diff --git a/src/Plugin/FileDownloadUrl.php b/src/Plugin/FileDownloadUrl.php
index 9f674f6..dc0669e 100644
--- a/src/Plugin/FileDownloadUrl.php
+++ b/src/Plugin/FileDownloadUrl.php
@@ -13,10 +13,7 @@ class FileDownloadUrl extends FieldItemList {
    */
   public function __construct(DataDefinitionInterface $definition, $name, TypedDataInterface $parent) {
     parent::__construct($definition, $name, $parent);
-    $values = array_map(function ($uri) {
-      return ['value' => $this::fileCreateRootRelativeUrl($uri['value'])];
-    }, $this->getUris());
-    $this->setValue($values);
+    $this->setValue($this->compute());
   }
 
   /**
@@ -32,7 +29,7 @@ class FileDownloadUrl extends FieldItemList {
    * @return string
    *   The transformed relative URL.
    */
-  public static function fileCreateRootRelativeUrl($uri) {
+  protected function fileCreateRootRelativeUrl($uri) {
     return file_url_transform_relative(file_create_url($uri));
   }
 
@@ -46,8 +43,21 @@ class FileDownloadUrl extends FieldItemList {
    * @return array
    *   The array of values.
    */
-  public function getUris() {
+  protected function getUris() {
     return $this->getEntity()->get('uri')->getValue();
   }
 
+  /**
+   * Computes the file download url field.
+   *
+   * @return array
+   *   The array of values to use for the computed field.
+   */
+  public function compute() {
+    $uri_values = $this->getUris();
+    return array_map(function ($uri_value) {
+      return ['value' => $this->fileCreateRootRelativeUrl($uri_value['value'])];
+    }, $uri_values);
+  }
+
 }
diff --git a/tests/src/Unit/Plugin/FileDownloadUrlTest.php b/tests/src/Unit/Plugin/FileDownloadUrlTest.php
index 47a2eb2..f88f1d4 100644
--- a/tests/src/Unit/Plugin/FileDownloadUrlTest.php
+++ b/tests/src/Unit/Plugin/FileDownloadUrlTest.php
@@ -42,12 +42,12 @@ class FileDownloadUrlTest extends UnitTestCase  {
   }
 
   /**
-   * @covers ::__construct
+   * @covers ::compute
    */
-  public function testConstructor() {
+  public function testCompute() {
     $field_list_mock = $this->getMockBuilder(FileDownloadUrl::class)
       ->setMethods(['getUris', 'fileCreateRootRelativeUrl'])
-      ->setConstructorArgs([$this->definition, 'lorem', $this->parent])
+      ->disableOriginalConstructor()
       ->getMock();
 
     $field_list_mock
@@ -55,14 +55,13 @@ class FileDownloadUrlTest extends UnitTestCase  {
       ->method('getUris')
       ->will($this->returnValue([['value' => 'public://url.to.file']]));
 
-    $uris = $field_list_mock->getUris();
     $field_list_mock
       ->expects($this->once())
       ->method('fileCreateRootRelativeUrl')
-      ->with($this->equalTo([['value' => 'public://url.to.file']]))
-      ->willReturn([['value' => '/url.to.file']]);
+      ->with($this->equalTo('public://url.to.file'))
+      ->willReturn('/url.to.file');
 
-    $this->assertArrayEquals([['value' => '/url.to.file']], $field_list_mock->getValue());
+    $this->assertArrayEquals([['value' => '/url.to.file']], $field_list_mock->compute());
   }
 
 }