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..a9753f5b 100644
--- a/app/src/main/java/htw/berlin/prog2/ha1/Calculator.java
+++ b/app/src/main/java/htw/berlin/prog2/ha1/Calculator.java
@@ -31,7 +31,7 @@ public String readScreen() {
     public void pressDigitKey(int digit) {
         if(digit > 9 || digit < 0) throw new IllegalArgumentException();
 
-        if(screen.equals("0") || latestValue == Double.parseDouble(screen)) screen = "";
+        if(screen.equals("0") || latestValue == Double.parseDouble(screen) && !screen.endsWith(".")) screen = "";
 
         screen = screen + digit;
     }
@@ -104,7 +104,13 @@ public void pressDotKey() {
      * entfernt und der Inhalt fortan als positiv interpretiert.
      */
     public void pressNegativeKey() {
-        screen = screen.startsWith("-") ? screen.substring(1) : "-" + screen;
+        if (screen.startsWith("-")){
+            screen.substring(1);
+        } else if (latestOperation.equals("-") || latestOperation.equals("+")){
+            screen = "-" + "0";
+        } else {
+            screen = "-" + screen;
+        }
     }
 
     /**
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..8138e3cb 100644
--- a/app/src/test/java/htw/berlin/prog2/ha1/CalculatorTest.java
+++ b/app/src/test/java/htw/berlin/prog2/ha1/CalculatorTest.java
@@ -40,6 +40,59 @@ void testSquareRoot() {
         assertEquals(expected, actual);
     }
 
-    //TODO hier weitere Tests erstellen
+    @Test
+    @DisplayName("should display result after subtracting two decimal numbers")
+    void testDecimalSubtraction() {
+        Calculator calc = new Calculator();
+
+        calc.pressDigitKey(2);
+        calc.pressDotKey();
+        calc.pressDigitKey(1);
+        calc.pressBinaryOperationKey("-");
+        calc.pressDigitKey(2);
+        calc.pressDotKey();
+        calc.pressDigitKey(1);
+        calc.pressEqualsKey();
+
+        String expected = "0";
+        String actual = calc.readScreen();
+
+        assertEquals(expected, actual);
+    }
+
+    @Test
+    @DisplayName("should display result after subtracting a positive number and a negative number")
+    void testPositiveSubtraction(){
+        Calculator calc = new Calculator();
+
+        calc.pressDigitKey(5);
+        calc.pressBinaryOperationKey("-");
+        calc.pressNegativeKey();
+        calc.pressDigitKey(2);
+        calc.pressEqualsKey();
+
+        String expected = "7";
+        String actual = calc.readScreen();
+
+        assertEquals(expected, actual);
+    }
+
+    @Test
+    @DisplayName("should display result after multiplying a decimal number and a natural number")
+    void testDecimalNumbers() {
+        Calculator calc = new Calculator();
+
+        calc.pressDigitKey(0);
+        calc.pressDotKey();
+        calc.pressDigitKey(9);
+        calc.pressBinaryOperationKey("x");
+        calc.pressDigitKey(3);
+        calc.pressEqualsKey();
+
+        String expected = "2.7";
+        String actual = calc.readScreen();
+
+        assertEquals(expected, actual);
+    }
 }