Update Matrix.java

src/main/java/matrix/Matrix.java

@@ -57,7 +57,9 @@ public interface Matrix<T> extends Iterable<T> {
     this.set(coordinate.x(), coordinate.y(), newValue);
   }
 
-  Matrix<T> subMatrix(Coordinate corner, int width, int height);
+  default Matrix<T> subMatrix(Coordinate corner, int width, int height){
+    return null ;
+  }
 
 
   /**