diff --git a/src/main/java/htw/berlin/prog2/ha1/Calculator.java b/src/main/java/htw/berlin/prog2/ha1/Calculator.java
index 84c04f2..04e9af4 100644
--- a/src/main/java/htw/berlin/prog2/ha1/Calculator.java
+++ b/src/main/java/htw/berlin/prog2/ha1/Calculator.java
@@ -33,7 +33,9 @@ public void pressDigitKey(int digit) {
 
         if(screen.equals("0") || latestValue == Double.parseDouble(screen)) screen = "";
 
+        if (screen.length() < 9) {
         screen = screen + digit;
+        }
     }
 
     /**
@@ -74,6 +76,10 @@ public void pressBinaryOperationKey(String operation)  {
     public void pressUnaryOperationKey(String operation) {
         latestValue = Double.parseDouble(screen);
         latestOperation = operation;
+        if (operation.equals("1/x") && Double.parseDouble(screen) == 0) {
+            screen = "Error";
+            return;
+        }
         var result = switch(operation) {
             case "√" -> Math.sqrt(Double.parseDouble(screen));
             case "%" -> Double.parseDouble(screen) / 100;
diff --git a/src/test/java/htw/berlin/prog2/ha1/CalculatorTest.java b/src/test/java/htw/berlin/prog2/ha1/CalculatorTest.java
index ddff0da..1a24238 100644
--- a/src/test/java/htw/berlin/prog2/ha1/CalculatorTest.java
+++ b/src/test/java/htw/berlin/prog2/ha1/CalculatorTest.java
@@ -1,10 +1,9 @@
 package htw.berlin.prog2.ha1;
 
+import static org.junit.jupiter.api.Assertions.assertEquals;
 import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.api.Test;
 
-import static org.junit.jupiter.api.Assertions.assertEquals;
-
 @DisplayName("Retro calculator")
 class CalculatorTest {
 
@@ -88,7 +87,48 @@ void testMultipleDecimalDots() {
         assertEquals(expected, actual);
     }
 
+    @Test 
+    @DisplayName("should display entered number after pressing digit keys")
+    void testEnteredNumber() {
+        Calculator calc = new Calculator();
+
+        calc.pressDigitKey(1);
+        calc.pressDigitKey(2);
+        calc.pressDigitKey(3);
+
+        String expected = "123";
+        String actual = calc.readScreen();
+
+        assertEquals(expected, actual);
+    }
+
+    @Test
+    @DisplayName("should display error when calculating inverse of zero")
+    void testInverseOfZero() {
+        Calculator calc = new Calculator();
 
-    //TODO hier weitere Tests erstellen
-}
+        calc.pressDigitKey(0);
+        calc.pressUnaryOperationKey("1/x");
+
+        String expected = "Error";
+        String actual = calc.readScreen();
+
+        assertEquals(expected, actual);
+    }
+
+    @Test
+    @DisplayName("should not allow more than 9 digits on display")
+    void testMaxDigits() {
+        Calculator calc = new Calculator();
+
+        for (int i = 0; i < 11; i++) {
+            calc.pressDigitKey(1);
+        }
+
+        String expected = "111111111";
+        String actual = calc.readScreen();
+
+        assertEquals(expected, actual);
+    }
 
+}
\ No newline at end of file