Link up the evaluation and the UI buttons.

src/main/java/org/nwapw/abacus/fx/AbacusController.java

@@ -1,10 +1,25 @@
 package org.nwapw.abacus.fx;
 
 import javafx.fxml.FXML;
+import javafx.scene.control.Button;
+import javafx.scene.control.TextField;
+import javafx.scene.text.Text;
 import org.nwapw.abacus.Abacus;
+import org.nwapw.abacus.number.NumberInterface;
+import org.nwapw.abacus.tree.TreeNode;
 
 public class AbacusController {
 
+    private static final String ERR_SYNTAX = "Syntax Error";
+    private static final String ERR_EVAL = "Evaluation Error";
+
+    @FXML
+    private Text outputText;
+    @FXML
+    private TextField inputField;
+    @FXML
+    private Button inputButton;
+
     private Abacus abacus;
 
     @FXML
@@ -12,4 +27,23 @@ public class AbacusController {
         abacus = new Abacus();
     }
 
+    @FXML
+    private void performCalculation(){
+        inputButton.setDisable(true);
+        TreeNode constructedTree = abacus.parseString(inputField.getText());
+        if(constructedTree == null){
+            outputText.setText(ERR_SYNTAX);
+            inputButton.setDisable(false);
+            return;
+        }
+        NumberInterface evaluatedNumber = abacus.evaluateTree(constructedTree);
+        if(constructedTree == null){
+            outputText.setText(ERR_EVAL);
+            inputButton.setDisable(false);
+            return;
+        }
+        inputButton.setDisable(false);
+        outputText.setText(evaluatedNumber.toString());
+    }
+
 }

src/main/resources/abacus.fxml

@@ -4,6 +4,7 @@
 <?import javafx.scene.layout.*?>
 <?import javafx.geometry.Insets?>
 
+<?import javafx.scene.text.Text?>
 <BorderPane xmlns="http://javafx.com/javafx"
             xmlns:fx="http://javafx.com/fxml"
             fx:controller="org.nwapw.abacus.fx.AbacusController">
@@ -15,9 +16,10 @@
                         <VBox>
                             <ScrollPane prefHeight="50" vbarPolicy="NEVER">
                                 <padding><Insets top="10" bottom="10" left="10" right="10"/></padding>
+                                <Text fx:id="outputText"/>
                             </ScrollPane>
-                            <TextField text="Input"/>
+                            <TextField fx:id="inputField" onAction="#performCalculation"/>
-                            <Button text="Calculate" maxWidth="Infinity"/>
+                            <Button fx:id="inputButton" text="Calculate" maxWidth="Infinity" onAction="#performCalculation"/>
                         </VBox>
                     </bottom>
                 </BorderPane>