Stage1 creational

src/Calc/App.java

@@ -7,8 +7,15 @@
 
 public class App {
 
-    public static void main(String[] args) {
-        new Calculator().setVisible(true);
+    public static void main(String[] args) {  
+        //deleting this line to test the singleton --> new Calculator().setVisible(true);
+        //testing the sigelton 
+        javax.swing.SwingUtilities.invokeLater(() -> {
+            Calculator calculator1 = Calculator.getInstance();
+            Calculator calculator2 = Calculator.getInstance();
+            System.out.println("same instance? " + (calculator1 == calculator2));
+            calculator1.setVisible(true);
+        });
     }
 
 }

src/Calc/Calculator.java

@@ -13,10 +13,61 @@ public final class Calculator extends javax.swing.JFrame {
     private String currentOperand;
     private String previousOperand;
     private String operation;
-
     private int x, y;
+
+        // --- Singleton Method ---
+    private static Calculator INSTANCE;
+
+    public static Calculator getInstance() {
+        if (INSTANCE == null) {
+            INSTANCE = new Calculator();
+        }
+        return INSTANCE;
+    }
+
+    // --- Factory Method ---
+    private interface Operation { float apply(float a, float b); }
+
+    private static class AddOperation implements Operation { 
+        @Override
+    public float apply(float a,float b){
+        return a+b;} 
+}
+
+    private static class SubOperation implements Operation { 
+        @Override
+        public float apply(float a,float b){
+            return a-b;}
+    }
+
+    private static class MultOperation implements Operation { 
+        @Override
+        public float apply(float a,float b){
+            return a*b;} 
+    }
+
+    private static class DivOperation  implements Operation { 
+        @Override
+        public float apply(float a,float b){
+            if (b == 0f) {
+                throw new ArithmeticException("Error! Division by zero is not accepted");
+            }
+            return a/b;} 
+    }
 
-    public Calculator() {
+    private static class OperationFactory {
+        static Operation getOperation(String op) {
+            switch (op) {
+                case "+": return new AddOperation();
+                case "-": return new SubOperation();
+                case "×": return new MultOperation();
+                case "÷": return new DivOperation();
+                default: throw new IllegalArgumentException("Unknown operation: " + op);
+            }
+        }
+    }
+
+    private Calculator() {
         initComponents();
         getContentPane().setSize(400, 700);
         this.clear();
@@ -109,17 +160,13 @@ public void chooseOperation(String operation) {
     }
 
     public void compute() {
-        float computation;
+
         if (this.currentOperand.equals("") || this.previousOperand.equals("")) {
             return;
         }
-
-        float curr = Float.parseFloat(this.currentOperand);
-        float prev = Float.parseFloat(this.previousOperand);
-        if (Float.isNaN(curr) || Float.isNaN(prev)) {
-            return;
-        }
-
+
+        //deleting the switch, we already have the operation interface
+        /*..
         switch (this.operation) {
             case "+" ->
                 computation = prev + curr;
@@ -139,8 +186,23 @@ public void compute() {
                 return;
             }
         }
-
-        this.currentOperand = (computation - (int) computation) != 0 ? Float.toString(computation) : Integer.toString((int) computation);
+..*/
+        try {
+            float curr = Float.parseFloat(this.currentOperand);
+            float prev = Float.parseFloat(this.previousOperand);
+            Operation op = OperationFactory.getOperation(this.operation);
+            float result  = op.apply(prev, curr);
+
+             this.currentOperand = (result  - (int) result ) != 0 
+                ? Float.toString(result ) 
+                : Integer.toString((int) result ); 
+
+        } catch (IllegalArgumentException | ArithmeticException ex) {
+            this.clear();
+            this.currentOperand = "Error";
+            return;
+        }
+
         this.previousOperand = "";
         this.operation = "";
     }