refactor create page form list (noSQL)

src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/PaginationService.java

@@ -9,6 +9,8 @@
 package ch.ethz.seb.sebserver.webservice.servicelayer;
 
 import java.util.Collection;
+import java.util.List;
+import java.util.function.Function;
 import java.util.function.Supplier;
 
 import org.mybatis.dynamic.sql.SqlTable;
@@ -75,4 +77,36 @@ public interface PaginationService {
             final String tableName,
             final Supplier<Result<Collection<T>>> delegate);
 
+    /** Use this to build a current Page from a given list of objects.
+     *
+     * @param <T> the Type if list entities
+     * @param pageNumber the number of the current page
+     * @param pageSize the size of a page
+     * @param sort the page sort flag
+     * @param all list of all entities, unsorted
+     * @param sorter a sorter function that sorts the list for specific type of entries
+     * @return current page of objects from the sorted list of entities */
+    default <T> Page<T> buildPageFromList(
+            final Integer pageNumber,
+            final Integer pageSize,
+            final String sort,
+            final Collection<T> all,
+            final Function<Collection<T>, List<T>> sorter) {
+
+        final List<T> sorted = sorter.apply(all);
+        final int _pageNumber = getPageNumber(pageNumber);
+        final int _pageSize = getPageSize(pageSize);
+        final int start = (_pageNumber - 1) * _pageSize;
+        int end = start + _pageSize;
+        if (sorted.size() < end) {
+            end = sorted.size();
+        }
+
+        return new Page<>(
+                sorted.size() / _pageSize,
+                _pageNumber,
+                sort,
+                sorted.subList(start, end));
+    }
+
 }

src/main/java/ch/ethz/seb/sebserver/webservice/weblayer/api/ExamAdministrationController.java

@@ -9,12 +9,12 @@
 package ch.ethz.seb.sebserver.webservice.weblayer.api;
 
 import java.io.IOException;
-import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.Comparator;
 import java.util.List;
 import java.util.Set;
+import java.util.function.Function;
 import java.util.stream.Collectors;
 
 import javax.servlet.ServletOutputStream;
@@ -158,16 +158,19 @@ public class ExamAdministrationController extends EntityController<Exam, Exam> {
                     EntityType.EXAM,
                     institutionId);
 
-            final List<Exam> exams = new ArrayList<>(
+            final Collection<Exam> exams = this.examDAO
-                    this.examDAO
+                    .allMatching(new FilterMap(
-                            .allMatching(new FilterMap(allRequestParams, request.getQueryString()), this::hasReadAccess)
+                            allRequestParams,
-                            .getOrThrow());
+                            request.getQueryString()),
+                            this::hasReadAccess)
+                    .getOrThrow();
 
-            return buildSortedExamPage(
+            return this.paginationService.buildPageFromList(
-                    this.paginationService.getPageNumber(pageNumber),
+                    pageNumber,
-                    this.paginationService.getPageSize(pageSize),
+                    pageSize,
                     sort,
-                    exams);
+                    exams,
+                    pageSort(sort));
         }
     }
 
@@ -543,39 +546,30 @@ public class ExamAdministrationController extends EntityController<Exam, Exam> {
         }
     }
 
-    public static Page<Exam> buildSortedExamPage(
+    static Function<Collection<Exam>, List<Exam>> pageSort(final String sort) {
-            final Integer pageNumber,
-            final Integer pageSize,
-            final String sort,
-            final List<Exam> exams) {
 
-        if (!StringUtils.isBlank(sort)) {
         final String sortBy = PageSortOrder.decode(sort);
+        return exams -> {
+            final List<Exam> list = exams.stream().collect(Collectors.toList());
+            if (StringUtils.isBlank(sort)) {
+                return list;
+            }
+
             if (sortBy.equals(Exam.FILTER_ATTR_NAME)) {
-                exams.sort(Comparator.comparing(exam -> exam.name));
+                list.sort(Comparator.comparing(exam -> exam.name));
             }
             if (sortBy.equals(Exam.FILTER_ATTR_TYPE)) {
-                exams.sort(Comparator.comparing(exam -> exam.type));
+                list.sort(Comparator.comparing(exam -> exam.type));
             }
             if (sortBy.equals(QuizData.FILTER_ATTR_START_TIME)) {
-                exams.sort(Comparator.comparing(exam -> exam.startTime));
+                list.sort(Comparator.comparing(exam -> exam.startTime));
-            }
             }
 
             if (PageSortOrder.DESCENDING == PageSortOrder.getSortOrder(sort)) {
-            Collections.reverse(exams);
+                Collections.reverse(list);
             }
-
+            return list;
-        final int start = (pageNumber - 1) * pageSize;
+        };
-        int end = start + pageSize;
-        if (exams.size() < end) {
-            end = exams.size();
-        }
-        return new Page<>(
-                exams.size() / pageSize,
-                pageNumber,
-                sort,
-                exams.subList(start, end));
     }
 
 }

src/main/java/ch/ethz/seb/sebserver/webservice/weblayer/api/ExamMonitoringController.java

@@ -8,10 +8,8 @@
 
 package ch.ethz.seb.sebserver.webservice.weblayer.api;
 
-import java.util.ArrayList;
 import java.util.Collection;
 import java.util.EnumSet;
-import java.util.List;
 import java.util.Objects;
 
 import javax.servlet.http.HttpServletRequest;
@@ -141,17 +139,18 @@ public class ExamMonitoringController {
             filterMap.putIfAbsent(API.PARAM_INSTITUTION_ID, String.valueOf(institutionId));
         }
 
-        final List<Exam> exams = new ArrayList<>(this.examSessionService
+        final Collection<Exam> exams = this.examSessionService
                 .getFilteredRunningExams(
                         filterMap,
                         exam -> this.hasRunningExamPrivilege(exam, institutionId))
-                .getOrThrow());
+                .getOrThrow();
 
-        return ExamAdministrationController.buildSortedExamPage(
+        return this.paginationService.buildPageFromList(
-                this.paginationService.getPageNumber(pageNumber),
+                pageNumber,
-                this.paginationService.getPageSize(pageSize),
+                pageSize,
                 sort,
-                exams);
+                exams,
+                ExamAdministrationController.pageSort(sort));
     }
 
     @RequestMapping(