diff --git a/cache/src/main/java/com/tomclaw/cache/DiskLruCache.java b/cache/src/main/java/com/tomclaw/cache/DiskLruCache.java
index 5106606..74381a6 100644
--- a/cache/src/main/java/com/tomclaw/cache/DiskLruCache.java
+++ b/cache/src/main/java/com/tomclaw/cache/DiskLruCache.java
@@ -134,7 +134,7 @@ public long getJournalSize() {
     }
 
     private static void assertKeyValid(String key) {
-        if (key == null || key.length() == 0) {
+        if (key == null || key.isEmpty()) {
             throw new IllegalArgumentException(String.format("Invalid key value: '%s'", key));
         }
     }
diff --git a/cache/src/main/java/com/tomclaw/cache/SimpleFileManager.java b/cache/src/main/java/com/tomclaw/cache/SimpleFileManager.java
index 6ce799b..2b106ea 100644
--- a/cache/src/main/java/com/tomclaw/cache/SimpleFileManager.java
+++ b/cache/src/main/java/com/tomclaw/cache/SimpleFileManager.java
@@ -5,7 +5,7 @@
 
 public class SimpleFileManager implements FileManager {
 
-    private File dir;
+    private final File dir;
 
     @SuppressWarnings("WeakerAccess")
     public SimpleFileManager(File dir) {