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 aaef8862..2a1526f5 100644
--- a/app/src/main/java/htw/berlin/prog2/ha1/Calculator.java
+++ b/app/src/main/java/htw/berlin/prog2/ha1/Calculator.java
@@ -14,13 +14,21 @@ public class Calculator {
 
     private String latestOperation = "";
 
+
     /**
      * @return den aktuellen Bildschirminhalt als String
      */
     public String readScreen() {
+
+        if(screen.equals("NaN")){
+            screen = "Error";
+        }
+
         return screen;
     }
 
+
+
     /**
      * Empfängt den Wert einer gedrückten Zifferntaste. Da man nur eine Taste auf einmal
      * drücken kann muss der Wert positiv und einstellig sein und zwischen 0 und 9 liegen.
@@ -34,6 +42,7 @@ public void pressDigitKey(int digit) {
         if(screen.equals("0") || latestValue == Double.parseDouble(screen)) screen = "";
 
         screen = screen + digit;
+
     }
 
     /**
@@ -60,6 +69,11 @@ public void pressClearKey() {
      * @param operation "+" für Addition, "-" für Substraktion, "x" für Multiplikation, "/" für Division
      */
     public void pressBinaryOperationKey(String operation)  {
+
+        if (!(latestOperation.equals(""))) {
+            pressEqualsKey();
+        }
+
         latestValue = Double.parseDouble(screen);
         latestOperation = operation;
     }
@@ -72,17 +86,19 @@ public void pressBinaryOperationKey(String operation)  {
      * @param operation "√" für Quadratwurzel, "%" für Prozent, "1/x" für Inversion
      */
     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);
             default -> throw new IllegalArgumentException();
         };
+
         screen = Double.toString(result);
         if(screen.contains(".") && screen.length() > 11) screen = screen.substring(0, 10);
-
     }
 
     /**
@@ -104,7 +120,14 @@ public void pressDotKey() {
      * entfernt und der Inhalt fortan als positiv interpretiert.
      */
     public void pressNegativeKey() {
-        screen = screen.startsWith("-") ? screen.substring(1) : "-" + screen;
+        //screen = screen.startsWith("-") ? screen.substring(1) : "-" + screen;
+
+        if(screen.startsWith("-")){
+            screen = screen.substring(1);
+
+        }   else{
+                screen = "-" + screen;
+        }
     }
 
     /**
@@ -117,6 +140,7 @@ 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);
@@ -128,4 +152,5 @@ 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 addc5f26..273dc471 100644
--- a/app/src/test/java/htw/berlin/prog2/ha1/CalculatorTest.java
+++ b/app/src/test/java/htw/berlin/prog2/ha1/CalculatorTest.java
@@ -4,6 +4,7 @@
 import org.junit.jupiter.api.Test;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 @DisplayName("Retro calculator")
 class CalculatorTest {
@@ -41,5 +42,54 @@ void testSquareRoot() {
     }
 
     //TODO hier weitere Tests erstellen
+
+    @Test
+    @DisplayName("should display result after adding a dot")
+    void testDot(){
+        Calculator calc = new Calculator();
+
+        calc.pressDigitKey(5);
+        calc.pressDotKey();
+        calc.pressDigitKey(1);
+
+        String expected = "5.1";
+        String actual = calc.readScreen();
+
+        assertEquals(expected, actual);
+    }
+
+
+    @Test
+    @DisplayName("should display result after adding three positive single-digit numbers")
+    void testAdditionThree(){
+        Calculator calc = new Calculator();
+
+        calc.pressDigitKey(5);
+        calc.pressBinaryOperationKey("+");
+        calc.pressDigitKey(1);
+        calc.pressBinaryOperationKey("+");
+        calc.pressDigitKey(4);
+        calc.pressEqualsKey();
+
+        String expected = "10";
+        String actual = calc.readScreen();
+
+        assertEquals(expected, actual);
+    }
+
+    @Test
+    @DisplayName("should display text after getting the square root of minus two")
+    void testNegaitveSquareRoot(){
+        Calculator calc = new Calculator();
+
+        calc.pressDigitKey(2);
+        calc.pressNegativeKey();
+        calc.pressUnaryOperationKey("√");
+
+        String expected = "Error";
+        String actual = calc.readScreen();
+
+        assertEquals(expected, actual);
+    }
 }