Allow empty pages in paged list

azure-client-runtime/src/main/java/com/microsoft/azure/PagedList.java

@@ -17,8 +17,6 @@
 import java.util.ListIterator;
 import java.util.NoSuchElementException;
 
-import javax.xml.bind.DataBindingException;
-
 /**
  * Defines a list response from a paging operation. The pages are
  * lazy initialized when an instance of this class is iterated.
@@ -28,10 +26,10 @@
 public abstract class PagedList<E> implements List<E> {
     /** The actual items in the list. */
     private List<E> items;
-    /** Stores the link to get the next page of items. */
-    private String nextPageLink;
     /** Stores the latest page fetched. */
     private Page<E> currentPage;
+    /** Cached page right after the current one. */
+    private Page<E> cachedPage;
 
     /**
      * Creates an instance of Pagedlist.
@@ -48,11 +46,26 @@ public PagedList() {
     public PagedList(Page<E> page) {
         this();
         List<E> retrievedItems = page.getItems();
-        if (retrievedItems != null && retrievedItems.size() != 0) {
+        if (retrievedItems != null) {
             items.addAll(retrievedItems);
         }
-        nextPageLink = page.getNextPageLink();
         currentPage = page;
+        cachePage(page.getNextPageLink());
+    }
+
+    private void cachePage(String nextPageLink) {
+        try {
+            while (nextPageLink != null) {
+                cachedPage = nextPage(nextPageLink);
+                nextPageLink = cachedPage.getNextPageLink();
+                if (hasNextPage()) {
+                    // a legit, non-empty page has been fetched, otherwise keep fetching
+                    break;
+                }
+            }
+        } catch (IOException ex) {
+            throw new RuntimeException(ex);
+        }
     }
 
     /**
@@ -71,22 +84,18 @@ public PagedList(Page<E> page) {
      * @return true if there are more pages to load. False otherwise.
      */
     public boolean hasNextPage() {
-        return this.nextPageLink != null;
+        return this.cachedPage != null && this.cachedPage.getItems() != null && !this.cachedPage.getItems().isEmpty();
     }
 
     /**
      * Loads a page from next page link.
      * The exceptions are wrapped into Java Runtime exceptions.
      */
     public void loadNextPage() {
-        try {
-            Page<E> nextPage = nextPage(this.nextPageLink);
-            this.nextPageLink = nextPage.getNextPageLink();
-            this.items.addAll(nextPage.getItems());
-            this.currentPage = nextPage;
-        } catch (IOException e) {
-            throw new DataBindingException(e.getMessage(), e);
-        }
+        this.currentPage = cachedPage;
+        cachedPage = null;
+        this.items.addAll(currentPage.getItems());
+        cachePage(currentPage.getNextPageLink());
     }
 
     /**
@@ -107,15 +116,6 @@ public Page<E> currentPage() {
         return currentPage;
     }
 
-    /**
-     * Gets the next page's link.
-     *
-     * @return the next page link.
-     */
-    public String nextPageLink() {
-        return nextPageLink;
-    }
-
     /**
      * The implementation of {@link ListIterator} for PagedList.
      */
@@ -141,17 +141,14 @@ public boolean hasNext() {
         public E next() {
             if (!itemsListItr.hasNext()) {
                 if (!hasNextPage()) {
-                    throw new NoSuchElementException();                        
+                    throw new NoSuchElementException();
                 } else {
                     int size = items.size();
                     loadNextPage();
                     itemsListItr = items.listIterator(size);
                 }
             }
-            if (itemsListItr.hasNext()) {
-                return itemsListItr.next();
-            }
-            return null;
+            return itemsListItr.next();
         }
 
         @Override

azure-client-runtime/src/test/java/com/microsoft/azure/PagedListTests.java

@@ -21,11 +21,11 @@ public class PagedListTests {
 
     @Before
     public void setupList() {
-        list = new PagedList<Integer>(new TestPage(0, 20)) {
+        list = new PagedList<Integer>(new TestPage(0, 21)) {
             @Override
             public Page<Integer> nextPage(String nextPageLink) throws CloudException, IOException {
                 int pageNum = Integer.parseInt(nextPageLink);
-                return new TestPage(pageNum, 20);
+                return new TestPage(pageNum, 21);
             }
         };
     }
@@ -119,9 +119,13 @@ public String getNextPageLink() {
 
         @Override
         public List<Integer> getItems() {
-            List<Integer> items = new ArrayList<>();
-            items.add(page);
-            return items;
+            if (page + 1 != max) {
+                List<Integer> items = new ArrayList<>();
+                items.add(page);
+                return items;
+            } else {
+                return new ArrayList<>();
+            }
         }
     }
 }