extends keyword in return type in java method signature

Question

I was going through the code of commons-chain

I found a lot of method signatures resembling this one:

public <CMD extends Command<K, V, C>> CMD getCommand(String commandID)

How is this signature any different from:

public Command getCommand(String commandID)

The only logical reason i could understand was to do type checking. but i still could not figure out the reason to do it from a design point of view.

Are there some more reasons why one would use and extends in return type of a java method?

Answer 1

The difference is three fold:

1. The type returned is typed (not a raw type)
2. The type returned can be a specific subclass
3. The method has access to the types K, V and C

And being a typed method, java can infer the type

All this means that this will compile (without casts):

Command<String, Integer, String> x = getCommand(commandID);

or

SomeTypedSubClassOfCommand x = getCommand(commandID);

The compiler will infer (ie figure out) the type for the method based on the type of the variable the result is being assigned to.

Further, you can explicitly specify a subclass with this syntax:

Command<String, Integer, String> x = ContainingClass.<CommandSubClass<String, Integer, String>>getCommand(commandID);