diff --git a/src/main/java/schule/ngb/zm/DrawableLayer.java b/src/main/java/schule/ngb/zm/DrawableLayer.java
index 0d04046..6273c15 100644
--- a/src/main/java/schule/ngb/zm/DrawableLayer.java
+++ b/src/main/java/schule/ngb/zm/DrawableLayer.java
@@ -1,11 +1,13 @@
 package schule.ngb.zm;
 
-import java.awt.*;
+import java.awt.Graphics2D;
+import java.util.Collections;
 import java.util.LinkedList;
+import java.util.List;
 
 public class DrawableLayer extends Layer {
 
-	protected LinkedList<Drawable> drawables = new LinkedList<>();
+	protected List<Drawable> drawables = new LinkedList<>();
 
 	protected boolean clearBeforeDraw = true;
 
@@ -43,7 +45,8 @@ public class DrawableLayer extends Layer {
 		}
 
 		synchronized( drawables ) {
-			for( Drawable d : drawables ) {
+			List<Drawable> it = Collections.unmodifiableList(drawables);
+			for( Drawable d : it ) {
 				if( d.isVisible() ) {
 					d.draw(drawing);
 				}
diff --git a/src/main/java/schule/ngb/zm/Spielemaschine.java b/src/main/java/schule/ngb/zm/Spielemaschine.java
index 47e53d1..bfed76b 100644
--- a/src/main/java/schule/ngb/zm/Spielemaschine.java
+++ b/src/main/java/schule/ngb/zm/Spielemaschine.java
@@ -1,7 +1,12 @@
 package schule.ngb.zm;
 
+import java.awt.Graphics2D;
+import java.util.Collections;
+import java.util.Iterator;
 import java.util.LinkedList;
+import java.util.List;
 
+@SuppressWarnings( "unused" )
 public class Spielemaschine extends Zeichenmaschine {
 
 	private LinkedList<Drawable> drawables;
@@ -78,9 +83,10 @@ public class Spielemaschine extends Zeichenmaschine {
 	@Override
 	public final void update( double delta ) {
 		synchronized( updatables ) {
-			for( Updatable updatable : updatables ) {
-				if( updatable.isActive() ) {
-					updatable.update(delta);
+			List<Updatable> it = Collections.unmodifiableList(updatables);
+			for( Updatable u: it ) {
+				if( u.isActive() ) {
+					u.update(delta);
 				}
 			}
 		}
@@ -92,12 +98,21 @@ public class Spielemaschine extends Zeichenmaschine {
 	public final void draw() {
 		mainLayer.clear();
 		synchronized( drawables ) {
-			for( Drawable drawable : drawables ) {
-				if( drawable.isVisible() ) {
-					drawable.draw(mainLayer.getGraphics());
+			List<Drawable> it = Collections.unmodifiableList(drawables);
+			for( Drawable d: it ) {
+				if( d.isVisible() ) {
+					d.draw(mainLayer.getGraphics());
 				}
 			}
 		}
 	}
 
+	static class GraphicsLayer extends Layer {
+
+		public Graphics2D getGraphics() {
+			return drawing;
+		}
+
+	}
+
 }
diff --git a/src/main/java/schule/ngb/zm/Zeichenleinwand.java b/src/main/java/schule/ngb/zm/Zeichenleinwand.java
index 137d4b2..f4e8e4c 100644
--- a/src/main/java/schule/ngb/zm/Zeichenleinwand.java
+++ b/src/main/java/schule/ngb/zm/Zeichenleinwand.java
@@ -1,12 +1,16 @@
 package schule.ngb.zm;
 
+import schule.ngb.zm.shapes.Shape;
+
 import java.awt.Canvas;
 import java.awt.Graphics;
 import java.awt.Graphics2D;
 import java.awt.Toolkit;
 import java.awt.image.BufferStrategy;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.LinkedList;
+import java.util.List;
 
 /**
  * Eine Leinwand ist die Hauptkomponente einer Zeichenmaschine. Sie besteht aus
@@ -195,7 +199,8 @@ public class Zeichenleinwand extends Canvas {
 
 	public void updateLayers( double delta ) {
 		synchronized( layers ) {
-			for( Layer layer : layers ) {
+			List<Layer> it = List.copyOf(layers);
+			for( Layer layer : it ) {
 				layer.update(delta);
 			}
 		}
@@ -232,7 +237,8 @@ public class Zeichenleinwand extends Canvas {
 	public void draw( Graphics graphics ) {
 		Graphics2D g2d = (Graphics2D) graphics.create();
 		synchronized( layers ) {
-			for( Layer layer : layers ) {
+			List<Layer> it = List.copyOf(layers);
+			for( Layer layer : it ) {
 				layer.draw(g2d);
 			}
 		}
@@ -256,7 +262,8 @@ public class Zeichenleinwand extends Canvas {
 						g2d.clearRect(0, 0, getWidth(), getHeight());
 
 						synchronized( layers ) {
-							for( Layer layer : layers ) {
+							List<Layer> it = List.copyOf(layers);
+							for( Layer layer : it ) {
 								layer.draw(g2d);
 							}
 						}
diff --git a/src/main/java/schule/ngb/zm/shapes/ShapesLayer.java b/src/main/java/schule/ngb/zm/shapes/ShapesLayer.java
index bc4e4c7..1459ef3 100644
--- a/src/main/java/schule/ngb/zm/shapes/ShapesLayer.java
+++ b/src/main/java/schule/ngb/zm/shapes/ShapesLayer.java
@@ -6,10 +6,7 @@ import schule.ngb.zm.anim.AnimationFacade;
 import schule.ngb.zm.anim.Easing;
 
 import java.awt.Graphics2D;
-import java.util.Collection;
-import java.util.Iterator;
-import java.util.LinkedList;
-import java.util.List;
+import java.util.*;
 import java.util.function.DoubleUnaryOperator;
 
 public class ShapesLayer extends Layer {
@@ -45,12 +42,12 @@ public class ShapesLayer extends Layer {
 		return null;
 	}
 
-	public java.util.List<Shape> getShapes() {
+	public List<Shape> getShapes() {
 		return shapes;
 	}
 
-	public <ST extends Shape> java.util.List<ST> getShapes( Class<ST> shapeClass ) {
-		java.util.List<ST> result = new LinkedList<>();
+	public <ST extends Shape> List<ST> getShapes( Class<ST> shapeClass ) {
+		List<ST> result = new LinkedList<>();
 		for( Shape s : shapes ) {
 			if( shapeClass.isInstance(s) ) {
 				result.add((ST) s);
@@ -60,7 +57,7 @@ public class ShapesLayer extends Layer {
 	}
 
 	public void add( Shape... shapes ) {
-		synchronized( shapes ) {
+		synchronized( this.shapes ) {
 			for( Shape s : shapes ) {
 				this.shapes.add(s);
 			}
@@ -68,7 +65,7 @@ public class ShapesLayer extends Layer {
 	}
 
 	public void add( Collection<Shape> shapes ) {
-		synchronized( shapes ) {
+		synchronized( this.shapes ) {
 			for( Shape s : shapes ) {
 				this.shapes.add(s);
 			}
@@ -76,16 +73,16 @@ public class ShapesLayer extends Layer {
 	}
 
 	public void remove( Shape... shapes ) {
-		synchronized( shapes ) {
-			for( Shape s: shapes ) {
+		synchronized( this.shapes ) {
+			for( Shape s : shapes ) {
 				this.shapes.remove(s);
 			}
 		}
 	}
 
 	public void remove( Collection<Shape> shapes ) {
-		synchronized( shapes ) {
-			for( Shape s: shapes ) {
+		synchronized( this.shapes ) {
+			for( Shape s : shapes ) {
 				this.shapes.remove(s);
 			}
 		}
@@ -99,16 +96,16 @@ public class ShapesLayer extends Layer {
 
 	public void showAll() {
 		synchronized( shapes ) {
-			for( Shape pShape : shapes ) {
-				pShape.show();
+			for( Shape s : shapes ) {
+				s.show();
 			}
 		}
 	}
 
 	public void hideAll() {
 		synchronized( shapes ) {
-			for( Shape pShape : shapes ) {
-				pShape.hide();
+			for( Shape s : shapes ) {
+				s.hide();
 			}
 		}
 	}
@@ -147,9 +144,10 @@ public class ShapesLayer extends Layer {
 		}
 
 		synchronized( shapes ) {
-			for( Shape pShape : shapes ) {
-				if( pShape.isVisible() ) {
-					pShape.draw(drawing);
+			List<Shape> it = List.copyOf(shapes);
+			for( Shape s : it ) {
+				if( s.isVisible() ) {
+					s.draw(drawing);
 				}
 			}
 		}