Java：如何使用接口参数类型将变量传递给函数

Question

Assume class A implements interface B , is it possible to pass List<A> into a function with parameter List<B> :

List<A> var1;

how to call (with some fix): func(var1)

void func(List<B> var){
}

Any suggestion would be appreciated. Thanks!

Answer 1

No it is not possible (at least, not without some grotesque casting). One is not the supertype of the other.

Answer 2

Sure: func(Collections.<B>unmodifiableList(var1)) . As long as you can guarantee that the list isn't being modified, that sort of call is safe.

Answer 3

No. List<A> is not a subtype of List<B> .

The reason is that if you have a List<B> , you can add objects to it whose class is any implementation of B . You can't do that to a List<A> . So there are operations that are fine for a List<B> but not a List<A> ; which means that a List<A> can't possibly be a kind of List<B> .

What you actually want, I think, is to use List<? extends B> List<? extends B> as the type of your parameter, instead of List<B> .

Answer 4

What a strange idea, trying to by-pass type checks.

Do classes A and B have the same properties ? If yes, consider copying them by org.apache.commons.beanutils. BeanUtils.copyProperties (src,dest) and wrap the List argument in some conversion method.

If you insist on passing something generally incompatible, something like this should work because of type erasure :

List x = var1;
func(x);