done

src/main/java/rocks/zipcode/calcskin/CalcEngine.java

@@ -6,18 +6,22 @@ public class CalcEngine {
     }
 
     public double add(double v, double v1) {
-        return Double.NaN;
+        return v + v1 ;
     }
 
-    public double subtract(double v, double v1) {
-        return Double.NaN;
-    }
+    public double subtract(double v, double v1) { return v - v1; }
 
-    public double multiply(double v, double v1) {
-        return Double.NaN;
-    }
+    public double multiply(double v, double v1) { return v * v1; }
 
     public double divide(double v, double v1) {
-        return Double.NaN;
+        return v / v1;
     }
+
+    public double square(double v){ return v*v;}
+
+    public double sqrt(double v){ return Math.sqrt(v);}
+
+    public double inverse(double v){
+        double inv = (double)1/v;
+        return inv; }
 }

src/main/java/rocks/zipcode/calcskin/CalcSkin.java

@@ -24,10 +24,10 @@ public class CalcSkin extends Application {
         launch(args);
     }
     private static final String[][] template = {
-            { "7", "8", "9", "/" },
-            { "4", "5", "6", "*" },
-            { "1", "2", "3", "-" },
-            { "0", "c", "=", "+" }
+            { "7", "8", "9", "/", "x^2"},
+            { "4", "5", "6", "*", "sqrt"},
+            { "1", "2", "3", "-", ""},
+            { "0", "c", "=", "+"}
     };
 
     private final Map<String, Button> accelerators = new HashMap<>();
@@ -84,14 +84,14 @@ public class CalcSkin extends Application {
         screen.setStyle("-fx-background-color: aquamarine;");
         screen.setAlignment(Pos.CENTER_RIGHT);
         screen.setEditable(false);
-        screen.textProperty().bind(Bindings.format("%.0f", currentValue));
+        screen.textProperty().bind(Bindings.format("%.3f", currentValue));
         return screen;
     }
 
     private TilePane createButtons() {
         TilePane buttons = new TilePane();
-        buttons.setVgap(7);
-        buttons.setHgap(7);
+        buttons.setVgap(10);
+        buttons.setHgap(10);
         buttons.setPrefColumns(template[0].length);
         for (String[] r: template) {
             for (String s: r) {
@@ -114,6 +114,12 @@ public class CalcSkin extends Application {
                 makeClearButton(button);
             } else if ("=".equals(s)) {
                 makeEqualsButton(button);
+            } else if("x^2".equals(s)){
+                makeSquareButton(button);
+            } else if("sqrt".equals(s)){
+                makeSqrtButton(button);
+            } else if("1/x".equals(s)){
+                makeInverseButton(button);
             }
         }
 
@@ -175,6 +181,39 @@ public class CalcSkin extends Application {
         });
     }
 
+    private void makeSquareButton(Button button){
+        button.setStyle("-fx-base: ghostwhite;");
+        button.setOnAction(new EventHandler<ActionEvent>() {
+            @Override
+            public void handle(ActionEvent actionEvent) {
+                currentValue.set(calcEngine.square(currentValue.get()));
+            }
+        });
+
+    }
+
+    private void makeInverseButton(Button button){
+        button.setStyle("-fx-base: ghostwhite;");
+        button.setOnAction(new EventHandler<ActionEvent>() {
+            @Override
+            public void handle(ActionEvent actionEvent) {
+                currentValue.set(calcEngine.inverse(currentValue.get()));
+            }
+        });
+
+    }
+
+    private void makeSqrtButton(Button button){
+        button.setStyle("-fx-base: ghostwhite;");
+        button.setOnAction(new EventHandler<ActionEvent>() {
+            @Override
+            public void handle(ActionEvent actionEvent) {
+                currentValue.set(calcEngine.sqrt(currentValue.get()));
+            }
+        });
+
+    }
+
     private void makeEqualsButton(Button button) {
         button.setStyle("-fx-base: ghostwhite;");
         button.setOnAction(new EventHandler<ActionEvent>() {