java 8 - How to skip even lines of a Stream<String> obtained from the Files.lines -

in case odd lines have meaningful data , there no character uniquely identifies lines. intention equivalent following example:

stream<domainobject> res = files.lines(src)      .filter(line -> isoddline())      .map(line -> todomainobject(line)) 

is there “clean” way it, without sharing global state?

a clean way go 1 level deeper , implement spliterator. on level can control iteration on stream elements , iterate on 2 items whenever downstream requests 1 item:

public class oddlines<t> extends spliterators.abstractspliterator<t>     implements consumer<t> {      public static <t> stream<t> oddlines(stream<t> source) {         return streamsupport.stream(new oddlines(source.spliterator()), false);     }     private static long odd(long l) { return l==long.max_value? l: (l+1)/2; }     spliterator<t> originallines;      oddlines(spliterator<t> source) {         super(odd(source.estimatesize()), source.characteristics());         originallines=source;     }      @override     public boolean tryadvance(consumer<? super t> action) {         if(originallines==null || !originallines.tryadvance(action))             return false;         if(!originallines.tryadvance(this)) originallines=null;         return true;     }      @override     public void accept(t t) {} } 

then can use like

stream<domainobject> res = oddlines.oddlines(files.lines(src))     .map(line -> todomainobject(line)); 

this solution has no side effects , retains advantages of stream api lazy evaluation. however, should clear hasn’t useful semantics unordered stream processing (beware subtle aspects using foreachordered rather foreach when performing terminal action on elements) , while supporting parallel processing in principle, it’s unlikely efficient…