在Java中重载方法时的奇怪行为

Question

I came across this weird (in my opinion) behavior today. Take this simple Test class:

public class Test {

public static void main(String[] args) {
    Test t = new Test();
    t.run();
}

private void run() {
    List<Object> list = new ArrayList<Object>();
    list.add(new Object());
    list.add(new Object());
    method(list);
}

public void method(Object o) {
    System.out.println("Object");
}

public void method(List<Object> o) {
    System.out.println("List of Objects");
}
}

It behaves the way you expect, printing "List of Objects". But if you change the following three lines:

List<String> list = new ArrayList<String>();
list.add("");
list.add("");

you will get "Object" instead.

I tried this a few other ways and got the same result. Is this a bug or is it a normal behavior? And if it is normal, can someone explain why?

Thanks.

Answer 1

It's a normal behaviour. List<String> is not a List<Object> , so method(Object) is the only applicable method.

If List<String> were a List<Object> , you would be able to violate type safety, for example, by adding Integer to the List<String> (and it can't be caught at runtime due to type erasure):

public void method(List<Object> o) { 
    o.add(new Integer(10));
}

Also note that arrays have a different behaviour - String[] is an Object[] , because array knows its element type and throws a runtime exception if you try to put a wrong object into it.

Answer 2

This is normal behaviour, because when you define a signature with generics, you specify a single class. (Unless you use wildcards , which you haven't, the link explains)...

So List<String> is not a List<Object> . It is, however a List<? extends Object> List<? extends Object> - give it a try and see.

Answer 3

Well this is expected - the list is not of type List<Object> . To get the results you are expecting use:

public void method(List<?> o)

as a wild card, this will match your list.

Answer 4

As expected because List<String> is not a List<Object> as stated in the other answer. To obtain the desired behavior, you need to use a wildcard:

public void method(List<? extends Object> o) { //or an unbounded wildcard List<?>
    System.out.println("List of Objects");
}

See also

• Java language guide/Generics/More Fun with wildcards

Answer 5

You need to understand java generics. List<String> is not List<Object> . Any simple way to explain why I cannot do List<Animal> animals = new ArrayList<Dog>()?