diff --git a/app/src/main/java/com/hippo/ehviewer/client/data/ListUrlBuilder.kt b/app/src/main/java/com/hippo/ehviewer/client/data/ListUrlBuilder.kt
index 83561597a..5ef525916 100644
--- a/app/src/main/java/com/hippo/ehviewer/client/data/ListUrlBuilder.kt
+++ b/app/src/main/java/com/hippo/ehviewer/client/data/ListUrlBuilder.kt
@@ -69,13 +69,8 @@ open class ListUrlBuilder(
     }
 
     fun setIndex(index: String?, isNext: Boolean = true) {
-        if (isNext) {
-            mNext = index
-            mPrev = null
-        } else {
-            mPrev = index
-            mNext = null
-        }
+        mNext = index?.takeIf { isNext }
+        mPrev = index?.takeUnless { isNext }
     }
 
     fun setJumpTo(jumpTo: String?) {
@@ -403,7 +398,7 @@ open class ListUrlBuilder(
                 sb.toString()
             }
 
-            else -> throw java.lang.IllegalStateException("Unexpected value: $mode")
+            else -> throw IllegalStateException("Unexpected value: $mode")
         }
     }
 
diff --git a/app/src/main/java/com/hippo/ehviewer/client/data/PreviewSet.kt b/app/src/main/java/com/hippo/ehviewer/client/data/PreviewSet.kt
index c33c90413..4a3ee723e 100644
--- a/app/src/main/java/com/hippo/ehviewer/client/data/PreviewSet.kt
+++ b/app/src/main/java/com/hippo/ehviewer/client/data/PreviewSet.kt
@@ -13,22 +13,15 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+package com.hippo.ehviewer.client.data
 
-package com.hippo.ehviewer.client.data;
+import android.os.Parcelable
+import com.hippo.widget.LoadImageView
 
-import android.os.Parcelable;
-
-import com.hippo.widget.LoadImageView;
-
-public abstract class PreviewSet implements Parcelable {
-
-    public abstract int size();
-
-    public abstract int getPosition(int index);
-
-    public abstract String getPageUrlAt(int index);
-
-    public abstract GalleryPreview getGalleryPreview(long gid, int index);
-
-    public abstract void load(LoadImageView view, long gid, int index);
-}
+abstract class PreviewSet : Parcelable {
+    abstract fun size(): Int
+    abstract fun getPosition(index: Int): Int
+    abstract fun getPageUrlAt(index: Int): String
+    abstract fun getGalleryPreview(gid: Long, index: Int): GalleryPreview
+    abstract fun load(view: LoadImageView, gid: Long, index: Int)
+}
\ No newline at end of file