Refactor the JXPathFilter

src/main/java/org/apache/commons/jxpath/ri/JXPathFilter.java

@@ -22,22 +22,18 @@
 
 /**
  * A filter to be used by JXPath, to evaluate the xpath string values to impose any restrictions.
- * This class implements specific filter interfaces, and implements methods in those. 
+ * This class implements specific filter interfaces, and implements methods in those.
  * For instance, it JXPathClassFilter interface, which is used to check if any restricted java classes are passed via xpath
  * JXPath uses this filter instance when an extension function instance is created.
  */
 public class JXPathFilter implements JXPathClassFilter {
-    private Set<String> allowedClassesList = null;
+    private final Set<String> allowedClasses;
 
     public JXPathFilter() {
-        init();
-    }
-
-    public void init() {
+        this.allowedClasses = new HashSet<>();
         final String allowedClasses = System.getProperty("jxpath.class.allow");
         if (allowedClasses != null && !allowedClasses.isEmpty()) {
-            allowedClassesList = new HashSet<>();
-            allowedClassesList.addAll(Arrays.asList(allowedClasses.split(",")));
+            this.allowedClasses.addAll(Arrays.asList(allowedClasses.split(",")));
         }
     }
 
@@ -50,10 +46,10 @@ public void init() {
      */
     @Override
     public boolean exposeToXPath(String className) {
-        if (allowedClassesList == null || allowedClassesList.size() < 1) {
+        if (allowedClasses.isEmpty()) {
             return false;
         }
 
-        return allowedClassesList.contains(className) || allowedClassesList.contains("*");
+        return allowedClasses.contains(className) || allowedClasses.contains("*");
     }
 }