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 84c04f21..ab4452c1 100644
--- a/app/src/main/java/htw/berlin/prog2/ha1/Calculator.java
+++ b/app/src/main/java/htw/berlin/prog2/ha1/Calculator.java
@@ -60,6 +60,10 @@ public void pressClearKey() {
      * @param operation "+" für Addition, "-" für Substraktion, "x" für Multiplikation, "/" für Division
      */
     public void pressBinaryOperationKey(String operation)  {
+        // Multiple numbers
+        if (!latestOperation.isEmpty()) {
+            pressEqualsKey();  // Perform the pending operation
+        }
         latestValue = Double.parseDouble(screen);
         latestOperation = operation;
     }
@@ -103,8 +107,12 @@ public void pressDotKey() {
      * aktualisiert und die Inhalt fortan als negativ interpretiert.
      * Zeigt der Bildschirm bereits einen negativen Wert mit führendem Minus an, dann wird dieses
      * entfernt und der Inhalt fortan als positiv interpretiert.
+     * Zeigt der Bildschirm eine Null an, so wird keine Anpassung vorgenommen, da die Negation von Null immer Null ist.
      */
     public void pressNegativeKey() {
+        if (screen.equals("0")) {
+            return;
+        }
         screen = screen.startsWith("-") ? screen.substring(1) : "-" + screen;
     }
 
@@ -130,4 +138,5 @@ public void pressEqualsKey() {
         if(screen.endsWith(".0")) screen = screen.substring(0,screen.length()-2);
         if(screen.contains(".") && screen.length() > 11) screen = screen.substring(0, 10);
     }
+
 }
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 ddff0daf..51e3c9ed 100644
--- a/app/src/test/java/htw/berlin/prog2/ha1/CalculatorTest.java
+++ b/app/src/test/java/htw/berlin/prog2/ha1/CalculatorTest.java
@@ -88,7 +88,51 @@ void testMultipleDecimalDots() {
         assertEquals(expected, actual);
     }
 
+    @Test
+    @DisplayName("should display the correct result after inverting a non-zero number")
+    void testInversion() {
+        Calculator calc = new Calculator();
+
+        calc.pressDigitKey(2);
+        calc.pressUnaryOperationKey("1/x");
+
+        String expected = "0.5";
+        String actual = calc.readScreen();
+
+        assertEquals(expected, actual);
+    }
+
+    @Test
+    @DisplayName("should display zero when attempting to negate zero")
+    void testNegateZero() {
+        Calculator calc = new Calculator();
+
+        calc.pressDigitKey(0);
+        calc.pressNegativeKey();
+
+        String expected = "0";
+        String actual = calc.readScreen();
+
+        assertEquals(expected, actual);
+    }
+
+    @Test
+    @DisplayName("should display result after adding four positive one-digit numbers")
+    void testMultiplePositiveAddition() {
+        Calculator calc = new Calculator();
+
+        calc.pressDigitKey(5);
+        calc.pressBinaryOperationKey("+");
+        calc.pressDigitKey(5);
+        calc.pressBinaryOperationKey("+");
+        calc.pressDigitKey(3);
+        calc.pressEqualsKey();
+
+        String expected = "13"; // 8
+        String actual = calc.readScreen();
+
+        assertEquals(expected, actual);
+    }
 
-    //TODO hier weitere Tests erstellen
 }