diff --git a/docs/book/10-Interfaces.md b/docs/book/10-Interfaces.md
index 927861c..a1f487a 100644
--- a/docs/book/10-Interfaces.md
+++ b/docs/book/10-Interfaces.md
@@ -857,7 +857,7 @@ package interfaces.filters;
 
 public class Waveform {
     private static long counter;
-    private final long id = count++;
+    private final long id = counter++;
     
     @Override
     public String toString() {
@@ -1041,7 +1041,7 @@ class FilterAdapter implements Processor {
     }
 }
 
-punlic class FilterProcessor {
+public class FilterProcessor {
     public static void main(String[] args) {
         Waveform w = new Waveform();
         Applicator.apply(new FilterAdapter(new LowPass(1.0)), w);