Fixed FilteringIterator.hasNext advances iterator every time it is ca…

…lled, with patch from Gabe Hicks (issue 23)

cqengine/trunk/src/main/java/com/googlecode/cqengine/resultset/filter/FilteringIterator.java

@@ -41,6 +41,9 @@ public FilteringIterator(Iterator<O> wrappedIterator) {
 
     @Override
     public boolean hasNext() {
+        if(nextObjectIsNull || nextObject != null) {
+            return true;
+        }
         while (wrappedIterator.hasNext()) {
             nextObject = wrappedIterator.next();
             if (isValid(nextObject)) {
@@ -54,14 +57,13 @@ public boolean hasNext() {
 
     @Override
     public O next() {
-        if (nextObjectIsNull) {
-            nextObjectIsNull = false;
-            return null;
-        }
-        if (nextObject == null) {
+        if(!hasNext()) {
             throw new NoSuchElementException();
         }
-        return nextObject;
+        O objectToReturn = nextObject;
+        nextObject = null;
+        nextObjectIsNull = false;
+        return objectToReturn;
     }
 
     public abstract boolean isValid(O object);

cqengine/trunk/src/test/java/com/googlecode/cqengine/resultset/filter/FilteringIteratorTest.java

@@ -0,0 +1,96 @@
+/**
+ * Copyright 2012 Niall Gallagher
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.googlecode.cqengine.resultset.filter;
+
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.NoSuchElementException;
+
+import static org.hamcrest.CoreMatchers.is;
+import static org.hamcrest.CoreMatchers.nullValue;
+import static org.junit.Assert.assertThat;
+
+public class FilteringIteratorTest {
+    @Test
+    public void testHasNextDoesNotAdvanceIterator(){
+        List<String> testList = Arrays.asList("abc", "bcd", "cde");
+        FilteringIterator<String> iterator = new FilteringIterator<String>(testList.iterator()) {
+            @Override
+            public boolean isValid(String object) {
+                return true;
+            }
+        };
+        iterator.hasNext();
+        iterator.hasNext();
+        iterator.hasNext();
+        assertThat(iterator.next(), is("abc"));
+    }
+
+    @Test
+    public void testNextPopulatedWithoutCallingHasNext(){
+        List<String> testList = Arrays.asList("abc", "bcd", "cde");
+        FilteringIterator<String> iterator = new FilteringIterator<String>(testList.iterator()) {
+            @Override
+            public boolean isValid(String object) {
+                return true;
+            }
+        };
+        assertThat(iterator.next(), is("abc"));
+    }
+
+    @Test
+    public void testDelegatedIteratorHasNulls() {
+        List<String> testList = Arrays.asList("abc", null, "cde");
+        FilteringIterator<String> iterator = new FilteringIterator<String>(testList.iterator()) {
+            @Override
+            public boolean isValid(String object) {
+                return true;
+            }
+        };
+        assertThat(iterator.next(), is("abc"));
+        assertThat(iterator.next(), nullValue());
+        assertThat(iterator.next(), is("cde"));
+        assertThat(iterator.hasNext(), is(false));
+    }
+
+    @Test
+    public void testFiltering() {
+        List<String> testList = Arrays.asList("aaa", "bbb", "aab", "bba");
+        FilteringIterator<String> iterator = new FilteringIterator<String>(testList.iterator()) {
+            @Override
+            public boolean isValid(String object) {
+                return object.startsWith("aa");
+            }
+        };
+        assertThat(iterator.next(), is("aaa"));
+        assertThat(iterator.next(), is("aab"));
+        assertThat(iterator.hasNext(), is(false));
+    }
+
+    @Test(expected = NoSuchElementException.class)
+    public void testEmptyDelegate() {
+        List<String> testList = Arrays.asList();
+        FilteringIterator<String> iterator = new FilteringIterator<String>(testList.iterator()) {
+            @Override
+            public boolean isValid(String object) {
+                return true;
+            }
+        };
+        iterator.next();
+    }
+}