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..00bf6146 100644
--- a/app/src/main/java/htw/berlin/prog2/ha1/Calculator.java
+++ b/app/src/main/java/htw/berlin/prog2/ha1/Calculator.java
@@ -60,10 +60,19 @@ public void pressClearKey() {
      * @param operation "+" für Addition, "-" für Substraktion, "x" für Multiplikation, "/" für Division
      */
     public void pressBinaryOperationKey(String operation)  {
-        latestValue = Double.parseDouble(screen);
-        latestOperation = operation;
+        if(!latestOperation.isEmpty()) { //fix1
+            pressEqualsKey();
+        }
+        if (operation.equals("+") || operation.equals("-") || operation.equals("x") || operation.equals("/")) { //fix2
+            latestValue = Double.parseDouble(screen);
+            latestOperation = operation;
+        } else {
+            screen = "Error";
+        }
     }
 
+
+
     /**
      * Empfängt den Wert einer gedrückten unären Operationstaste, also eine der drei Operationen
      * Quadratwurzel, Prozent, Inversion, welche nur einen Operanden benötigen.
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..bb06f053 100644
--- a/app/src/test/java/htw/berlin/prog2/ha1/CalculatorTest.java
+++ b/app/src/test/java/htw/berlin/prog2/ha1/CalculatorTest.java
@@ -90,5 +90,55 @@ void testMultipleDecimalDots() {
 
 
     //TODO hier weitere Tests erstellen
+
+    @Test
+    @DisplayName("should display result after subtracting two positive multi-digit numbers")
+    void testPositiveSubtraction() {
+        Calculator calc = new Calculator();
+
+        calc.pressDigitKey(2);
+        calc.pressDigitKey(0);
+        calc.pressBinaryOperationKey("-");
+        calc.pressDigitKey(2);
+        calc.pressDigitKey(0);
+        calc.pressEqualsKey();
+
+        String expected = "0";
+        String actual = calc.readScreen();
+
+        assertEquals(expected, actual);
+    }
+
+    @Test
+    @DisplayName("should display result after adding three positive multi-digit numbers")
+    void testdoublePositiveAddition() {
+        Calculator calc = new Calculator();
+
+        calc.pressDigitKey(2);
+        calc.pressBinaryOperationKey("+");
+        calc.pressDigitKey(3);
+        calc.pressBinaryOperationKey("+");
+        calc.pressDigitKey(5);
+        calc.pressEqualsKey();
+
+        String expected = "10";
+        String actual = calc.readScreen();
+
+        assertEquals(expected, actual);
+    }
+
+    @Test
+    @DisplayName("should display Error when attempting an unsupported operation")
+    void testUnsupportedOperation() {
+        Calculator calc = new Calculator();
+
+        calc.pressDigitKey(2);
+        calc.pressBinaryOperationKey("^");
+
+        String expected = "Error";
+        String actual = calc.readScreen();
+
+        assertEquals(expected, actual);
+    }
 }