diff --git a/src/main/java/org/ids_mannheim/PaddedSlidingWindowQueue.java b/src/main/java/org/ids_mannheim/PaddedSlidingWindowQueue.java
index 7c141bd..68c4b87 100644
--- a/src/main/java/org/ids_mannheim/PaddedSlidingWindowQueue.java
+++ b/src/main/java/org/ids_mannheim/PaddedSlidingWindowQueue.java
@@ -32,9 +32,10 @@
     }
 
     @Override
-    public void add(String k) {
+    public boolean add(String k) {
         super.add(k);
         needsFlush = true;
+        return true;
     }
 
     @Override
diff --git a/src/main/java/org/ids_mannheim/SlidingWindowQueue.java b/src/main/java/org/ids_mannheim/SlidingWindowQueue.java
index 32e0f60..39f0471 100644
--- a/src/main/java/org/ids_mannheim/SlidingWindowQueue.java
+++ b/src/main/java/org/ids_mannheim/SlidingWindowQueue.java
@@ -2,7 +2,7 @@
 
 import java.util.LinkedList;
 
-public class SlidingWindowQueue extends LinkedList {
+public class SlidingWindowQueue extends LinkedList<String> {
     final int maxSize;
     final Increaser flush;
     final boolean with_lemma_and_pos;
@@ -14,12 +14,13 @@
         this.with_lemma_and_pos = with_lemma_and_pos;
     }
 
-    public void add(String k) {
-        boolean r = super.add(k);
+    public boolean add(String k) {
+        super.add(k);
         if (size() == maxSize) {
             this.flush.accept(String.join("\t", this));
             remove(0);
         }
+        return true;
     }
 
     public void reset(int fold) {