diff --git a/app/src/main/java/xdsopl/robot36/ImageView.java b/app/src/main/java/xdsopl/robot36/ImageView.java
index a60395d..0412291 100644
--- a/app/src/main/java/xdsopl/robot36/ImageView.java
+++ b/app/src/main/java/xdsopl/robot36/ImageView.java
@@ -46,7 +46,7 @@ public class ImageView extends SurfaceView implements SurfaceHolder.Callback {
     }
 
     public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
-        synchronized (holder) {
+        synchronized (this.holder) {
             canvasWidth = width;
             canvasHeight = height;
         }
@@ -54,13 +54,13 @@ public class ImageView extends SurfaceView implements SurfaceHolder.Callback {
     }
 
     public void surfaceCreated(SurfaceHolder holder) {
-        synchronized (holder) {
+        synchronized (this.holder) {
             cantTouchThis = false;
         }
     }
 
     public void surfaceDestroyed(SurfaceHolder holder) {
-        synchronized (holder) {
+        synchronized (this.holder) {
             cantTouchThis = true;
         }
     }
diff --git a/app/src/main/java/xdsopl/robot36/SpectrumView.java b/app/src/main/java/xdsopl/robot36/SpectrumView.java
index 6a8aaef..51c7577 100644
--- a/app/src/main/java/xdsopl/robot36/SpectrumView.java
+++ b/app/src/main/java/xdsopl/robot36/SpectrumView.java
@@ -41,7 +41,7 @@ public class SpectrumView extends SurfaceView implements SurfaceHolder.Callback
     }
 
     public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
-        synchronized (holder) {
+        synchronized (this.holder) {
             canvasWidth = width;
             canvasHeight = height;
         }
@@ -49,13 +49,13 @@ public class SpectrumView extends SurfaceView implements SurfaceHolder.Callback
     }
 
     public void surfaceCreated(SurfaceHolder holder) {
-        synchronized (holder) {
+        synchronized (this.holder) {
             cantTouchThis = false;
         }
     }
 
     public void surfaceDestroyed(SurfaceHolder holder) {
-        synchronized (holder) {
+        synchronized (this.holder) {
             cantTouchThis = true;
         }
     }
diff --git a/app/src/main/java/xdsopl/robot36/VUMeterView.java b/app/src/main/java/xdsopl/robot36/VUMeterView.java
index 6b03aff..2624786 100644
--- a/app/src/main/java/xdsopl/robot36/VUMeterView.java
+++ b/app/src/main/java/xdsopl/robot36/VUMeterView.java
@@ -17,7 +17,6 @@
 package xdsopl.robot36;
 
 import android.content.Context;
-import android.graphics.Bitmap;
 import android.graphics.Canvas;
 import android.graphics.Color;
 import android.graphics.Paint;
@@ -40,7 +39,7 @@ public class VUMeterView extends SurfaceView implements SurfaceHolder.Callback {
     }
 
     public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
-        synchronized (holder) {
+        synchronized (this.holder) {
             canvasWidth = width;
             canvasHeight = height;
         }
@@ -48,13 +47,13 @@ public class VUMeterView extends SurfaceView implements SurfaceHolder.Callback {
     }
 
     public void surfaceCreated(SurfaceHolder holder) {
-        synchronized (holder) {
+        synchronized (this.holder) {
             cantTouchThis = false;
         }
     }
 
     public void surfaceDestroyed(SurfaceHolder holder) {
-        synchronized (holder) {
+        synchronized (this.holder) {
             cantTouchThis = true;
         }
     }