diff --git a/app/src/androidTest/java/com/simplemobiletools/calculator/MainActivityTest.java b/app/src/androidTest/java/com/simplemobiletools/calculator/MainActivityTest.java
index 8ad4c0f..c38f35b 100644
--- a/app/src/androidTest/java/com/simplemobiletools/calculator/MainActivityTest.java
+++ b/app/src/androidTest/java/com/simplemobiletools/calculator/MainActivityTest.java
@@ -3,6 +3,8 @@ package com.simplemobiletools.calculator;
 import android.support.test.rule.ActivityTestRule;
 import android.support.test.runner.AndroidJUnit4;
 
+import com.simplemobiletools.calculator.activities.MainActivity;
+
 import org.junit.Rule;
 import org.junit.Test;
 import org.junit.runner.RunWith;
diff --git a/app/src/main/AndroidManifest.xml b/app/src/main/AndroidManifest.xml
index e34234c..db8fad0 100644
--- a/app/src/main/AndroidManifest.xml
+++ b/app/src/main/AndroidManifest.xml
@@ -9,7 +9,7 @@
         android:label="@string/app_name"
         android:theme="@style/AppTheme">
         <activity
-            android:name=".MainActivity"
+            android:name=".activities.MainActivity"
             android:screenOrientation="portrait">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN"/>
@@ -28,12 +28,12 @@
         </activity>
 
         <activity
-            android:name=".AboutActivity"
+            android:name=".activities.AboutActivity"
             android:label="@string/about"
             android:screenOrientation="portrait"/>
 
         <activity
-            android:name=".LicenseActivity"
+            android:name=".activities.LicenseActivity"
             android:label="@string/third_party_licences"
             android:screenOrientation="portrait"/>
 
diff --git a/app/src/main/java/com/simplemobiletools/calculator/CalculatorImpl.java b/app/src/main/java/com/simplemobiletools/calculator/CalculatorImpl.java
index 8cc7ddc..a1f9916 100644
--- a/app/src/main/java/com/simplemobiletools/calculator/CalculatorImpl.java
+++ b/app/src/main/java/com/simplemobiletools/calculator/CalculatorImpl.java
@@ -1,62 +1,63 @@
 package com.simplemobiletools.calculator;
 
 public class CalculatorImpl {
-    private String displayedValue;
-    private String displayedFormula;
-    private double baseValue;
-    private double secondValue;
-    private boolean resetValue;
-    private String lastKey;
-    private String lastOperation;
-    private Calculator callback;
-    private boolean isFirstOperation;
+    private String mDisplayedValue;
+    private String mDisplayedFormula;
+    private String mLastKey;
+    private String mLastOperation;
+    private Calculator mCallback;
+
+    private boolean mIsFirstOperation;
+    private boolean mResetValue;
+    private double mBaseValue;
+    private double mSecondValue;
 
     public CalculatorImpl(Calculator calculator) {
-        callback = calculator;
+        mCallback = calculator;
         resetValues();
         setValue("0");
         setFormula("");
     }
 
     public CalculatorImpl(Calculator calculatorInterface, String value) {
-        callback = calculatorInterface;
+        mCallback = calculatorInterface;
         resetValues();
-        displayedValue = value;
+        mDisplayedValue = value;
         setFormula("");
     }
 
     private void resetValueIfNeeded() {
-        if (resetValue)
-            displayedValue = "0";
+        if (mResetValue)
+            mDisplayedValue = "0";
 
-        resetValue = false;
+        mResetValue = false;
     }
 
     private void resetValues() {
-        baseValue = 0;
-        secondValue = 0;
-        resetValue = false;
-        lastKey = "";
-        lastOperation = "";
-        displayedValue = "";
-        displayedFormula = "";
-        isFirstOperation = true;
+        mBaseValue = 0;
+        mSecondValue = 0;
+        mResetValue = false;
+        mLastKey = "";
+        mLastOperation = "";
+        mDisplayedValue = "";
+        mDisplayedFormula = "";
+        mIsFirstOperation = true;
     }
 
     public void setValue(String value) {
-        callback.setValue(value);
-        displayedValue = value;
+        mCallback.setValue(value);
+        mDisplayedValue = value;
     }
 
     private void setFormula(String value) {
-        callback.setFormula(value);
-        displayedFormula = value;
+        mCallback.setFormula(value);
+        mDisplayedFormula = value;
     }
 
     private void updateFormula() {
-        final String first = Formatter.doubleToString(baseValue);
-        final String second = Formatter.doubleToString(secondValue);
-        final String sign = getSign(lastOperation);
+        final String first = Formatter.doubleToString(mBaseValue);
+        final String second = Formatter.doubleToString(mSecondValue);
+        final String sign = getSign(mLastOperation);
 
         if (sign.equals("√")) {
             setFormula(sign + first);
@@ -65,8 +66,8 @@ public class CalculatorImpl {
         }
     }
 
-    public void setLastKey(String lastKey) {
-        this.lastKey = lastKey;
+    public void setLastKey(String mLastKey) {
+        this.mLastKey = mLastKey;
     }
 
     public void addDigit(int number) {
@@ -87,11 +88,11 @@ public class CalculatorImpl {
 
     private void updateResult(double value) {
         setValue(Formatter.doubleToString(value));
-        baseValue = value;
+        mBaseValue = value;
     }
 
     public String getDisplayedNumber() {
-        return displayedValue;
+        return mDisplayedValue;
     }
 
     public double getDisplayedNumberAsDouble() {
@@ -99,28 +100,28 @@ public class CalculatorImpl {
     }
 
     public String getDisplayedFormula() {
-        return displayedFormula;
+        return mDisplayedFormula;
     }
 
     public void handleResult() {
-        secondValue = getDisplayedNumberAsDouble();
+        mSecondValue = getDisplayedNumberAsDouble();
         calculateResult();
-        baseValue = getDisplayedNumberAsDouble();
+        mBaseValue = getDisplayedNumberAsDouble();
     }
 
     public void calculateResult() {
-        if (!isFirstOperation)
+        if (!mIsFirstOperation)
             updateFormula();
 
-        switch (lastOperation) {
+        switch (mLastOperation) {
             case Constants.PLUS:
-                updateResult(baseValue + secondValue);
+                updateResult(mBaseValue + mSecondValue);
                 break;
             case Constants.MINUS:
-                updateResult(baseValue - secondValue);
+                updateResult(mBaseValue - mSecondValue);
                 break;
             case Constants.MULTIPLY:
-                updateResult(baseValue * secondValue);
+                updateResult(mBaseValue * mSecondValue);
                 break;
             case Constants.DIVIDE:
                 divideNumbers();
@@ -132,44 +133,44 @@ public class CalculatorImpl {
                 powerNumbers();
                 break;
             case Constants.ROOT:
-                updateResult(Math.sqrt(baseValue));
+                updateResult(Math.sqrt(mBaseValue));
                 break;
             default:
                 break;
         }
-        isFirstOperation = false;
+        mIsFirstOperation = false;
     }
 
     private void divideNumbers() {
         double resultValue = 0;
-        if (secondValue != 0)
-            resultValue = baseValue / secondValue;
+        if (mSecondValue != 0)
+            resultValue = mBaseValue / mSecondValue;
 
         updateResult(resultValue);
     }
 
     private void moduloNumbers() {
         double resultValue = 0;
-        if (secondValue != 0)
-            resultValue = baseValue % secondValue;
+        if (mSecondValue != 0)
+            resultValue = mBaseValue % mSecondValue;
 
         updateResult(resultValue);
     }
 
     private void powerNumbers() {
-        double resultValue = Math.pow(baseValue, secondValue);
+        double resultValue = Math.pow(mBaseValue, mSecondValue);
         if (Double.isInfinite(resultValue) || Double.isNaN(resultValue))
             resultValue = 0;
         updateResult(resultValue);
     }
 
     public void handleOperation(String operation) {
-        if (lastKey.equals(Constants.DIGIT))
+        if (mLastKey.equals(Constants.DIGIT))
             handleResult();
 
-        resetValue = true;
-        lastKey = operation;
-        lastOperation = operation;
+        mResetValue = true;
+        mLastKey = operation;
+        mLastOperation = operation;
 
         if (operation.equals(Constants.ROOT))
             calculateResult();
@@ -177,7 +178,7 @@ public class CalculatorImpl {
 
     public void handleClear() {
         final String oldValue = getDisplayedNumber();
-        String newValue;
+        String newValue = "0";
         final int len = oldValue.length();
         int minLen = 1;
         if (oldValue.contains("-"))
@@ -185,13 +186,11 @@ public class CalculatorImpl {
 
         if (len > minLen)
             newValue = oldValue.substring(0, len - 1);
-        else
-            newValue = "0";
 
         newValue = newValue.replaceAll("\\.$", "");
         newValue = formatString(newValue);
         setValue(newValue);
-        baseValue = Formatter.stringToDouble(newValue);
+        mBaseValue = Formatter.stringToDouble(newValue);
     }
 
     public void handleReset() {
@@ -201,15 +200,15 @@ public class CalculatorImpl {
     }
 
     public void handleEquals() {
-        if (lastKey.equals(Constants.EQUALS))
+        if (mLastKey.equals(Constants.EQUALS))
             calculateResult();
 
-        if (!lastKey.equals(Constants.DIGIT))
+        if (!mLastKey.equals(Constants.DIGIT))
             return;
 
-        secondValue = getDisplayedNumberAsDouble();
+        mSecondValue = getDisplayedNumberAsDouble();
         calculateResult();
-        lastKey = Constants.EQUALS;
+        mLastKey = Constants.EQUALS;
     }
 
     public void decimalClicked() {
@@ -246,9 +245,9 @@ public class CalculatorImpl {
     }
 
     public void numpadClicked(int id) {
-        if (lastKey.equals(Constants.EQUALS))
-            lastOperation = Constants.EQUALS;
-        lastKey = Constants.DIGIT;
+        if (mLastKey.equals(Constants.EQUALS))
+            mLastOperation = Constants.EQUALS;
+        mLastKey = Constants.DIGIT;
         resetValueIfNeeded();
 
         switch (id) {
diff --git a/app/src/main/java/com/simplemobiletools/calculator/MyWidgetConfigure.java b/app/src/main/java/com/simplemobiletools/calculator/MyWidgetConfigure.java
index 317fb5d..1715cde 100644
--- a/app/src/main/java/com/simplemobiletools/calculator/MyWidgetConfigure.java
+++ b/app/src/main/java/com/simplemobiletools/calculator/MyWidgetConfigure.java
@@ -19,20 +19,20 @@ import butterknife.OnClick;
 import yuku.ambilwarna.AmbilWarnaDialog;
 
 public class MyWidgetConfigure extends AppCompatActivity {
-    @BindView(R.id.btn_reset) View resetBtn;
-    @BindView(R.id.config_bg_color) View bgColorPicker;
-    @BindView(R.id.config_bg_seekbar) SeekBar bgSeekBar;
-    @BindView(R.id.config_text_color) View textColorPicker;
-    @BindView(R.id.config_calc) View background;
-    @BindView(R.id.config_save) Button saveBtn;
-    @BindView(R.id.result) TextView result;
-    @BindView(R.id.formula) TextView formula;
-    private int widgetId;
+    @BindView(R.id.btn_reset) View mResetBtn;
+    @BindView(R.id.config_bg_color) View mBgColorPicker;
+    @BindView(R.id.config_bg_seekbar) SeekBar mBgSeekBar;
+    @BindView(R.id.config_text_color) View mTextColorPicker;
+    @BindView(R.id.config_calc) View mBackground;
+    @BindView(R.id.config_save) Button mSaveBtn;
+    @BindView(R.id.result) TextView mResult;
+    @BindView(R.id.formula) TextView mFormula;
 
-    private int bgColor;
-    private int bgColorWithoutTransparency;
-    private float bgAlpha;
-    private int textColor;
+    private int mBgColor;
+    private int mBgColorWithoutTransparency;
+    private int mWidgetId;
+    private int mTextColor;
+    private float mBgAlpha;
 
     @Override
     public void onCreate(Bundle savedInstanceState) {
@@ -45,98 +45,98 @@ public class MyWidgetConfigure extends AppCompatActivity {
         final Intent intent = getIntent();
         final Bundle extras = intent.getExtras();
         if (extras != null)
-            widgetId = extras.getInt(AppWidgetManager.EXTRA_APPWIDGET_ID, AppWidgetManager.INVALID_APPWIDGET_ID);
+            mWidgetId = extras.getInt(AppWidgetManager.EXTRA_APPWIDGET_ID, AppWidgetManager.INVALID_APPWIDGET_ID);
 
-        if (widgetId == AppWidgetManager.INVALID_APPWIDGET_ID)
+        if (mWidgetId == AppWidgetManager.INVALID_APPWIDGET_ID)
             finish();
     }
 
     private void initVariables() {
         final SharedPreferences prefs = getSharedPreferences(Constants.PREFS, Context.MODE_PRIVATE);
-        bgColor = prefs.getInt(Constants.WIDGET_BG_COLOR, 1);
-        if (bgColor == 1) {
-            bgColor = Color.BLACK;
-            bgAlpha = .2f;
+        mBgColor = prefs.getInt(Constants.WIDGET_BG_COLOR, 1);
+        if (mBgColor == 1) {
+            mBgColor = Color.BLACK;
+            mBgAlpha = .2f;
         } else {
-            bgAlpha = Color.alpha(bgColor) / (float) 255;
+            mBgAlpha = Color.alpha(mBgColor) / (float) 255;
         }
 
-        resetBtn.setVisibility(View.VISIBLE);
-        bgColorWithoutTransparency = Color.rgb(Color.red(bgColor), Color.green(bgColor), Color.blue(bgColor));
-        bgSeekBar.setOnSeekBarChangeListener(bgSeekbarChangeListener);
-        bgSeekBar.setProgress((int) (bgAlpha * 100));
+        mResetBtn.setVisibility(View.VISIBLE);
+        mBgColorWithoutTransparency = Color.rgb(Color.red(mBgColor), Color.green(mBgColor), Color.blue(mBgColor));
+        mBgSeekBar.setOnSeekBarChangeListener(bgSeekbarChangeListener);
+        mBgSeekBar.setProgress((int) (mBgAlpha * 100));
         updateBackgroundColor();
 
-        textColor = prefs.getInt(Constants.WIDGET_TEXT_COLOR, getResources().getColor(R.color.colorPrimary));
+        mTextColor = prefs.getInt(Constants.WIDGET_TEXT_COLOR, getResources().getColor(R.color.colorPrimary));
         updateTextColor();
 
-        formula.setText("15,937*5");
-        result.setText("79,685");
+        mFormula.setText("15,937*5");
+        mResult.setText("79,685");
     }
 
     @OnClick(R.id.config_save)
     public void saveConfig() {
         final AppWidgetManager appWidgetManager = AppWidgetManager.getInstance(this);
         final RemoteViews views = new RemoteViews(getPackageName(), R.layout.activity_main);
-        views.setInt(R.id.calculator_holder, "setBackgroundColor", bgColor);
-        appWidgetManager.updateAppWidget(widgetId, views);
+        views.setInt(R.id.calculator_holder, "setBackgroundColor", mBgColor);
+        appWidgetManager.updateAppWidget(mWidgetId, views);
 
         storeWidgetBackground();
         requestWidgetUpdate();
 
         final Intent resultValue = new Intent();
-        resultValue.putExtra(AppWidgetManager.EXTRA_APPWIDGET_ID, widgetId);
+        resultValue.putExtra(AppWidgetManager.EXTRA_APPWIDGET_ID, mWidgetId);
         setResult(RESULT_OK, resultValue);
         finish();
     }
 
     private void storeWidgetBackground() {
         final SharedPreferences prefs = getSharedPreferences(Constants.PREFS, Context.MODE_PRIVATE);
-        prefs.edit().putInt(Constants.WIDGET_BG_COLOR, bgColor).apply();
-        prefs.edit().putInt(Constants.WIDGET_TEXT_COLOR, textColor).apply();
+        prefs.edit().putInt(Constants.WIDGET_BG_COLOR, mBgColor).apply();
+        prefs.edit().putInt(Constants.WIDGET_TEXT_COLOR, mTextColor).apply();
     }
 
     private void requestWidgetUpdate() {
         final Intent intent = new Intent(AppWidgetManager.ACTION_APPWIDGET_UPDATE, null, this, MyWidgetProvider.class);
-        intent.putExtra(AppWidgetManager.EXTRA_APPWIDGET_IDS, new int[]{widgetId});
+        intent.putExtra(AppWidgetManager.EXTRA_APPWIDGET_IDS, new int[]{mWidgetId});
         sendBroadcast(intent);
     }
 
     private void updateBackgroundColor() {
-        bgColor = adjustAlpha(bgColorWithoutTransparency, bgAlpha);
-        background.setBackgroundColor(bgColor);
-        bgColorPicker.setBackgroundColor(bgColor);
-        saveBtn.setBackgroundColor(bgColor);
+        mBgColor = adjustAlpha(mBgColorWithoutTransparency, mBgAlpha);
+        mBackground.setBackgroundColor(mBgColor);
+        mBgColorPicker.setBackgroundColor(mBgColor);
+        mSaveBtn.setBackgroundColor(mBgColor);
     }
 
     private void updateTextColor() {
-        textColorPicker.setBackgroundColor(textColor);
-        saveBtn.setTextColor(textColor);
+        mTextColorPicker.setBackgroundColor(mTextColor);
+        mSaveBtn.setTextColor(mTextColor);
 
         int[] viewIds =
                 new int[]{R.id.btn_0, R.id.btn_1, R.id.btn_2, R.id.btn_3, R.id.btn_4, R.id.btn_5, R.id.btn_6, R.id.btn_7, R.id.btn_8,
                         R.id.btn_9, R.id.btn_modulo, R.id.btn_power, R.id.btn_root, R.id.btn_clear, R.id.btn_reset, R.id.btn_divide,
                         R.id.btn_multiply, R.id.btn_minus, R.id.btn_plus, R.id.btn_decimal, R.id.btn_equals};
-        result.setTextColor(textColor);
-        formula.setTextColor(textColor);
+        mResult.setTextColor(mTextColor);
+        mFormula.setTextColor(mTextColor);
 
         Button btn;
         for (int i : viewIds) {
             btn = (Button) findViewById(i);
-            btn.setTextColor(textColor);
+            btn.setTextColor(mTextColor);
         }
     }
 
     @OnClick(R.id.config_bg_color)
     public void pickBackgroundColor() {
-        AmbilWarnaDialog dialog = new AmbilWarnaDialog(this, bgColorWithoutTransparency, new AmbilWarnaDialog.OnAmbilWarnaListener() {
+        AmbilWarnaDialog dialog = new AmbilWarnaDialog(this, mBgColorWithoutTransparency, new AmbilWarnaDialog.OnAmbilWarnaListener() {
             @Override
             public void onCancel(AmbilWarnaDialog dialog) {
             }
 
             @Override
             public void onOk(AmbilWarnaDialog dialog, int color) {
-                bgColorWithoutTransparency = color;
+                mBgColorWithoutTransparency = color;
                 updateBackgroundColor();
             }
         });
@@ -146,14 +146,14 @@ public class MyWidgetConfigure extends AppCompatActivity {
 
     @OnClick(R.id.config_text_color)
     public void pickTextColor() {
-        AmbilWarnaDialog dialog = new AmbilWarnaDialog(this, textColor, new AmbilWarnaDialog.OnAmbilWarnaListener() {
+        AmbilWarnaDialog dialog = new AmbilWarnaDialog(this, mTextColor, new AmbilWarnaDialog.OnAmbilWarnaListener() {
             @Override
             public void onCancel(AmbilWarnaDialog dialog) {
             }
 
             @Override
             public void onOk(AmbilWarnaDialog dialog, int color) {
-                textColor = color;
+                mTextColor = color;
                 updateTextColor();
             }
         });
@@ -164,7 +164,7 @@ public class MyWidgetConfigure extends AppCompatActivity {
     private SeekBar.OnSeekBarChangeListener bgSeekbarChangeListener = new SeekBar.OnSeekBarChangeListener() {
         @Override
         public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
-            bgAlpha = (float) progress / (float) 100;
+            mBgAlpha = (float) progress / (float) 100;
             updateBackgroundColor();
         }
 
diff --git a/app/src/main/java/com/simplemobiletools/calculator/MyWidgetProvider.java b/app/src/main/java/com/simplemobiletools/calculator/MyWidgetProvider.java
index b00f06a..049b220 100644
--- a/app/src/main/java/com/simplemobiletools/calculator/MyWidgetProvider.java
+++ b/app/src/main/java/com/simplemobiletools/calculator/MyWidgetProvider.java
@@ -11,20 +11,23 @@ import android.graphics.Color;
 import android.view.View;
 import android.widget.RemoteViews;
 
+import com.simplemobiletools.calculator.activities.MainActivity;
+
 public class MyWidgetProvider extends AppWidgetProvider implements Calculator {
-    private static int[] widgetIds;
-    private static RemoteViews remoteViews;
-    private static CalculatorImpl calc;
-    private static AppWidgetManager widgetManager;
-    private static Intent intent;
-    private static Context cxt;
-    private static SharedPreferences prefs;
+    private static RemoteViews mRemoteViews;
+    private static CalculatorImpl mCalc;
+    private static AppWidgetManager mWidgetManager;
+    private static Intent mIntent;
+    private static Context mContext;
+    private static SharedPreferences mPrefs;
+
+    private static int[] mWidgetIds;
 
     @Override
     public void onUpdate(Context context, AppWidgetManager appWidgetManager, int[] appWidgetIds) {
         initVariables(context);
 
-        intent = new Intent(context, MyWidgetProvider.class);
+        mIntent = new Intent(context, MyWidgetProvider.class);
         setupIntent(Constants.DECIMAL, R.id.btn_decimal);
         setupIntent(Constants.ZERO, R.id.btn_0);
         setupIntent(Constants.ONE, R.id.btn_1);
@@ -56,45 +59,45 @@ public class MyWidgetProvider extends AppWidgetProvider implements Calculator {
     }
 
     private void setupIntent(String action, int id) {
-        intent.setAction(action);
-        PendingIntent pendingIntent = PendingIntent.getBroadcast(cxt, 0, intent, 0);
-        remoteViews.setOnClickPendingIntent(id, pendingIntent);
+        mIntent.setAction(action);
+        PendingIntent pendingIntent = PendingIntent.getBroadcast(mContext, 0, mIntent, 0);
+        mRemoteViews.setOnClickPendingIntent(id, pendingIntent);
     }
 
     private void setupAppOpenIntent(int id) {
-        final Intent intent = new Intent(cxt, MainActivity.class);
-        final PendingIntent pendingIntent = PendingIntent.getActivity(cxt, 0, intent, 0);
-        remoteViews.setOnClickPendingIntent(id, pendingIntent);
+        final Intent intent = new Intent(mContext, MainActivity.class);
+        final PendingIntent pendingIntent = PendingIntent.getActivity(mContext, 0, intent, 0);
+        mRemoteViews.setOnClickPendingIntent(id, pendingIntent);
     }
 
     private void initVariables(Context context) {
-        cxt = context;
+        mContext = context;
         updateWidgetIds();
-        prefs = initPrefs(cxt);
-        final int defaultColor = cxt.getResources().getColor(R.color.text_grey);
-        final int newBgColor = prefs.getInt(Constants.WIDGET_BG_COLOR, defaultColor);
-        final int newTextColor = prefs.getInt(Constants.WIDGET_TEXT_COLOR, Color.WHITE);
+        mPrefs = initPrefs(mContext);
+        final int defaultColor = mContext.getResources().getColor(R.color.text_grey);
+        final int newBgColor = mPrefs.getInt(Constants.WIDGET_BG_COLOR, defaultColor);
+        final int newTextColor = mPrefs.getInt(Constants.WIDGET_TEXT_COLOR, Color.WHITE);
 
-        remoteViews = new RemoteViews(cxt.getPackageName(), R.layout.activity_main);
-        remoteViews.setViewVisibility(R.id.btn_reset, View.VISIBLE);
-        remoteViews.setInt(R.id.calculator_holder, "setBackgroundColor", newBgColor);
+        mRemoteViews = new RemoteViews(mContext.getPackageName(), R.layout.activity_main);
+        mRemoteViews.setViewVisibility(R.id.btn_reset, View.VISIBLE);
+        mRemoteViews.setInt(R.id.calculator_holder, "setBackgroundColor", newBgColor);
 
         updateTextColors(newTextColor);
-        widgetManager = AppWidgetManager.getInstance(cxt);
+        mWidgetManager = AppWidgetManager.getInstance(mContext);
 
         final String displayValue = "0";
-        calc = new CalculatorImpl(this, displayValue);
+        mCalc = new CalculatorImpl(this, displayValue);
     }
 
     private void updateWidgetIds() {
-        final ComponentName component = new ComponentName(cxt, MyWidgetProvider.class);
-        widgetManager = AppWidgetManager.getInstance(cxt);
-        widgetIds = widgetManager.getAppWidgetIds(component);
+        final ComponentName component = new ComponentName(mContext, MyWidgetProvider.class);
+        mWidgetManager = AppWidgetManager.getInstance(mContext);
+        mWidgetIds = mWidgetManager.getAppWidgetIds(component);
     }
 
     private void updateWidget() {
-        for (int widgetId : widgetIds) {
-            widgetManager.updateAppWidget(widgetId, remoteViews);
+        for (int widgetId : mWidgetIds) {
+            mWidgetManager.updateAppWidget(widgetId, mRemoteViews);
         }
     }
 
@@ -103,13 +106,13 @@ public class MyWidgetProvider extends AppWidgetProvider implements Calculator {
     }
 
     private void updateTextColors(int color) {
-        int[] viewIds =
+        final int[] viewIds =
                 new int[]{R.id.formula, R.id.result, R.id.btn_0, R.id.btn_1, R.id.btn_2, R.id.btn_3, R.id.btn_4, R.id.btn_5, R.id.btn_6,
                         R.id.btn_7, R.id.btn_8, R.id.btn_9, R.id.btn_modulo, R.id.btn_power, R.id.btn_root, R.id.btn_clear, R.id.btn_reset,
                         R.id.btn_divide, R.id.btn_multiply, R.id.btn_minus, R.id.btn_plus, R.id.btn_decimal, R.id.btn_equals};
 
         for (int i : viewIds) {
-            remoteViews.setInt(i, "setTextColor", color);
+            mRemoteViews.setInt(i, "setTextColor", color);
         }
     }
 
@@ -146,52 +149,52 @@ public class MyWidgetProvider extends AppWidgetProvider implements Calculator {
     }
 
     private void myAction(String action, Context context) {
-        if (calc == null || remoteViews == null || widgetManager == null || prefs == null || cxt == null) {
+        if (mCalc == null || mRemoteViews == null || mWidgetManager == null || mPrefs == null || mContext == null) {
             initVariables(context);
         }
 
         switch (action) {
             case Constants.DECIMAL:
-                calc.numpadClicked(R.id.btn_decimal);
+                mCalc.numpadClicked(R.id.btn_decimal);
                 break;
             case Constants.ZERO:
-                calc.numpadClicked(R.id.btn_0);
+                mCalc.numpadClicked(R.id.btn_0);
                 break;
             case Constants.ONE:
-                calc.numpadClicked(R.id.btn_1);
+                mCalc.numpadClicked(R.id.btn_1);
                 break;
             case Constants.TWO:
-                calc.numpadClicked(R.id.btn_2);
+                mCalc.numpadClicked(R.id.btn_2);
                 break;
             case Constants.THREE:
-                calc.numpadClicked(R.id.btn_3);
+                mCalc.numpadClicked(R.id.btn_3);
                 break;
             case Constants.FOUR:
-                calc.numpadClicked(R.id.btn_4);
+                mCalc.numpadClicked(R.id.btn_4);
                 break;
             case Constants.FIVE:
-                calc.numpadClicked(R.id.btn_5);
+                mCalc.numpadClicked(R.id.btn_5);
                 break;
             case Constants.SIX:
-                calc.numpadClicked(R.id.btn_6);
+                mCalc.numpadClicked(R.id.btn_6);
                 break;
             case Constants.SEVEN:
-                calc.numpadClicked(R.id.btn_7);
+                mCalc.numpadClicked(R.id.btn_7);
                 break;
             case Constants.EIGHT:
-                calc.numpadClicked(R.id.btn_8);
+                mCalc.numpadClicked(R.id.btn_8);
                 break;
             case Constants.NINE:
-                calc.numpadClicked(R.id.btn_9);
+                mCalc.numpadClicked(R.id.btn_9);
                 break;
             case Constants.EQUALS:
-                calc.handleEquals();
+                mCalc.handleEquals();
                 break;
             case Constants.CLEAR:
-                calc.handleClear();
+                mCalc.handleClear();
                 break;
             case Constants.RESET:
-                calc.handleReset();
+                mCalc.handleReset();
                 break;
             case Constants.PLUS:
             case Constants.MINUS:
@@ -200,7 +203,7 @@ public class MyWidgetProvider extends AppWidgetProvider implements Calculator {
             case Constants.MODULO:
             case Constants.POWER:
             case Constants.ROOT:
-                calc.handleOperation(action);
+                mCalc.handleOperation(action);
                 break;
             default:
                 break;
@@ -209,7 +212,7 @@ public class MyWidgetProvider extends AppWidgetProvider implements Calculator {
 
     @Override
     public void setValue(String value) {
-        remoteViews.setTextViewText(R.id.result, value);
+        mRemoteViews.setTextViewText(R.id.result, value);
         updateWidget();
     }
 
@@ -218,16 +221,15 @@ public class MyWidgetProvider extends AppWidgetProvider implements Calculator {
 
     }
 
-    @Override
     public void setFormula(String value) {
-        remoteViews.setTextViewText(R.id.formula, value);
+        mRemoteViews.setTextViewText(R.id.formula, value);
         updateWidget();
     }
 
     @Override
     public void onDeleted(Context context, int[] appWidgetIds) {
         super.onDeleted(context, appWidgetIds);
-        if (cxt != null)
+        if (mContext != null)
             updateWidgetIds();
     }
 }
diff --git a/app/src/main/java/com/simplemobiletools/calculator/AboutActivity.java b/app/src/main/java/com/simplemobiletools/calculator/activities/AboutActivity.java
similarity index 58%
rename from app/src/main/java/com/simplemobiletools/calculator/AboutActivity.java
rename to app/src/main/java/com/simplemobiletools/calculator/activities/AboutActivity.java
index 6ffb6ed..9976cc0 100644
--- a/app/src/main/java/com/simplemobiletools/calculator/AboutActivity.java
+++ b/app/src/main/java/com/simplemobiletools/calculator/activities/AboutActivity.java
@@ -1,13 +1,16 @@
-package com.simplemobiletools.calculator;
+package com.simplemobiletools.calculator.activities;
 
 import android.content.Intent;
 import android.content.res.Resources;
 import android.os.Bundle;
 import android.support.v7.app.AppCompatActivity;
+import android.support.v7.appcompat.BuildConfig;
 import android.text.Html;
 import android.text.method.LinkMovementMethod;
 import android.widget.TextView;
 
+import com.simplemobiletools.calculator.R;
+
 import java.util.Calendar;
 
 import butterknife.BindView;
@@ -15,17 +18,17 @@ import butterknife.ButterKnife;
 import butterknife.OnClick;
 
 public class AboutActivity extends AppCompatActivity {
-    @BindView(R.id.about_copyright) TextView copyright;
-    @BindView(R.id.about_version) TextView version;
-    @BindView(R.id.about_email) TextView emailTV;
-    private Resources res;
+    @BindView(R.id.about_copyright) TextView mCopyright;
+    @BindView(R.id.about_version) TextView mVersion;
+    @BindView(R.id.about_email) TextView mEmailTV;
+    private static Resources mRes;
 
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_about);
         ButterKnife.bind(this);
-        res = getResources();
+        mRes = getResources();
 
         setupEmail();
         setupVersion();
@@ -33,23 +36,23 @@ public class AboutActivity extends AppCompatActivity {
     }
 
     private void setupEmail() {
-        final String email = res.getString(R.string.email);
-        final String appName = res.getString(R.string.app_name);
+        final String email = mRes.getString(R.string.email);
+        final String appName = mRes.getString(R.string.app_name);
         final String href = "<a href=\"mailto:" + email + "?subject=" + appName + "\">" + email + "</a>";
-        emailTV.setText(Html.fromHtml(href));
-        emailTV.setMovementMethod(LinkMovementMethod.getInstance());
+        mEmailTV.setText(Html.fromHtml(href));
+        mEmailTV.setMovementMethod(LinkMovementMethod.getInstance());
     }
 
     private void setupVersion() {
         final String versionName = BuildConfig.VERSION_NAME;
-        final String versionText = String.format(res.getString(R.string.version), versionName);
-        version.setText(versionText);
+        final String versionText = String.format(mRes.getString(R.string.version), versionName);
+        mVersion.setText(versionText);
     }
 
     private void setupCopyright() {
         final int year = Calendar.getInstance().get(Calendar.YEAR);
-        final String copyrightText = String.format(res.getString(R.string.copyright), year);
-        copyright.setText(copyrightText);
+        final String copyrightText = String.format(mRes.getString(R.string.copyright), year);
+        mCopyright.setText(copyrightText);
     }
 
     @OnClick(R.id.about_license)
diff --git a/app/src/main/java/com/simplemobiletools/calculator/LicenseActivity.java b/app/src/main/java/com/simplemobiletools/calculator/activities/LicenseActivity.java
similarity index 93%
rename from app/src/main/java/com/simplemobiletools/calculator/LicenseActivity.java
rename to app/src/main/java/com/simplemobiletools/calculator/activities/LicenseActivity.java
index 9d87b44..5618b4c 100644
--- a/app/src/main/java/com/simplemobiletools/calculator/LicenseActivity.java
+++ b/app/src/main/java/com/simplemobiletools/calculator/activities/LicenseActivity.java
@@ -1,14 +1,17 @@
-package com.simplemobiletools.calculator;
+package com.simplemobiletools.calculator.activities;
 
 import android.content.Intent;
 import android.net.Uri;
 import android.os.Bundle;
 import android.support.v7.app.AppCompatActivity;
 
+import com.simplemobiletools.calculator.R;
+
 import butterknife.ButterKnife;
 import butterknife.OnClick;
 
 public class LicenseActivity extends AppCompatActivity {
+
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
diff --git a/app/src/main/java/com/simplemobiletools/calculator/MainActivity.java b/app/src/main/java/com/simplemobiletools/calculator/activities/MainActivity.java
similarity index 66%
rename from app/src/main/java/com/simplemobiletools/calculator/MainActivity.java
rename to app/src/main/java/com/simplemobiletools/calculator/activities/MainActivity.java
index 72486b2..71d52a7 100644
--- a/app/src/main/java/com/simplemobiletools/calculator/MainActivity.java
+++ b/app/src/main/java/com/simplemobiletools/calculator/activities/MainActivity.java
@@ -1,4 +1,4 @@
-package com.simplemobiletools.calculator;
+package com.simplemobiletools.calculator.activities;
 
 import android.content.ClipData;
 import android.content.ClipboardManager;
@@ -7,11 +7,17 @@ import android.content.res.Resources;
 import android.os.Bundle;
 import android.support.v7.app.AppCompatActivity;
 import android.view.Menu;
-import android.view.MenuInflater;
 import android.view.MenuItem;
 import android.view.View;
 import android.widget.TextView;
 
+import com.simplemobiletools.calculator.Calculator;
+import com.simplemobiletools.calculator.CalculatorImpl;
+import com.simplemobiletools.calculator.Constants;
+import com.simplemobiletools.calculator.Formatter;
+import com.simplemobiletools.calculator.R;
+import com.simplemobiletools.calculator.Utils;
+
 import butterknife.BindView;
 import butterknife.ButterKnife;
 import butterknife.OnClick;
@@ -19,26 +25,26 @@ import butterknife.OnLongClick;
 import me.grantland.widget.AutofitHelper;
 
 public class MainActivity extends AppCompatActivity implements Calculator {
-    @BindView(R.id.result) TextView result;
-    @BindView(R.id.formula) TextView formula;
+    @BindView(R.id.result) TextView mResult;
+    @BindView(R.id.formula) TextView mFormula;
 
-    private CalculatorImpl calc;
+    private static CalculatorImpl mCalc;
 
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
         ButterKnife.bind(this);
-        calc = new CalculatorImpl(this);
+
+        mCalc = new CalculatorImpl(this);
         setupResultView();
-        AutofitHelper.create(result);
-        AutofitHelper.create(formula);
+        AutofitHelper.create(mResult);
+        AutofitHelper.create(mFormula);
     }
 
     @Override
     public boolean onCreateOptionsMenu(Menu menu) {
-        MenuInflater inflater = getMenuInflater();
-        inflater.inflate(R.menu.menu, menu);
+        getMenuInflater().inflate(R.menu.menu, menu);
         return true;
     }
 
@@ -56,62 +62,62 @@ public class MainActivity extends AppCompatActivity implements Calculator {
 
     private void setupResultView() {
         final Resources res = getResources();
-        result.setBackgroundColor(res.getColor(android.R.color.white));
-        result.setTextColor(res.getColor(R.color.text_grey));
+        mResult.setBackgroundColor(res.getColor(android.R.color.white));
+        mResult.setTextColor(res.getColor(R.color.text_grey));
 
-        formula.setBackgroundColor(res.getColor(android.R.color.white));
-        formula.setTextColor(res.getColor(R.color.text_grey));
+        mFormula.setBackgroundColor(res.getColor(android.R.color.white));
+        mFormula.setTextColor(res.getColor(R.color.text_grey));
     }
 
     @OnClick(R.id.btn_plus)
     public void plusClicked() {
-        calc.handleOperation(Constants.PLUS);
+        mCalc.handleOperation(Constants.PLUS);
     }
 
     @OnClick(R.id.btn_minus)
     public void minusClicked() {
-        calc.handleOperation(Constants.MINUS);
+        mCalc.handleOperation(Constants.MINUS);
     }
 
     @OnClick(R.id.btn_multiply)
     public void multiplyClicked() {
-        calc.handleOperation(Constants.MULTIPLY);
+        mCalc.handleOperation(Constants.MULTIPLY);
     }
 
     @OnClick(R.id.btn_divide)
     public void divideClicked() {
-        calc.handleOperation(Constants.DIVIDE);
+        mCalc.handleOperation(Constants.DIVIDE);
     }
 
     @OnClick(R.id.btn_modulo)
     public void moduloClicked() {
-        calc.handleOperation(Constants.MODULO);
+        mCalc.handleOperation(Constants.MODULO);
     }
 
     @OnClick(R.id.btn_power)
     public void powerClicked() {
-        calc.handleOperation(Constants.POWER);
+        mCalc.handleOperation(Constants.POWER);
     }
 
     @OnClick(R.id.btn_root)
     public void rootClicked() {
-        calc.handleOperation(Constants.ROOT);
+        mCalc.handleOperation(Constants.ROOT);
     }
 
     @OnClick(R.id.btn_clear)
     public void clearClicked() {
-        calc.handleClear();
+        mCalc.handleClear();
     }
 
     @OnLongClick(R.id.btn_clear)
     public boolean clearLongClicked() {
-        calc.handleReset();
+        mCalc.handleReset();
         return true;
     }
 
     @OnClick(R.id.btn_equals)
     public void equalsClicked() {
-        calc.handleEquals();
+        mCalc.handleEquals();
     }
 
     @OnClick({R.id.btn_decimal, R.id.btn_0, R.id.btn_1, R.id.btn_2, R.id.btn_3, R.id.btn_4, R.id.btn_5, R.id.btn_6, R.id.btn_7, R.id.btn_8,
@@ -121,7 +127,7 @@ public class MainActivity extends AppCompatActivity implements Calculator {
     }
 
     public void numpadClicked(int id) {
-        calc.numpadClicked(id);
+        mCalc.numpadClicked(id);
     }
 
     @OnLongClick(R.id.formula)
@@ -137,11 +143,9 @@ public class MainActivity extends AppCompatActivity implements Calculator {
     }
 
     private void copyToClipboard(boolean copyResult) {
-        String value;
+        String value = mFormula.getText().toString();
         if (copyResult) {
-            value = result.getText().toString();
-        } else {
-            value = formula.getText().toString();
+            value = mResult.getText().toString();
         }
 
         final ClipboardManager clipboard = (ClipboardManager) getSystemService(CLIPBOARD_SERVICE);
@@ -152,22 +156,21 @@ public class MainActivity extends AppCompatActivity implements Calculator {
 
     @Override
     public void setValue(String value) {
-        result.setText(value);
+        mResult.setText(value);
     }
 
     // used only by Robolectric
     @Override
     public void setValueDouble(double d) {
-        calc.setValue(Formatter.doubleToString(d));
-        calc.setLastKey(Constants.DIGIT);
+        mCalc.setValue(Formatter.doubleToString(d));
+        mCalc.setLastKey(Constants.DIGIT);
     }
 
-    @Override
     public void setFormula(String value) {
-        formula.setText(value);
+        mFormula.setText(value);
     }
 
     public CalculatorImpl getCalc() {
-        return calc;
+        return mCalc;
     }
 }
diff --git a/app/src/main/res/layout/activity_main.xml b/app/src/main/res/layout/activity_main.xml
index 11143b0..91270e8 100644
--- a/app/src/main/res/layout/activity_main.xml
+++ b/app/src/main/res/layout/activity_main.xml
@@ -8,7 +8,7 @@
     android:background="@color/buttons_background"
     android:orientation="vertical"
     android:theme="@style/MainTheme"
-    tools:context=".MainActivity">
+    tools:context=".activities.MainActivity">
 
     <TextView
         android:id="@+id/formula"