I'm reading about wildcard and I'm a little confused.I don't really understand why the ifPresent method needs ? super T .
public void ifPresent(Consumer<? super T> consumer)
Why doesn't just T work?
I've read about PECS and it's clear how it works for class methods, for example,but why is it here in the interface, I'll be glad to help.
The most likely reason is to allow Consumers of a type broader than T (which is a type that is compatible with T) to be able to be used for the consumer argument.
Here's an example:
Optional<String> opt = Optional.of("abc");
Optional<Integer> intOptional = Optional.of(2);
Consumer<Object> printConsumer = System.out::println;
opt.ifPresent(printConsumer);
intOptional.ifPresent(printConsumer);
printConsumer is just a function that prints objects. So it works with both String and Integer types.
If ifPresent took only Consumer<T>, one would not be able to write code as the above, because one can't use Consumer<Object> where Consumer<String> or Consumer<Integer> is required. But in cases where a consumer instance already exists and is compatible with multiple T types, Consumer<? super T> makes it possible to reuse the consumer instance.