This question already has answers here:
Name of Class <T> of a generic Class
(2 answers)
Closed 9 years ago.
I want to find the class name of the argument sent to the generic class as follows.
public abstract class RootClass<T extends Iface> {
protected ApplicationContext applicationContext;
public T getIfaceBean() {
return applicationContext.getBean(T.class);
}
}
But it looks like I can't do T.class (due to Type Erasure?).
So, Is such an action possible with Java Generics ?
How can I achieve this type of functionality with Java Generics?
Because of Type Erasure, you can't say T.class because T doesn't exist at runtime.
The best you can do is to take a parameter of type Class<T> to get the Class object:
public T getIfaceBean(Class<T> clazz) {
Related
This question already has answers here:
Why does this Java method appear to have two return types?
(3 answers)
Closed 6 months ago.
I am new to java. I am trying to debug a code and not able to understand one line.
public interface CommandDispatcher {
<T extends BaseCommand> void registerHandler(Class<T> type, CommandHandlerMethod<T> handler);
void send(BaseCommand command);
}
I know generics but not able to understand below line.
<T extends BaseCommand> void registerHandler(Class<T> type, CommandHandlerMethod<T>)
what is <T extends BaseCommand> before void also I am not ablr to understand Class<T>
Can somebody explain me to understand the above line. Consider BaseCommand is an interface.
<T extends BaseCommand> means at the calling side Type T can be BaseCommand OR derived from BaseCommand class/interface.
Class<T> type means the first argument should be the type of Class T.
class Command extends BaseCommand{
}
then you can call like this
registerHandle( Command.class, ...);
So that inside implementation one can create instance of Type Command.
This question already has answers here:
Is List<Dog> a subclass of List<Animal>? Why are Java generics not implicitly polymorphic?
(19 answers)
Closed 5 years ago.
I have a method that should receives a Class<A>, something like:
protected void method(final Class<A> clazz) {
}
Then when I try to call like method(A.class) it do works, but if I try method(B.class), where B is child of A, I get the message "incompatible types".
The solution is change to Class<B>, but I have another childs that I like to pass as argument to this method(), like C.class.
One solution would be to use a bounded wildcard when defining the parameter:
protected void method(final Class<? extends A> clazz) {
// Code here...
}
With this, you are allowed to pass A's class directly or any class that extends from A.
Or as Pavlo suggested:
protected <T extends A> void method(final Class<T> clazz) {
// Code here...
}
Both will work, but it would help to know what you plan to do with clazz so we can select one over the other.
This question already has answers here:
Whats the use of saying <? extends SomeObject> instead of <SomeObject>
(5 answers)
Closed 6 years ago.
I have seen some code as follows
public interface IBean {
}
and its usage at some places as
public void persist(List<? extends IBean> beansList) {
}
However same can achieved with following code
public void persist(List<IBean> beansList) {
}
So what is the difference between both methods, both are excepting objects that must inherit IBean interface?
Here are the bean classes
public class Category implement IBean {
//related fields
}
public class Product implement IBean {
//related fields
}
You can pass a List<Category> to public void persist(List<? extends IBean> beansList), but you cannot pass a List<Category> to public void persist(List<IBean> beansList).
On the other hand, you can pass a List<IBean> to both methods.
The reason is that generics are invariant. This means for example that you can't use a List<Integer> where a List<Number> is expected.
But when turning to wildcards, you can circumvent that restriction. Therefore, when you really have a List<Product> you will not be able to pass that into a method that expects List<IBean> - you would have to somehow convert the list first. To be precise: you would do a "hard" cast; as there is no point in "converting" generic lists, as type erasure kicks in at runtime anyway!
By using the wildcard on the method definition, you can allow for passing Lists that use "real" sub classes of the extended type; without the need of ugly casts.
This question already has answers here:
Is List<Dog> a subclass of List<Animal>? Why are Java generics not implicitly polymorphic?
(19 answers)
Closed 7 years ago.
Why does Java not support automatic up-casting for template argument types?
For example, the following class will not compile unless the newly created Derived instance will be manually casted to a Base instance:
public class Example implements Iterable<Base> {
#Override
public Iterator<Base> iterator() {
return Arrays.asList(new Derived()).iterator();
}
private class Base {
}
private class Derived extends Base {
}
}
No need to cast.
The problem here is that Arrays.asList(new Derived()) naturally tries to create a List<Derived>,
and then calling .iterator() on a List<Derived> naturally gives an Iterator<Derived>,
which is not a sub-type of Iterator<Base>, so you get a compilation error.
You can specify that you want a List<Derived>, using Arrays.<Base>asList.
This works,
because you can certainly put a Derived instance into a List<Base>,
and then calling .iterator() on a List<Base> naturally gives an Iterator<Base>.
class Example implements Iterable<Base> {
#Override
public Iterator<Base> iterator() {
return Arrays.<Base>asList(new Derived()).iterator();
}
}
This question already has answers here:
Get generic type of class at runtime
(30 answers)
How do I get a class instance of generic type T?
(23 answers)
Closed 7 years ago.
How can I find the class of XOperation via Reflection API in Java?
public interface Operation<P extends Parameters> {
}
public class XParameters implements Parameters<XOperation> {
}
I'm trying to implement this method.
public <O extends Operation<P>, P extends Parameters> O getOperationByParametersClass(Class<P> parametersClass) {
// TODO
}
You can introduce a member in Parameters that will hold the Class of the type-parameter.
For example:
abstract class Parameters<T> {
protected Class<T> type;
}
Then, in the getOperationByParametersClass() method, you should pass an instance of Parameters (including subclasses):
public <O extends Operation, P extends Parameters<O>> Class<O>
getOperationByParametersClass(P parametersInstance) {
return parametersInstance.type;
}