OAK-11563 : added util to replace Iterables.getFirst in oak-commons (#2150)

Co-authored-by: Rishabh Kumar <[email protected]>

Author: rishabhdaim

oak-commons/src/main/java/org/apache/jackrabbit/oak/commons/collections/IterableUtils.java

@@ -390,4 +390,22 @@ public static <T> Iterable<T> limit(final Iterable<T> iterable, final int limitS
     public static String toString(final Iterable<?> iterable) {
         return org.apache.commons.collections4.IterableUtils.toString(iterable);
     }
+
+    /**
+     * Returns the first element of the specified iterable, or the default value if the iterable is empty.
+     * <p>
+     * The iterable is only traversed enough to get the first element. If the iterable is empty,
+     * the default value is returned instead.
+     *
+     * @param <T> the type of elements in the iterable
+     * @param iterable the iterable to get the first element from, may be null
+     * @param defaultValue the value to return if the iterable is empty
+     * @return the first element in the iterable or the default value if the iterable is empty
+     * @throws NullPointerException if the iterable is null
+     */
+    public static <T> T getFirst(final Iterable<T> iterable, final  T defaultValue) {
+        Objects.requireNonNull(iterable, "Iterable must not be null.");
+        final Iterator<T> iterator = iterable.iterator();
+        return iterator.hasNext() ? iterator.next() : defaultValue;
+    }
 }

oak-commons/src/test/java/org/apache/jackrabbit/oak/commons/collections/IterableUtilsTest.java

@@ -863,4 +863,51 @@ public void testToStringWithSpecialCharacters() {
         String result = IterableUtils.toString(iterable);
         Assert.assertEquals("[a,b, c\"d, e\nf]", result);
     }
+
+    @Test
+    public void testGetFirstWithNonEmptyIterable() {
+        List<String> list = Arrays.asList("a", "b", "c");
+        String result = IterableUtils.getFirst(list, "default");
+        Assert.assertEquals("a", result);
+    }
+
+    @Test
+    public void testGetFirstWithEmptyIterable() {
+        List<String> list = Collections.emptyList();
+        String result = IterableUtils.getFirst(list, "default");
+        Assert.assertEquals("default", result);
+    }
+
+    @Test
+    public void testGetFirstWithNullIterable() {
+        Assert.assertThrows(NullPointerException.class, () -> IterableUtils.getFirst(null, "default"));
+    }
+
+    @Test
+    public void testGetFirstWithSingleElement() {
+        List<Integer> list = Collections.singletonList(42);
+        Integer result = IterableUtils.getFirst(list, 0);
+        Assert.assertEquals(Integer.valueOf(42), result);
+    }
+
+    @Test
+    public void testGetFirstWithNullFirstElement() {
+        List<String> list = Arrays.asList(null, "b", "c");
+        String result = IterableUtils.getFirst(list, "default");
+        Assert.assertNull(result);
+    }
+
+    @Test
+    public void testGetFirstWithNullDefaultValue() {
+        List<String> list = Collections.emptyList();
+        String result = IterableUtils.getFirst(list, null);
+        Assert.assertNull(result);
+    }
+
+    @Test
+    public void testGetFirstWithCustomIterable() {
+        Iterable<Integer> customIterable = () -> Arrays.asList(5, 10, 15).iterator();
+        Integer result = IterableUtils.getFirst(customIterable, 0);
+        Assert.assertEquals(Integer.valueOf(5), result);
+    }
 }