Question

http://docs.oracle.com/javase/tutorial/java/generics/genmethods.html
Quoting from there-

A more realistic use of generic methods might be something like the following, which defines a static method that stuffs references to a single item into multiple boxes:

public static <U> void fillBoxes(U u, List<Box<U>> boxes) { 
for (Box<U> box: boxes {
  box.add(u); }

}

Here, what does this List<Box<U>> do? How does this work?

Further,

To use this method, your code would look something like the following:

 Crayon red = ...;
 List<Box<Crayon>> crayonBoxes = ...;

The complete syntax for invoking this method is:

Box.<Crayon>fillBoxes(red, crayonBoxes);

I couldn’t understand all these.

Answer 1

List is a generic collection.
Box is another generic.

You can declare Box<Crayon> to construct a Box of Crayons

List<Box> is a collection of Boxes but Box is generic, need a Type for the box List<Box<Crayon>>.

Just follow the generic type U. FillBoxes method is static public static <U> void fillBoxes(U u, List<Box<U>> boxes). Static methods doesn’t need a instance, if there is not a instance you can’t declare the type of the box Box<Crayon>, you have to pass the type in the call of the method Box.<Crayon>fillBoxes, knowing that type, the compiler knows that the first parameter in FillBoxes (U u) is a Crayon type and the second is a List of Boxes of Crayons (List<Box<U>> boxes).

The method will fill a list of Crayon Boxes with red crayons 🙂

Ufff this was hard for a non Java programmer and non native english speaker…