slightly less broken

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

     private DoubleProperty currentValue = new SimpleDoubleProperty();
     private CalcEngine calcEngine = new CalcEngine();
 
-    private enum Op { NOOP, ADD, SUBTRACT, MULTIPLY, DIVIDE, EXPONENT }
+    private enum Op { NOOP, ADD, SUBTRACT, MULTIPLY, DIVIDE, EXPONENT, SQUARE, SQRROOT, INVERSE }
 
     private Op curOp   = Op.NOOP;
     private Op stackOp = Op.NOOP;
         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("%.2f", currentValue));
         return screen;
     }
 
                 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);
+            } else if ("xⁿ".equals((s))) {
+                makeSingleIntegerButtons(button);
+            } else if ("x²".equals((s))) {
+                makeSingleIntegerButtons(button);
+            } else if ("√x".equals((s))) {
+                makeSingleIntegerButtons(button);
+            } else if ("x⁻¹".equals((s))) {
+                makeSingleIntegerButtons(button);
             }
         }
 
             case "-": triggerOp.set(Op.SUBTRACT); break;
             case "*": triggerOp.set(Op.MULTIPLY); break;
             case "/": triggerOp.set(Op.DIVIDE);   break;
+            case "xⁿ": triggerOp.set(Op.EXPONENT); 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 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>() {
                     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;
+                    case EXPONENT: currentValue.set(calcEngine.exp(previousValue.get(),currentValue.get())); break;
                 }
             }
         });
     }
-}
-
 
+    private void makeSingleIntegerButtons(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 SQRROOT: currentValue.set(calcEngine.sqrt(currentValue.get())); break;
+                    case INVERSE: currentValue.set(calcEngine.inv(currentValue.get())); break;
+                }
+            }
+        });
+    }
+}

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

 
 // You probably should make NO changes here. Keep it simple.
 public class MainApplication {
-    public static void main(String[] args){
+    public static void main(String[] args) {
         CalcSkin.launchCalc(args);
     }