Clash Royale CLAN TAG#URR8PPP

Java Generics - way around instantiating wildcard type

I am trying to build a lib that supports deployment of custom implementations of a specific object.

When the user is setting things up with Class A, I want them to be able to do something like this:

/* this code will not be part of the lib */
ClassA a = new ClassA()
a.setClassCImpl(ClassD.class)


Now when Class B gets instantiated it needs to know to use the custom implementation Class D

/* this code will be part of the lib */
Class<? extends ClassC> classCImpl;

ClassB()
classCImpl = new ??? // this needs to be an instance of Class D



There will be many instances of Class B. I'd rather not need Class B to hold onto an instance of Class A, but I'm not sure this will be possible without using static stuff in Class A.

Class

a.setClassCImpl()

newInstance()

Class

getDeclaredConstructor().newInstance()

Class<? extends ClassC> classCImpl;

ClassC classCImpl;

Class

ClassB()

Class

ClassA

Class B

? extends ClassC

ClassC

2 Answers
2

This sort of pattern is a good use case for using annotations. Some frameworks implement this kind of behavior but you can quite easily achieve it.

If you can allow users to extend B annotations are really convenient.

For example declare your annotation like that :

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)

public @interface MeaningfulName
public Class<? extends ClassC> value(); // you can define a default



Now when your users extend B they can add this annotation

@MeaningfulName(ClassD.class)
public class CustomB extends ClassB
// whatever your class does



Then you can extract this at runtime using getAnnotation() and setup some caching maybe to avoid frequent lookups. After obtaining the Class attribute you can get the appropriate instance using your desired strategy (singleton, factory, straight up reflection etc.).

If you just want to stick to a single B class, you can specify various combinations of locations users could use it using @Target and other ElementTypes, although with different ways to extract the annotation.

@Target

ClassB

ClassC

ClassA:

ClassA
static Class<? extends ClassC> impl = ClassD.class



ClassB:

ClassC classCImpl;

ClassB()
classCImpl = ClassA.impl.getDeclaredConstructor().newInstance();



I think this will work.

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