use .json file extension

src/Main.java

@@ -13,7 +13,7 @@ import View.MainView;
 public class Main {
 
 	public static void main(String[] args) {
-		Reader r = new Reader( "save_err1.graph" );
+		Reader r = new Reader( "save_err1.json" );
 		LayeredGraphNode graph = r.readInputGraph();
 	    //RandomGraphGenerator r = new RandomGraphGenerator( 0.1, 0.2, 5,5, 5, 5, 2 );
 	    //LayeredGraphNode graph = r.createRandomNode( null, 0 );

src/View/MainView.java

@@ -63,7 +63,7 @@ public class MainView {
             public void keyPressed(KeyEvent e) {
                 if( e.getKeyCode() == KeyEvent.VK_S )
                 {
-                    Writer w = new Writer( "save.graph" );
+                    Writer w = new Writer( "save.json" );
                     w.writeOutputGraph( graph );
                 }
                 if( e.getKeyCode() == KeyEvent.VK_LEFT )