diff --git a/src/Core/JVM/ConstantPool.php b/src/Core/JVM/ConstantPool.php
index 27079fa1..05ad72b9 100644
--- a/src/Core/JVM/ConstantPool.php
+++ b/src/Core/JVM/ConstantPool.php
@@ -51,6 +51,9 @@ public function __construct(ReaderInterface $reader, int $entries)
         }
     }
 
+    /**
+     * @return StructureInterface[]
+     */
     public function getEntries(): array
     {
         return $this->entries;
@@ -94,31 +97,49 @@ private function read($entryTag): ?StructureInterface
         throw new ReadEntryException('Entry tag ' . sprintf('0x%04X', $entryTag) . ' is not defined.');
     }
 
+    /**
+     * @return boolean
+     */
     public function offsetExists($offset)
     {
         return isset($this->entries[$offset]);
     }
 
+    /**
+     * @return StructureInterface
+     */
     public function offsetGet($offset)
     {
         return $this->entries[$offset];
     }
 
+    /**
+     * @return int
+     */
     public function count()
     {
         return count($this->entries);
     }
 
+    /**
+     * @throws ReadOnlyException
+     */
     public function offsetSet($offset, $value)
     {
         throw new ReadOnlyException('You cannot rewrite datum. The Constant Pool is read-only.');
     }
 
+    /**
+     * @throws ReadOnlyException
+     */
     public function offsetUnset($offset)
     {
         throw new ReadOnlyException('You cannot rewrite datum. The Constant Pool is read-only.');
     }
 
+    /**
+     * @return \ArrayIterator<StructureInterface>
+     */
     public function getIterator()
     {
         return new \ArrayIterator($this->entries);
diff --git a/src/Packages/java/lang/System.php b/src/Packages/java/lang/System.php
index 9883a0a3..a5038a17 100644
--- a/src/Packages/java/lang/System.php
+++ b/src/Packages/java/lang/System.php
@@ -3,6 +3,7 @@
 
 use PHPJava\Exceptions\NotImplementedException;
 use PHPJava\Packages\java\lang\_Object;
+use PHPJava\Packages\PHPJava\Kernel\Behavior\System as SystemBehavior;
 
 // use PHPJava\Packages\java\lang\System\Logger;
 // use PHPJava\Packages\java\util\Map;
@@ -194,12 +195,11 @@ public static function static_getSecurityManager($a = null)
      *
      * @param mixed $a
      * @return mixed
-     * @throws NotImplementedException
-     * @see https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/package-summary.html#identityHashCode
+     * @see https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/System.html#identityHashCode(java.lang.Object)
      */
     public static function static_identityHashCode($a = null)
     {
-        throw new NotImplementedException(__METHOD__);
+        return SystemBehavior::identityHashCode($a);
     }
 
     /**
diff --git a/src/Packages/java/lang/_String.php b/src/Packages/java/lang/_String.php
index 11ad1f11..6bb66611 100644
--- a/src/Packages/java/lang/_String.php
+++ b/src/Packages/java/lang/_String.php
@@ -19,6 +19,7 @@
  */
 class _String extends _Object implements CharSequence
 {
+    private $hash = 0;
     private $object = null;
 
     /**
@@ -223,8 +224,7 @@ public function endsWith($a = null)
      *
      * @param mixed $a
      * @return mixed
-     * @throws NotImplementedException
-     * @see https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/package-summary.html#equals
+     * @see https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/String.html#equals(java.lang.Object)
      */
     public function equals($a = null)
     {
@@ -304,14 +304,24 @@ public function getChars($a = null, $b = null, $c = null, $d = null)
     /**
      * Returns a hash code for this string.
      *
-     * @param mixed $a
      * @return mixed
-     * @throws NotImplementedException
-     * @see https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/package-summary.html#hashCode
+     * @see https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/String.html#hashCode()
      */
-    public function hashCode($a = null)
+    public function hashCode()
     {
-        throw new NotImplementedException(__METHOD__);
+        $count = $this->length();
+
+        $h = $this->hash;
+        if ($h || !$count) {
+            return $h;
+        }
+
+        $str = $this->toString();
+        for ($i = 0; $i < $count; ++$i) {
+            $h = 31 * $h + ord($str[$i]);
+        }
+
+        return $this->hash = $h;
     }
 
     /**
diff --git a/src/Packages/java/util/Objects.php b/src/Packages/java/util/Objects.php
index 3a76db66..d9520b99 100644
--- a/src/Packages/java/util/Objects.php
+++ b/src/Packages/java/util/Objects.php
@@ -120,12 +120,15 @@ public static function static_hash($a = null)
      *
      * @param mixed $a
      * @return mixed
-     * @throws NotImplementedException
-     * @see https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/package-summary.html#hashCode
+     * @see https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/Objects.html#hashCode(java.lang.Object)
      */
     public static function static_hashCode($a = null)
     {
-        throw new NotImplementedException(__METHOD__);
+        if ($a === null) {
+            return 0;
+        }
+
+        return $a->hashCode();
     }
 
     /**