问题
I have a group of classes that all implement a validation interface which has the method isValid(). I want to put a group of objects--all of different classes--into an ArrayList, loop through them and call isValid() on each.
Here's my code
Email email = new email();
Address address = new Address();
ArrayList<? extends Validation> myValidationObjects = new ArrayList();
But when I try to do:
myValidationObjects.add(email);
I get:
The method add(capture#2-of ? extends Validation) in the type ArrayList is not applicable for the arguments (Email)
Both Email and Address implement Validation.
According to this document, I should be able to use extends for both interfaces and subclasses.
回答1:
You can use:
List<Validation> myValidationObjects = new ArrayList<>(); // Java 7
List<Validation> myValidationObjects = new ArrayList<Validation>(); // pre Java 7
Now you can add any instance of a class that implements Validation to that list.
回答2:
List<? extends Validation> myValidationObjects
Incorrect reading
"myValidationObjects is list of objects that extend Validation."
Correct reading
"myValidationObjects can be a list of any type that extends Validation. For example, it could be a List<RangeValidation> or a List<RegexValidation>."
Since there is no object you can legitimately add to both a List<RangeValidation> and a List<RegexValidation>, Java prevents you to call add on a variable of such type.
Your case is in fact the simpler one: you need the definite type List<Validation>.
回答3:
The declaration ArrayList<? extends Validation> means a list of an unknown class that extends Validation. Email is not compatible with this unknown class.
You can use ArrayList<Validation> for your list.
回答4:
If a generic class's T is <? extends Foo>, then the only thing you can pass to a method that takes T is null -- not any subclass that extends Foo.
The reason is that List<? extends Validation> doesn't mean "a list of things that extend Validation". You can get that with just List<Validation>. Instead, it means "a list of some type, such that that type extends Validation."
It's a subtle distinction, but basically the idea is that List<? extends T> is a subtype of List<T>, and you therefore don't want to be able to insert anything into it. Think of this case:
List<FooValidation> foos = new ArrayList<>();
List<? extends Validation> validations = foos; // this is allowed
validations.add(new BarValidation()); // not allowed! this is your question
FooValidation foo = foos.get(0);
If the third line were allowed, then the last line would throw a ClassCastException.
来源：https://stackoverflow.com/questions/14164501/java-arraylist-of-extends-interface