diff --git a/src/DataObject.php b/src/DataObject.php
index 1f3317ed..34a0f16d 100644
--- a/src/DataObject.php
+++ b/src/DataObject.php
@@ -198,7 +198,7 @@ public function dump($depth = 3, ?\SplObjectStorage $dumped = null)
      * @see     IteratorAggregate::getIterator()
      * @since   1.0
      */
-    public function getIterator()
+    public function getIterator(): \ArrayIterator
     {
         return new \ArrayIterator($this->dump(0));
     }
@@ -206,11 +206,11 @@ public function getIterator()
     /**
      * Gets the data properties in a form that can be serialised to JSON format.
      *
-     * @return  mixed
+     * @return  \stdClass
      *
      * @since   1.0
      */
-    public function jsonSerialize()
+    public function jsonSerialize(): \stdClass
     {
         return $this->dump();
     }
@@ -304,7 +304,7 @@ protected function setProperty($property, $value)
      *
      * @since   1.0
      */
-    public function count()
+    public function count(): int
     {
         return \count($this->properties);
     }
diff --git a/src/DataSet.php b/src/DataSet.php
index 95d97e85..5b48b09c 100644
--- a/src/DataSet.php
+++ b/src/DataSet.php
@@ -256,7 +256,7 @@ public function toArray($associative = true, ...$keys)
      *
      * @since   1.0
      */
-    public function count()
+    public function count(): int
     {
         return \count($this->objects);
     }
@@ -268,7 +268,7 @@ public function count()
      *
      * @since   1.0
      */
-    public function clear()
+    public function clear(): DataSet
     {
         $this->objects = [];
         $this->rewind();
@@ -279,11 +279,11 @@ public function clear()
     /**
      * Get the current data object in the set.
      *
-     * @return  DataObject|false  The current object, or false if the array is empty or the pointer is beyond the end of the elements.
+     * @return  DataObject|bool  The current object, or false if the array is empty or the pointer is beyond the end of the elements.
      *
      * @since   1.0
      */
-    public function current()
+    public function current(): DataObject|bool
     {
         return is_scalar($this->current) ? $this->objects[$this->current] : false;
     }
@@ -350,11 +350,11 @@ public function jsonSerialize()
     /**
      * Gets the key of the current object in the iterator.
      *
-     * @return  integer|false  The object key on success; false on failure.
+     * @return  int|bool|null  The object key on success; false on failure.
      *
      * @since   1.0
      */
-    public function key()
+    public function key(): int|bool|null
     {
         return $this->current;
     }
@@ -397,7 +397,7 @@ public function walk(callable $funcname)
      *
      * @since   1.0
      */
-    public function next()
+    public function next(): void
     {
         // Get the object offsets.
         $keys = $this->keys();
@@ -430,7 +430,7 @@ public function next()
      *
      * @since   1.0
      */
-    public function offsetExists($offset)
+    public function offsetExists($offset): bool
     {
         return isset($this->objects[$offset]);
     }
@@ -444,7 +444,7 @@ public function offsetExists($offset)
      *
      * @since   1.0
      */
-    public function offsetGet($offset)
+    public function offsetGet($offset): DataObject|null
     {
         return $this->objects[$offset] ?? null;
     }
@@ -460,7 +460,7 @@ public function offsetGet($offset)
      * @since   1.0
      * @throws  \InvalidArgumentException if an object is not an instance of DataObject.
      */
-    public function offsetSet($offset, $object)
+    public function offsetSet($offset, $object): void
     {
         if (!($object instanceof DataObject)) {
             throw new \InvalidArgumentException(
@@ -489,7 +489,7 @@ public function offsetSet($offset, $object)
      *
      * @since   1.0
      */
-    public function offsetUnset($offset)
+    public function offsetUnset($offset): void
     {
         if (!isset($this[$offset])) {
             // Do nothing if the offset does not exist.
@@ -522,7 +522,7 @@ public function offsetUnset($offset)
      *
      * @since   1.0
      */
-    public function rewind()
+    public function rewind(): void
     {
         // Set the current position to the first object.
         if (empty($this->objects)) {
@@ -540,7 +540,7 @@ public function rewind()
      *
      * @since   1.0
      */
-    public function valid()
+    public function valid(): bool
     {
         // Check the current position.
         if (!is_scalar($this->current) || !isset($this->objects[$this->current])) {