but ugly. And a big stumbling block, at least for beginners. EDIT: after reading it again, I'd add: /rant end. Sorry, but I actually care.
I think what you in fact need is the following:
public class A {
public static Set<A> getAllFromX() {
...
}
}
public class B extends A {
public static Set<B> getAllFromX() {
...
}
}
public class C extends A {
public static Set<C> getAllFromX() {
...
}
}
(Just as the valueOf() and values() methods in enums, which is redefined in every Enum subclass, because static methods can't be inherited)
In this case, each class has its own static method doing whatever it wants. But your question doesn't make much sense because it says:
The static method in class A must get the value the non-static method returns (for the subclass the static method is inherited from, of course).
Indeed, the static method is not inherited by the subclass. Static methods are never inherited. If you define a static method foo() in A, and call
B.foo();
the compiler doesn't refuse to compile it, but it translates it to
A.foo();
So, there's no way to do in foo() something that depends on the class on which foo() is called, since it's always A.
You can always use reflection to invoke a method using class name e.g.
Object objectX = ClassX.class.newInstance();
//get your method passing argument types as second param
Method method = ClassX.class.getDeclaredMethod("methodX", null);
//invoke your method passing arguments as second param
method.invoke(objectX, null);
Since you mentioned your static method doesn't use any instance but you are using reflection to get the instance hence I am really not sure, how does it fit in your requirement though.
I think making it as an implemented method (non-static) in your abstract class is a better choice. That way you implement it once but its available in in all your 10 extending classes.
I think your problem is one of larger design. A different object should be responsible for retrieving instances of A or its subclasses. As you can see, relying on a static method to be replaced by subclasses does not work well. Without knowing more about the problem domain, it's hard to give a good answer, but I would consider something similar to the Abstract Factory pattern.
Broadly speaking: Define an abstract class, AFactory, with a method Collection getInstances(). Extend AFactory for each of the concrete subclasses of A you need to return and implement that logic in the overridden getInstances() method as appropriate. You may also provide a static method on the abstract AFactory, getFactory(Class), to get the appropriate factory subtype at runtime.
I have a class called Base which has a method called execute(). There are about 100 classes which derive from this base class and provide their own implementation of execute(). Now, I have some common logic which I want to put in Base.SomeMethod(). This method needs to be called at the end of execute(). My question whether it is possible to call this without changing each and every derived class's execute() method?
public class Base {
public final void execute() {
doExecute();
someMethod();
}
protected abstract void doExecute();
public void someMethod() {
}
}
This solution prevents the super code smell.
Yes, but you have to change the callers then. Callers will have to call a doExecute() (find a better name for it though) method, which you define in your base class as final, and which calls execute(), then the common code.
Another option is aspect-oriented programming, but I wouldn't recommend it for this purpose, that is, to "hack" code.
The question is: why is changing the name of a method in a 100 or so classes such a problem? It's a click of the mouse with an IDE.
Not that I'm aware of. Next time you should consider that you might want to add some common action for all extended classes, and call for super.execute()!
Only by using something that instruments your code; this isn't possible with pure Java.
Let me state your problem as i understand : Animal class has Breath() method which has implementation and due to inheritance all the subclasses has this member and unless there is very different way of breathing nobody will override.
Now at the end of Breath method you want to call CloseEyes() method of animal class and may be that is true that some or all of the subclasses overrides CloseEyes() method.
So your problem : Everytime any animal breath you want to them to CloseEyes but from Animal class and not from the derived classes.
If there are already CloseEyes() methods in many derived classes then you are actually doing something wrong in calling base class's CloseEyes().
If you still want only base class's method to be called then why do you need same method name- you just say AnimalEyeClose() , make it private and have it in Animal class.