CODEWITHSUNDEEP
scala
MCP
MCP

Reputation: 4546

Scala: def method[A](some params) vs def method(some params)

I'm having trouble finding what this is called and what the difference is exactly between defining a method like this:

def method[A](//...//) {}

or

def method(//...//) {}

Also, is def method[A] the same as def method[Any]?

Help would be greatly appreciated.

Upvotes: 1

Views: 79

Answers (1)

Rex Kerr
Rex Kerr

Reputation: 167921

def method[A] defines a generic type A. Used like this, with no refinement, it is a wildcard that can be satisfied by any type. (You can specify that only some subtypes are allowed with syntax like [A <: Foo], where Foo is a class or trait you've defined.)

Why would you want to do this? Most likely, you want a method that returns the same type that it takes as an argument (or some variation thereof--maybe it takes a list of that type and returns an array of that type):

def method[A](a: A): A = ...

Since you don't know anything about A, it is as generic as Any.

Now, there's nothing special about A. You could put anything there, even Any:

def method[Any](a: Any) ...    // Don't do this!

But this would be supremely confusing, because this isn't your standard Any type that is at the top of the inheritance hierarchy--it's a generic type just like A, but with a longer name.

If you don't need to use a generic type, omit the [A]. For example:

def printed[A](a: A): A = { println(a); a }   // Generic necessary
def printMe(a: Any) { println(a) }            // Not necessary, returns Unit

Upvotes: 7

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