Fix Subject interface violation

Author: danmichaelo

src/Fields/Field.php

@@ -37,32 +37,48 @@ public function __get($key)
     }
 
     /**
-     * Return concatenated string of the given subfields.
-     *
-     * @param string[] $codes
-     * @param string   $glue
-     * @return string
+     * @param string|string[] $codes
+     * @return array
      */
-    protected function toString($codes, $glue = ' ')
+    protected function getSubfieldValues($codes)
     {
+        if (!is_array($codes)) {
+            $codes = [$codes];
+        }
         $parts = [];
         foreach ($this->field->getSubfields() as $sf) {
             if (in_array($sf->getCode(), $codes)) {
                 $parts[] = trim($sf->getData());
             }
         }
 
-        return trim(implode($glue, $parts));
+        return $parts;
+    }
+
+    /**
+     * Return concatenated string of the given subfields.
+     *
+     * @param string[] $codes
+     * @param string   $glue
+     * @return string
+     */
+    protected function toString($codes, $glue = ' ')
+    {
+        return trim(implode($glue, $this->getSubfieldValues($codes)));
     }
 
     /**
      * Return the data value of the *first* subfield with a given code.
+     *
+     * @param string $code
+     * @param mixed $default
+     * @return mixed
      */
-    public function sf($code)
+    public function sf($code, $default = '')
     {
         $subfield = $this->getSubfield($code);
         if (!$subfield) {
-            return null;
+            return $default;
         }
 
         return trim($subfield->getData());

src/Fields/Subject.php

@@ -7,6 +7,7 @@
 class Subject extends Field implements FieldInterface, SubjectInterface
 {
     public static $glue = ' : ';
+    public static $termComponentCodes = ['a', 'b', 'x', 'y', 'z'];
 
     const PERSONAL_NAME = '600';
     const CORPORATION_NAME = '601';
@@ -80,9 +81,14 @@ public function getControlNumber()
         return $value ?: null;
     }
 
+    public function getParts()
+    {
+        return $this->getSubfields(self::$termComponentCodes);
+    }
+
     public function getTerm()
     {
-        return $this->toString(['a', 'b', 'x', 'y', 'z'], self::$glue);
+        return $this->toString(self::$termComponentCodes, self::$glue);
     }
 
     public function __toString()