I broked it

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

@@ -5,19 +5,26 @@ public class CalcEngine {
     CalcEngine() {
     }
 
-    public double add(double v, double v1) {
-        return Double.NaN;
+    public double add(double x, double y) {
+        return x+y;
     }
 
-    public double subtract(double v, double v1) {
-        return Double.NaN;
+    public double subtract(double x, double y) {
+        return x-y;
     }
 
-    public double multiply(double v, double v1) {
-        return Double.NaN;
-    }
+    public double multiply(double x, double y) { return x*y; }
 
-    public double divide(double v, double v1) {
-        return Double.NaN;
+    public double divide(double x, double y) {
+        return x/y;
     }
+
+    public double squ(double x){ return Math.pow(x,2); }
+
+    public double sqrt(double x){ return Math.sqrt(x); }
+
+    public double exp(double x, double y){ return Math.pow(x,y); }
+
+    public double inv(double x){ return (1/x); }
+
 }

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", "/", "x²" },
+            { "4", "5", "6", "*", "√x" },
+            { "1", "2", "3", "-", "x⁻¹" },
+            { "0", "c", "=", "+", "xⁿ" }
     };
 
     private final Map<String, Button> accelerators = new HashMap<>();
@@ -36,7 +36,7 @@ public class CalcSkin extends Application {
     private DoubleProperty currentValue = new SimpleDoubleProperty();
     private CalcEngine calcEngine = new CalcEngine();
 
-    private enum Op { NOOP, ADD, SUBTRACT, MULTIPLY, DIVIDE }
+    private enum Op { NOOP, ADD, SUBTRACT, MULTIPLY, DIVIDE, EXPONENT }
 
     private Op curOp   = Op.NOOP;
     private Op stackOp = Op.NOOP;
@@ -114,6 +114,12 @@ public class CalcSkin extends Application {
                 makeClearButton(button);
             } else if ("=".equals(s)) {
                 makeEqualsButton(button);
+            } else if ("x²".equals(s)) {
+                makeSingleIntegerButton(button);
+            } else if ("x⁻¹".equals(s)) {
+                makeSingleIntegerButton(button);
+            } else if ("√x".equals(s)) {
+                makeSingleIntegerButton(button);
             }
         }
 
@@ -127,12 +133,16 @@ public class CalcSkin extends Application {
             case "-": triggerOp.set(Op.SUBTRACT); break;
             case "*": triggerOp.set(Op.MULTIPLY); break;
             case "/": triggerOp.set(Op.DIVIDE);   break;
+            case "x²": triggerOp.set(Op.SQUARE);  break;
+            case "√x": triggerOp.set(Op.SQRROOT);  break;
+            case "x⁻¹": triggerOp.set(Op.INVERSE);  break;
+            case "xⁿ": triggerOp.set(Op.EXPONENT);  break;
         }
         return triggerOp;
     }
 
     private void makeOperandButton(Button button, final ObjectProperty<Op> triggerOp) {
-        button.setStyle("-fx-base: lightgray;");
+        button.setStyle("-fx-base: azure;");
         button.setOnAction(new EventHandler<ActionEvent>() {
             @Override
             public void handle(ActionEvent actionEvent) {
@@ -175,6 +185,19 @@ public class CalcSkin extends Application {
         });
     }
 
+    private void makeSingleIntegerButton(Button button) {
+        button.setStyle("-fx-base: azure");
+        button.setOnAction(new EventHandler<ActionEvent>() {
+            @Override
+            public void handle(ActionEvent actionEvent) {
+            switch (stackOp) {
+                case SQUARE:    currentValue.set(calcEngine.squ(currentValue.get())); break;
+                case INVERSE:   currentValue.set(calcEngine.inv(currentValue.get())); break;
+                case SQRROOT:   currentValue.set(calcEngine.sqrt(currentValue.get())); break;
+            }
+        }});
+    }
+
     private void makeEqualsButton(Button button) {
         button.setStyle("-fx-base: ghostwhite;");
         button.setOnAction(new EventHandler<ActionEvent>() {
@@ -185,6 +208,7 @@ public class CalcSkin extends Application {
                     case SUBTRACT: currentValue.set(calcEngine.subtract(previousValue.get(), currentValue.get())); break;
                     case MULTIPLY: currentValue.set(calcEngine.multiply(previousValue.get(), currentValue.get())); break;
                     case DIVIDE:   currentValue.set(calcEngine.divide(previousValue.get(), currentValue.get())); break;
+                    case EXPONENT: currentValue.set(calcEngine.exp(previousValue.get(), currentValue.get())); break;
                 }
             }
         });

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

@@ -5,7 +5,6 @@ import rocks.zipcode.calcskin.CalcSkin;
 // You probably should make NO changes here. Keep it simple.
 public class MainApplication {
     public static void main(String[] args){
-
         CalcSkin.launchCalc(args);
     }