JCH
Java Community | Help. Code. Learn.•13mo ago
i hate SQL so much its unreal

any help to understand the generics?

hey guys. can smb explain generics to me? i know how they work in theory and i can for example write my own generic method, but when it comes to reading already written generic methods, im lost and i cant understand a thing. i have this code: 
public interface Stream<T> extends BaseStream<T, Stream<T>> {
    Stream<T> filter(Predicate<? super T> predicate);
    <R> Stream<R> map(Function<? super T, ? extends R> mapper);
        //other methods
public interface Stream<T> extends BaseStream<T, Stream<T>> {
    Stream<T> filter(Predicate<? super T> predicate);
    <R> Stream<R> map(Function<? super T, ? extends R> mapper);
        //other methods
and i have the following questions: 1. why does map function has <R> but filter function doesnt? 2. why does map return R and not T? filter method returns a stream of type T (i.e. objects), but map cant? thanks in advance.
3 Replies
JavaBot
JavaBot•13mo ago
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Hey @bambyzas! Please use /close or the Close Post button above when your problem is solved. Please remember to follow the help guidelines. This post will be automatically closed after 300 minutes of inactivity.
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imp_o_rt
imp_o_rt•13mo ago
1. Filter has no R because it doesn't change the stream elements; only tests them against some criteria 2. map returns a Stream of R because you can use it to convert your original T type to a new, different, type R
JavaBot
JavaBot•13mo ago
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