diff --git a/app/src/main/java/htw/berlin/prog2/ha1/Calculator.java b/app/src/main/java/htw/berlin/prog2/ha1/Calculator.java
index 84c04f2..4215c2c 100644
--- a/app/src/main/java/htw/berlin/prog2/ha1/Calculator.java
+++ b/app/src/main/java/htw/berlin/prog2/ha1/Calculator.java
@@ -9,10 +9,10 @@
 public class Calculator {
 
     private String screen = "0";
-
-    private double latestValue;
-
+    private Double latestValue = 0.0;
     private String latestOperation = "";
+    private Double lastOperand;
+    private String lastOperator;
 
     /**
      * @return den aktuellen Bildschirminhalt als String
@@ -62,6 +62,8 @@ public void pressClearKey() {
     public void pressBinaryOperationKey(String operation)  {
         latestValue = Double.parseDouble(screen);
         latestOperation = operation;
+        lastOperand = null;
+        lastOperator = latestOperation;
     }
 
     /**
@@ -75,15 +77,15 @@ public void pressUnaryOperationKey(String operation) {
         latestValue = Double.parseDouble(screen);
         latestOperation = operation;
         var result = switch(operation) {
-            case "√" -> Math.sqrt(Double.parseDouble(screen));
-            case "%" -> Double.parseDouble(screen) / 100;
-            case "1/x" -> 1 / Double.parseDouble(screen);
+            case "√" -> Math.sqrt(latestValue);
+            case "%" -> latestValue/ 100;
+            case "1/x" -> 1 / latestValue;
+            case "±" -> -latestValue;
             default -> throw new IllegalArgumentException();
         };
         screen = Double.toString(result);
         if(screen.equals("NaN")) screen = "Error";
         if(screen.contains(".") && screen.length() > 11) screen = screen.substring(0, 10);
-
     }
 
     /**
@@ -118,16 +120,27 @@ public void pressNegativeKey() {
      * und das Ergebnis direkt angezeigt.
      */
     public void pressEqualsKey() {
-        var result = switch(latestOperation) {
-            case "+" -> latestValue + Double.parseDouble(screen);
-            case "-" -> latestValue - Double.parseDouble(screen);
-            case "x" -> latestValue * Double.parseDouble(screen);
-            case "/" -> latestValue / Double.parseDouble(screen);
+        // Falls keine gültige Operation vorhanden ist, nichts tun
+        if (latestOperation == null && lastOperator == null) {
+            return;
+        }
+        // Letzten Operanden und Operator speichern, falls noch nicht gesetzt
+        if (lastOperand == null) {
+            lastOperand = Double.parseDouble(screen);
+            lastOperator = latestOperation;
+        }
+        var result = switch(lastOperator) {
+            case "+" -> latestValue + lastOperand;
+            case "-" -> latestValue - lastOperand;
+            case "x" -> latestValue * lastOperand;
+            case "/" -> (lastOperand != 0) ? latestValue / lastOperand : Double.POSITIVE_INFINITY;
             default -> throw new IllegalArgumentException();
         };
         screen = Double.toString(result);
         if(screen.equals("Infinity")) screen = "Error";
         if(screen.endsWith(".0")) screen = screen.substring(0,screen.length()-2);
         if(screen.contains(".") && screen.length() > 11) screen = screen.substring(0, 10);
+
+        latestValue = result;
     }
-}
+}
\ No newline at end of file
diff --git a/app/src/test/java/htw/berlin/prog2/ha1/CalculatorTest.java b/app/src/test/java/htw/berlin/prog2/ha1/CalculatorTest.java
index ddff0da..a4611b7 100644
--- a/app/src/test/java/htw/berlin/prog2/ha1/CalculatorTest.java
+++ b/app/src/test/java/htw/berlin/prog2/ha1/CalculatorTest.java
@@ -89,6 +89,55 @@ void testMultipleDecimalDots() {
     }
 
 
-    //TODO hier weitere Tests erstellen
+   @Test
+   @DisplayName(value = "should display the product of the numbers")
+    void testProductOfNumbers() {
+        Calculator calc = new Calculator();
+
+        calc.pressDigitKey(6);
+        calc.pressDigitKey(0);
+        calc.pressBinaryOperationKey("x");
+        calc.pressDigitKey(4);
+        calc.pressEqualsKey();
+
+        String expected = "240";
+        String actual = calc.readScreen();
+
+        assertEquals(expected, actual);
+   }
+
+    @Test
+    @DisplayName("should display correct result when equals is pressed multiple times after subtraction")
+    void testRepeatedEqualsSubtraction() {
+        Calculator calc = new Calculator();
+
+        calc.pressDigitKey(5);
+        calc.pressBinaryOperationKey("-");
+        calc.pressDigitKey(1);
+        calc.pressEqualsKey();  // Ergebnis sollte 4 sein
+        calc.pressEqualsKey();  // Ergebnis sollte 3 sein
+        calc.pressEqualsKey();  // Ergebnis sollte 2 sein
+
+        String expected = "2"; // Erwartetes Ergebnis nach dreimaligem Drücken von "=": 5 - 1 - 1 - 1 = 2
+        String actual = calc.readScreen();
+
+        assertEquals(expected, actual);
+    }
+
+    @Test
+    @DisplayName("should change the sign of the number when pressing the negation key")
+    void testNegationKey() {
+        Calculator calc = new Calculator();
+
+        calc.pressDigitKey(5);
+        calc.pressDotKey();
+        calc.pressDigitKey(5);
+        calc.pressUnaryOperationKey("±");
+
+        String expected = "-5.5"; // Erwartet: Die Zahl sollte das Vorzeichen wechseln
+        String actual = calc.readScreen();
+
+        assertEquals(expected, actual);
+    }
 }