Question

Browsing through Guava libraries I saw this weird signature on the readLines method of the Files class:

public static <T> T readLines(File file,
                          Charset charset,
                          LineProcessor<T> callback)

I know a little bit about generics in Java, but this baffled me.

What does the double T mean here? And why is the first one in angled brackets?

After some answers, I am still not clear as to why I should use a T inside the brackets. Why for example can’t it just be:

 public static <> T readLines()

or

 public static <K> T readLines()

Or does the Java syntax dictate that the same letter must be used?

Now this is even wierder:

static <T> void fromArrayToCollection(T[] a, Collection<T> c) {

how can a method have a generic-return type and be void?

Answer 1

I am still not clear as to why I should use a T inside the brackets. Why for example can’t it just be:

 public static <> T readLines()

or

 public static <K> T readLines()

Or does the java syntax dictate that the SAME letter must be used?

The <T> or <K> is the type parameter. If you write <K> T, then the T isn’t a type parameter – rather, you’re using the specific class T. This won’t work if you don’t have a class that’s literally named T in scope.

Now this is even wierder:

static <T> void fromArrayToCollection(T[] a, Collection<T> c) {

how can a method have a generic-return type and be void?

It doesn’t; the <T> is not a “generic return type”, it’s just the type parameter to the method. You’re saying that the method is generic, and T is the type parameter. The return type of the method is void.