java - Reusing part of Stream mapping and filtering to compose two different results -

i'd know if there way of reusing common stream operation varies in end different outputs. example bellow i'm trying compact one-step operation:

public static departmentinfo extractdepartmentinfo(baselinepolicy resource) throws resourceprocessorerror {     function<exception, exception> rpe = e -> new resourceprocessorerror(e.getmessage());     list<string> parents =          objects.requirenonnull(             exceptions.trying(                 () -> arrays.aslist(exceptions.dangerous(resource::getparentids).expecting(cmexception.class).throwing(rpe))                             .stream()                             .map(cid -> exceptions.dangerous(cid, resource.getcmserver()::getpolicy).expecting(cmexception.class).throwing(rpe))                             .filter(policy -> pagepolicy.class.isassignablefrom(policy.getclass()))                             .map(pagepolicy.class::cast)                             .filter(page -> exceptions.dangerous(page,                                                                  p -> boolean.valueof(p.getcomponentnotnull(componentconstants.polopoly_client,                                                                                                              componentconstants.is_home_department,                                                                                                             boolean.false.tostring())).booleanvalue())                                                       .expecting(cmexception.class).throwing(rpe))                             .map(page -> exceptions.dangerous(page, p -> p.getexternalid().getexternalid()).expecting(cmexception.class).throwing(rpe)), resourceprocessorerror.class)                             .collect(collectors.tolist()));     string externalid = parents.get(parents.size()-1).tostring();     list<string> list =          objects.requirenonnull(             exceptions.trying(                 () -> arrays.aslist(exceptions.dangerous(resource::getparentids).expecting(cmexception.class).throwing(rpe))                             .stream()                             .map(cid -> exceptions.dangerous(cid, resource.getcmserver()::getpolicy).expecting(cmexception.class).throwing(rpe))                             .filter(policy -> pagepolicy.class.isassignablefrom(policy.getclass()))                             .map(pagepolicy.class::cast)                             .map(page ->                                  exceptions.dangerous(page,                                          p -> p.getchildpolicy(path_segment) != null &&                                               stringutils.haslength(singlevalued.class.cast(p.getchildpolicy(path_segment)).getvalue())?                                               singlevalued.class.cast(p.getchildpolicy(path_segment)).getvalue(): p.getname()).expecting(cmexception.class).throwing(rpe))                             .filter(val -> val != null && !val.isempty()), resourceprocessorerror.class)                             .collect(collectors.tolist()));     if(list.size() > 3) {         list = list.sublist(list.size() - 3, list.size()-1);     }     switch(list.size()) {         case 0: {             throw new resourceprocessorerror("br.com.oesp.xmlrender.error.noproduct");         }         case 1: {             return departmentinfo.withproduct(list.get(0), externalid);         }         case 2: {             return departmentinfo.withproduct(list.get(0), externalid).withdepartment(list.get(1));         }         default: {             return departmentinfo.withproduct(list.get(0), externalid).withdepartment(list.get(1)).withsubdepartment(list.get(2));         }     } } 

notice first step repeated both:

list<string> parents =      objects.requirenonnull(         exceptions.trying(             () -> arrays.aslist(exceptions.dangerous(resource::getparentids).expecting(cmexception.class).throwing(rpe))                         .stream()                         .map(cid -> exceptions.dangerous(cid, resource.getcmserver()::getpolicy).expecting(cmexception.class).throwing(rpe))                         .filter(policy -> pagepolicy.class.isassignablefrom(policy.getclass()))                         .map(pagepolicy.class::cast) 

it's not problem reading specially because i'm redoing heavy operation twice, meanwhile in more imperative way i'd once.

there 2 things you're trying do:

• avoid redundant work of creating input array
• avoid redundant code of map/filter/map

the first easy:

    list<id> list = arrays.aslist(exceptions.dangerous(resource::getparentids)                     .expecting(cmexception.class)                     .throwing(rpe)); 

now can pull streams source twice without rematerializing it.

the next bit function list stream:

function<list<id>, stream<something>> asstream =      list -> list.stream().map(...).filter(...).map(...); 

now, start stream this:

asstream.apply(list).morestuff().morestuff()