diff --git a/src/collections_me.c b/src/collections_me.c
index ff45b20..7d42c3c 100644
--- a/src/collections_me.c
+++ b/src/collections_me.c
@@ -92,7 +92,6 @@ ZEND_BEGIN_ARG_INFO(associate_by_to_arginfo, 0)
 ZEND_END_ARG_INFO()
 
 ZEND_BEGIN_ARG_INFO(copy_of_arginfo, 0)
-    ZEND_ARG_INFO(0, elements)
     ZEND_ARG_TYPE_INFO(0, new_size, IS_LONG, 0)
 ZEND_END_ARG_INFO()
 
diff --git a/src/collections_methods.c b/src/collections_methods.c
index 1c83ca2..8442907 100644
--- a/src/collections_methods.c
+++ b/src/collections_methods.c
@@ -63,6 +63,7 @@
 #define ERR_BAD_ARGUMENT_TYPE() PHP_COLLECTIONS_ERROR(E_WARNING, "Bad argument type")
 #define ERR_BAD_KEY_TYPE() PHP_COLLECTIONS_ERROR(E_WARNING, "Key must be integer or string")
 #define ERR_BAD_CALLBACK_RETVAL() PHP_COLLECTIONS_ERROR(E_WARNING, "Bad callback return value")
+#define ERR_BAD_SIZE() PHP_COLLECTIONS_ERROR(E_WARNING, "Size must be non-negative")
 
 #define ELEMENTS_VALIDATE(elements) \
     if (IS_COLLECTION_P(elements)) { \
@@ -94,6 +95,11 @@
         RETVAL_OBJ(obj); \
     } while (0)
 
+#define RETURN_NEW_COLLECTION(collection) { \
+        RETVAL_NEW_COLLECTION(collection); \
+        return; \
+    }
+
 PHP_METHOD(Collection, __construct) {}
 
 PHP_METHOD(Collection, addAll)
@@ -105,9 +111,11 @@ PHP_METHOD(Collection, addAll)
     ELEMENTS_VALIDATE(elements);
     zval rv;
     zval* current = COLLECTION_FETCH_EX();
-    ZEND_HASH_FOREACH_VAL_IND(Z_ARRVAL_P(elements), zval* val)
-        zend_hash_next_index_insert(Z_ARRVAL_P(current), val);
-    ZEND_HASH_FOREACH_END();
+    ZEND_HASH_FILL_PACKED(Z_ARRVAL_P(current))
+        ZEND_HASH_FOREACH_VAL_IND(Z_ARRVAL_P(elements), zval* val)
+            ZEND_HASH_FILL_ADD(val);
+        ZEND_HASH_FOREACH_END();
+    ZEND_HASH_FILL_END();
 }
 
 PHP_METHOD(Collection, all)
@@ -312,17 +320,68 @@ PHP_METHOD(Collection, containsAll)
 
 PHP_METHOD(Collection, containsKey)
 {
-    
+    zval* key;
+    ZEND_PARSE_PARAMETERS_START(1, 1)
+        Z_PARAM_ZVAL(key)
+    ZEND_PARSE_PARAMETERS_END();
+    zval rv;
+    zval* current = COLLECTION_FETCH_EX();
+    if (Z_TYPE_P(key) == IS_LONG)
+        RETURN_BOOL(zend_hash_index_exists(Z_ARRVAL_P(current), Z_LVAL_P(key)));
+    if (Z_TYPE_P(key) == IS_STRING)
+        RETURN_BOOL(zend_hash_exists(Z_ARRVAL_P(current), Z_STR_P(key)));
+    ERR_BAD_KEY_TYPE();
+    RETVAL_FALSE;
 }
 
 PHP_METHOD(Collection, containsValue)
 {
-    
+    zval* element;
+    ZEND_PARSE_PARAMETERS_START(1, 1)
+        Z_PARAM_ZVAL(element)
+    ZEND_PARSE_PARAMETERS_END();
+    zval rv;
+    zval* current = COLLECTION_FETCH_EX();
+    INIT_EQUAL_CHECK_FUNC(element);
+    ZEND_HASH_FOREACH_VAL_IND(Z_ARRVAL_P(current), zval* val)
+        if (equal_check_func(element, val))
+            RETURN_TRUE;
+    ZEND_HASH_FOREACH_END();
+    RETURN_FALSE;
 }
 
 PHP_METHOD(Collection, copyOf)
 {
-    
+    zend_long new_size = -1;
+    ZEND_PARSE_PARAMETERS_START(0, 1)
+        Z_PARAM_OPTIONAL
+        Z_PARAM_LONG(new_size);
+    ZEND_PARSE_PARAMETERS_END();
+    zval rv;
+    zval* current = COLLECTION_FETCH_EX();
+    if (EX_NUM_ARGS() == 0) {
+        ARRAY_CLONE(new_collection, current);
+        RETURN_NEW_COLLECTION(new_collection);
+    }
+    if (UNEXPECTED(new_size < 0)) {
+        ERR_BAD_SIZE();
+        RETURN_NULL();
+    }
+    if (new_size == 0)
+    {
+        ARRAY_NEW(new_collection, 0);
+        RETURN_NEW_COLLECTION(new_collection);
+    }
+    ARRAY_NEW_EX(new_collection, current);
+    ZEND_HASH_FOREACH_BUCKET(Z_ARRVAL_P(current), Bucket* bucket)
+        if (bucket->key)
+            zend_hash_add(new_collection, bucket->key, &bucket->val);
+        else
+            zend_hash_index_add(new_collection, bucket->h, &bucket->val);
+        if (--new_size == 0)
+            break;
+    ZEND_HASH_FOREACH_END();
+    RETVAL_NEW_COLLECTION(new_collection);
 }
 
 PHP_METHOD(Collection, copyOfRange)
diff --git a/stubs/Collection.php b/stubs/Collection.php
index 742773c..1a1e7db 100644
--- a/stubs/Collection.php
+++ b/stubs/Collection.php
@@ -90,7 +90,7 @@ class Collection implements ArrayAccess, Countable
     function containsAll($other) {}
 
     /**
-     * Checks if the array contains the given key.
+     * Checks if the collection contains the given key.
      *
      * @param int|string $key
      * @return bool
@@ -98,7 +98,7 @@ class Collection implements ArrayAccess, Countable
     function containsKey($key) {}
 
     /**
-     * Check if the array maps one or more keys to the specified value.
+     * Check if the collection maps one or more keys to the specified value.
      *
      * @param mixed $element
      * @return bool
@@ -106,16 +106,15 @@ class Collection implements ArrayAccess, Countable
     function containsValue($element) {}
 
     /**
-     * Returns new array which is a copy of the original array.
+     * Returns new collection which is a copy of the original collection.
      *
-     * @param array|Collection $elements
      * @param int $new_size[optional]
      * @return Collection
      */
-    function copyOf($elements, $new_size) {}
+    function copyOf($new_size) {}
 
     /**
-     * Returns new array which is a copy of range of original array.
+     * Returns new collection which is a copy of range of original collection.
      *
      * @param int $from_index
      * @param int $to_index
@@ -124,22 +123,22 @@ class Collection implements ArrayAccess, Countable
     function copyOfRange($from_index, $to_index) {}
 
     /**
-     * Returns the number of elements in this array.
+     * Returns the number of elements in this collection.
      *
      * @return int
      */
     function count() {}
 
     /**
-     * Returns a collection containing only distinct elements from the given array.
+     * Returns a collection containing only distinct elements from the given collection.
      *
      * @return Collection
      */
     function distinct() {}
 
     /**
-     * Returns a collection containing only elements from the given array having distinct keys
-     * returned by the given selector function.
+     * Returns a collection containing only elements from the given collection having distinct
+     * keys returned by the given selector function.
      *
      * @param callable $selector ($value, $key) -> $key
      * @return Collection
@@ -181,7 +180,7 @@ class Collection implements ArrayAccess, Countable
     function dropWhile($predicate) {}
 
     /**
-     * Fills original array with the provided value.
+     * Fills original collection with the provided value.
      *
      * @param mixed $element
      * @param int $from_index[optional]
@@ -306,7 +305,7 @@ class Collection implements ArrayAccess, Countable
 
     /**
      * Returns an element at the given key or the result of calling the `default` function
-     * if the index is out of bounds of this array.
+     * if the index is out of bounds of this collection.
      *
      * @param int|string $key
      * @param callable $default[optional] ($key) -> $value
@@ -316,7 +315,7 @@ class Collection implements ArrayAccess, Countable
 
     /**
      * Groups values returned by the value_transform function applied to each element of the
-     * original array by the key returned by the given key_selector function applied to the element
+     * original collection by the key returned by the given key_selector function applied to the element
      * and returns a collection where each group key is associated with a list of corresponding values.
      *
      * @param callable $key_selector ($value, $key) -> $new_key
@@ -338,7 +337,7 @@ class Collection implements ArrayAccess, Countable
     function groupByTo($destination, $key_selector, $value_transform) {}
 
     /**
-     * Returns first key of element, or null if the array does not contain element.
+     * Returns first key of element, or null if the collection does not contain element.
      *
      * @param mixed $element
      * @return int|string|null
@@ -346,8 +345,8 @@ class Collection implements ArrayAccess, Countable
     function indexOf($element) {}
 
     /**
-     * Returns key of the first element matching the given predicate, or null if the array does
-     * not contain such element.
+     * Returns key of the first element matching the given predicate, or null if the collection
+     * does not contain such element.
      *
      * @param callable $predicate ($value) -> bool
      * @return int|string|null
@@ -355,8 +354,8 @@ class Collection implements ArrayAccess, Countable
     function indexOfFirst($predicate) {}
 
     /**
-     * Returns key of the last element matching the given predicate, or null if the array does
-     * not contain such element.
+     * Returns key of the last element matching the given predicate, or null if the collection
+     * does not contain such element.
      *
      * @param callable $predicate ($value) -> bool
      * @return int|string|null
@@ -380,14 +379,14 @@ class Collection implements ArrayAccess, Countable
     function intersect($other) {}
 
     /**
-     * Returns true if the array is empty.
+     * Returns true if the collection is empty.
      *
      * @return bool
      */
     function isEmpty() {}
 
     /**
-     * Returns true if the array is not empty.
+     * Returns true if the collection is not empty.
      */
     function isNotEmpty() {}
 
@@ -407,7 +406,7 @@ class Collection implements ArrayAccess, Countable
     function last($predicate) {}
 
     /**
-     * Returns last key of element, or null if the array does not contain element.
+     * Returns last key of element, or null if the collection does not contain element.
      *
      * @param mixed $element
      * @return int|string|null
diff --git a/tests/009-contains-key-value.phpt b/tests/009-contains-key-value.phpt
new file mode 100644
index 0000000..f63f44a
--- /dev/null
+++ b/tests/009-contains-key-value.phpt
@@ -0,0 +1,11 @@
+--TEST--
+Test Collection::containsKey() and Collection::containsValue().
+--FILE--
+<?php
+$collection = Collection::init(['foo' => 'bar', 12 => 6, 'baz' => 27]);
+if (!$collection->containsKey('foo') || $collection->containsKey('Baz'))
+    echo 'Collection::containsKey() failed.', PHP_EOL;
+if (!$collection->containsValue(27) || $collection->containsValue(3))
+    echo 'Collection::containsValue() failed.', PHP_EOL;
+?>
+--EXPECT--
diff --git a/tests/010-copy-of.phpt b/tests/010-copy-of.phpt
new file mode 100644
index 0000000..5fd2aa0
--- /dev/null
+++ b/tests/010-copy-of.phpt
@@ -0,0 +1,12 @@
+--TEST--
+Test Collection::copyOf();
+--FILE--
+<?php
+$array = ['a' => 2, 3 => 'd', 'e' => 'f'];
+$collection = Collection::init($array);
+$collection1 = $collection->copyOf(2);
+$collection2 = $collection->copyOf();
+if ($collection1->toArray() != array_slice($array, 0, 2, true) || $collection2->toArray() != $array)
+    echo 'Collection::copyOf() failed.', PHP_EOL;
+?>
+--EXPECT--