Provide a convenience method to get items set via setItems().

Fixes #8357

server/src/main/java/com/vaadin/data/HasDataProvider.java

@@ -36,13 +36,6 @@ import com.vaadin.data.provider.DataProvider;
  */
 public interface HasDataProvider<T> extends HasItems<T> {
 
-    /**
-     * Returns the source of data items used by this listing.
-     *
-     * @return the data provider, not null
-     */
-    public DataProvider<T, ?> getDataProvider();
-
     /**
      * Sets the data provider for this listing. The data provider is queried for
      * displayed items as needed.

server/src/main/java/com/vaadin/data/HasFilterableDataProvider.java

@@ -37,13 +37,6 @@ import com.vaadin.server.SerializableFunction;
  */
 public interface HasFilterableDataProvider<T, F> extends HasItems<T> {
 
-    /**
-     * Returns the source of data items used by this listing.
-     *
-     * @return the data provider, not <code>null</code>
-     */
-    public DataProvider<T, ?> getDataProvider();
-
     /**
      * Sets the data provider for this listing. The data provider is queried for
      * displayed items as needed.

server/src/main/java/com/vaadin/data/HasItems.java

@@ -22,6 +22,8 @@ import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
 import com.vaadin.data.provider.BackEndDataProvider;
+import com.vaadin.data.provider.DataProvider;
+import com.vaadin.data.provider.ListDataProvider;
 import com.vaadin.ui.Component;
 
 /**
@@ -33,9 +35,33 @@ import com.vaadin.ui.Component;
  *            the type of the displayed item
  */
 public interface HasItems<T> extends Component, Serializable {
+
+    /**
+     * Returns the source of data items used by this listing.
+     *
+     * @return the data provider, not null
+     */
+    public DataProvider<T, ?> getDataProvider();
+
     /**
      * Sets the data items of this component provided as a collection.
      * <p>
+     * The provided items are wrapped into a {@link ListDataProvider} and this
+     * instance is used as a data provider for the
+     * {@link #setDataProvider(DataProvider)} method. It means that the items
+     * collection can be accessed later on via
+     * {@link ListDataProvider#getItems()}:
+     * 
+     * <pre>
+     * <code>
+     * HasDataProvider<String> listing = new CheckBoxGroup<>();
+     * listing.setItems(Arrays.asList("a","b"));
+     * ...
+     * 
+     * Collection<String> collection = ((ListDataProvider<String>)listing.getDataProvider()).getItems();
+     * </code>
+     * </pre>
+     * <p>
      * The provided collection instance may be used as-is. Subsequent
      * modification of the collection might cause inconsistent data to be shown
      * in the component unless it is explicitly instructed to read the data
@@ -49,7 +75,26 @@ public interface HasItems<T> extends Component, Serializable {
 
     /**
      * Sets the data items of this listing.
-     *
+     * <p>
+     * The provided items are wrapped into a {@link ListDataProvider} and this
+     * instance is used as a data provider for the
+     * {@link #setDataProvider(DataProvider)} method. It means that the items
+     * collection can be accessed later on via
+     * {@link ListDataProvider#getItems()}:
+     * 
+     * <pre>
+     * <code>
+     * HasDataProvider<String> listing = new CheckBoxGroup<>();
+     * listing.setItems(Arrays.asList("a","b"));
+     * ...
+     * 
+     * Collection<String> collection = ((ListDataProvider<String>)listing.getDataProvider()).getItems();
+     * </code>
+     * </pre>
+     * <p>
+     * 
+     * @see #setItems(Collection)
+     * 
      * @param items
      *            the data items to display
      */
@@ -65,6 +110,25 @@ public interface HasItems<T> extends Component, Serializable {
      * instead of its array and Collection variations, doesn't save any memory.
      * If you have a large data set to bind, using a lazy data provider is
      * recommended. See {@link BackEndDataProvider} for more info.
+     * <p>
+     * The provided items are wrapped into a {@link ListDataProvider} and this
+     * instance is used as a data provider for the
+     * {@link #setDataProvider(DataProvider)} method. It means that the items
+     * collection can be accessed later on via
+     * {@link ListDataProvider#getItems()}:
+     * 
+     * <pre>
+     * <code>
+     * HasDataProvider<String> listing = new CheckBoxGroup<>();
+     * listing.setItems(Arrays.asList("a","b"));
+     * ...
+     * 
+     * Collection<String> collection = ((ListDataProvider<String>)listing.getDataProvider()).getItems();
+     * </code>
+     * </pre>
+     * <p>
+     *
+     * @see #setItems(Collection)
      *
      * @param streamOfItems
      *            the stream of data items to display, not {@code null}

server/src/main/java/com/vaadin/data/provider/ListDataProvider.java

@@ -72,6 +72,15 @@ public class ListDataProvider<T>
         sortOrder = null;
     }
 
+    /**
+     * Returns the underlying data items.
+     * 
+     * @return the underlying data items
+     */
+    public Collection<T> getItems() {
+        return backend;
+    }
+
     @Override
     public Stream<T> fetch(Query<T, SerializablePredicate<T>> query) {
         Stream<T> stream = getFilteredStream(query);

server/src/test/java/com/vaadin/ui/AbstractMultiSelectTest.java

@@ -39,6 +39,7 @@ import org.junit.runners.Parameterized.Parameters;
 import org.mockito.Mockito;
 
 import com.vaadin.data.HasValue.ValueChangeEvent;
+import com.vaadin.data.provider.DataProvider;
 import com.vaadin.event.selection.MultiSelectionEvent;
 import com.vaadin.event.selection.MultiSelectionListener;
 import com.vaadin.server.ServerRpcManager;
@@ -268,6 +269,11 @@ public class AbstractMultiSelectTest<S extends AbstractMultiSelect<String>> {
                 throw new UnsupportedOperationException(
                         "Not implemented for this test");
             }
+
+            @Override
+            public DataProvider<String, ?> getDataProvider() {
+                return null;
+            }
         };
 
         Assert.assertSame(set, select.getValue());
@@ -337,6 +343,11 @@ public class AbstractMultiSelectTest<S extends AbstractMultiSelect<String>> {
                 throw new UnsupportedOperationException(
                         "Not implemented for this test");
             }
+
+            @Override
+            public DataProvider<String, ?> getDataProvider() {
+                return null;
+            }
         };
 
         AtomicReference<ValueChangeEvent<?>> event = new AtomicReference<>();

server/src/test/java/com/vaadin/ui/AbstractSingleSelectTest.java

@@ -272,6 +272,11 @@ public class AbstractSingleSelectTest {
                 throw new UnsupportedOperationException(
                         "Not needed in this test");
             }
+
+            @Override
+            public DataProvider<String, ?> getDataProvider() {
+                return null;
+            }
         };
 
         AtomicReference<ValueChangeEvent<?>> event = new AtomicReference<>();