diff --git a/app/src/main/java/calculator/simplemobiletools/com/simple_calculator/CalculatorImpl.java b/app/src/main/java/calculator/simplemobiletools/com/simple_calculator/CalculatorImpl.java
index fbde6dc..ee17b17 100644
--- a/app/src/main/java/calculator/simplemobiletools/com/simple_calculator/CalculatorImpl.java
+++ b/app/src/main/java/calculator/simplemobiletools/com/simple_calculator/CalculatorImpl.java
@@ -6,8 +6,8 @@ public class CalculatorImpl {
     private double baseValue;
     private double secondValue;
     private boolean resetValue;
-    private int lastKey;
-    private int lastOperation;
+    private String lastKey;
+    private String lastOperation;
     private Calculator callback;
 
     public CalculatorImpl(Calculator calculatorInterface) {
@@ -26,12 +26,12 @@ public class CalculatorImpl {
         baseValue = 0;
         secondValue = 0;
         resetValue = false;
-        lastKey = 0;
-        lastOperation = 0;
+        lastKey = "";
+        lastOperation = "";
         callback.setValue("0");
     }
 
-    public void setLastKey(int lastKey) {
+    public void setLastKey(String lastKey) {
         this.lastKey = lastKey;
     }
 
@@ -116,18 +116,18 @@ public class CalculatorImpl {
         updateResult(resultValue);
     }
 
-    public void handleOperation(int operation) {
-        if (lastKey == operation)
+    public void handleOperation(String operation) {
+        if (lastKey.equals(operation))
             return;
 
-        if (lastKey == Constants.DIGIT)
+        if (lastKey.equals(Constants.DIGIT))
             handleResult();
 
         resetValue = true;
         lastKey = operation;
         lastOperation = operation;
 
-        if (operation == Constants.ROOT)
+        if (operation.equals(Constants.ROOT))
             calculateResult();
     }
 
@@ -156,10 +156,10 @@ public class CalculatorImpl {
     }
 
     public void handleEquals() {
-        if (lastKey == Constants.EQUALS)
+        if (lastKey.equals(Constants.EQUALS))
             calculateResult();
 
-        if (lastKey != Constants.DIGIT)
+        if (!lastKey.equals(Constants.DIGIT))
             return;
 
         secondValue = getDisplayedNumberAsDouble();
@@ -182,7 +182,7 @@ public class CalculatorImpl {
     }
 
     public void numpadClicked(View view) {
-        if (lastKey == Constants.EQUALS)
+        if (lastKey.equals(Constants.EQUALS))
             lastOperation = Constants.EQUALS;
         lastKey = Constants.DIGIT;
         resetValueIfNeeded();
diff --git a/app/src/main/java/calculator/simplemobiletools/com/simple_calculator/Constants.java b/app/src/main/java/calculator/simplemobiletools/com/simple_calculator/Constants.java
index 07551b4..f9d1ff9 100644
--- a/app/src/main/java/calculator/simplemobiletools/com/simple_calculator/Constants.java
+++ b/app/src/main/java/calculator/simplemobiletools/com/simple_calculator/Constants.java
@@ -1,13 +1,13 @@
 package calculator.simplemobiletools.com.simple_calculator;
 
 public class Constants {
-    public static final int DIGIT = 0;
-    public static final int EQUALS = 1;
-    public static final int PLUS = 2;
-    public static final int MINUS = 3;
-    public static final int MULTIPLY = 4;
-    public static final int DIVIDE = 5;
-    public static final int MODULO = 6;
-    public static final int POWER = 7;
-    public static final int ROOT = 8;
+    public static final String DIGIT = "digit";
+    public static final String EQUALS = "equals";
+    public static final String PLUS = "plus";
+    public static final String MINUS = "minus";
+    public static final String MULTIPLY = "multiply";
+    public static final String DIVIDE = "divide";
+    public static final String MODULO = "modulo";
+    public static final String POWER = "power";
+    public static final String ROOT = "root";
 }
diff --git a/app/src/test/java/calculator/simplemobiletools/com/simple_calculator/MainActivityTest.java b/app/src/test/java/calculator/simplemobiletools/com/simple_calculator/MainActivityTest.java
index 04e615b..e14b44b 100644
--- a/app/src/test/java/calculator/simplemobiletools/com/simple_calculator/MainActivityTest.java
+++ b/app/src/test/java/calculator/simplemobiletools/com/simple_calculator/MainActivityTest.java
@@ -148,11 +148,11 @@ public class MainActivityTest {
         activity.setValueDouble(d);
     }
 
-    private void handleOperation(int operation) {
+    private void handleOperation(String operation) {
         activity.getCalc().handleOperation(operation);
     }
 
-    private String calcResult(double baseValue, int operation, double secondValue) {
+    private String calcResult(double baseValue, String operation, double secondValue) {
         setDouble(baseValue);
         handleOperation(operation);
         setDouble(secondValue);