Add error style to result display

src/main/java/seedu/address/ui/MainWindow.java

@@ -181,7 +181,7 @@ private CommandResult executeCommand(String commandText) throws CommandException
         try {
             CommandResult commandResult = logic.execute(commandText);
             logger.info("Result: " + commandResult.getFeedbackToUser());
-            resultDisplay.setFeedbackToUser(commandResult.getFeedbackToUser());
+            resultDisplay.setFeedbackSuccessToUser(commandResult.getFeedbackToUser());
 
             if (commandResult.isShowHelp()) {
                 handleHelp();
@@ -194,7 +194,7 @@ private CommandResult executeCommand(String commandText) throws CommandException
             return commandResult;
         } catch (CommandException | ParseException e) {
             logger.info("Invalid command: " + commandText);
-            resultDisplay.setFeedbackToUser(e.getMessage());
+            resultDisplay.setFeedbackErrorToUser(e.getMessage());
             throw e;
         }
     }

src/main/java/seedu/address/ui/ResultDisplay.java

@@ -2,6 +2,7 @@
 
 import static java.util.Objects.requireNonNull;
 
+import javafx.collections.ObservableList;
 import javafx.fxml.FXML;
 import javafx.scene.control.TextArea;
 import javafx.scene.layout.Region;
@@ -11,6 +12,8 @@
  */
 public class ResultDisplay extends UiPart<Region> {
 
+    public static final String ERROR_STYLE_CLASS = "error";
+
     private static final String FXML = "ResultDisplay.fxml";
 
     @FXML
@@ -20,7 +23,37 @@ public ResultDisplay() {
         super(FXML);
     }
 
-    public void setFeedbackToUser(String feedbackToUser) {
+    /**
+     * Sets the command box style to use the default style.
+     */
+    private void setStyleToDefault() {
+        resultDisplay.getStyleClass().remove(ERROR_STYLE_CLASS);
+    }
+
+    /**
+     * Sets the command box style to indicate a failed command.
+     */
+    private void setStyleToIndicateCommandFailure() {
+        ObservableList<String> styleClass = resultDisplay.getStyleClass();
+
+        if (styleClass.contains(ERROR_STYLE_CLASS)) {
+            return;
+        }
+
+        styleClass.add(ERROR_STYLE_CLASS);
+    }
+
+    public void setFeedbackErrorToUser(String feedbackToUser) {
+        setStyleToIndicateCommandFailure();
+        setFeedbackToUser(feedbackToUser);
+    }
+
+    public void setFeedbackSuccessToUser(String feedbackToUser) {
+        setStyleToDefault();
+        setFeedbackToUser(feedbackToUser);
+    }
+
+    private void setFeedbackToUser(String feedbackToUser) {
         requireNonNull(feedbackToUser);
         resultDisplay.setText(feedbackToUser);
     }

src/test/java/guitests/guihandles/ResultDisplayHandle.java

@@ -1,5 +1,6 @@
 package guitests.guihandles;
 
+import javafx.collections.ObservableList;
 import javafx.scene.control.TextArea;
 
 /**
@@ -19,4 +20,11 @@ public ResultDisplayHandle(TextArea resultDisplayNode) {
     public String getText() {
         return getRootNode().getText();
     }
+
+    /**
+     * Returns the list of style classes present in the command box.
+     */
+    public ObservableList<String> getStyleClass() {
+        return getRootNode().getStyleClass();
+    }
 }

src/test/java/seedu/address/ui/ResultDisplayTest.java

@@ -2,6 +2,8 @@
 
 import static org.junit.Assert.assertEquals;
 
+import java.util.ArrayList;
+
 import org.junit.Before;
 import org.junit.Test;
 
@@ -12,24 +14,91 @@ public class ResultDisplayTest extends GuiUnitTest {
     private ResultDisplay resultDisplay;
     private ResultDisplayHandle resultDisplayHandle;
 
+    private ArrayList<String> defaultStyleOfResultDisplay;
+    private ArrayList<String> errorStyleOfResultDisplay;
+
     @Before
     public void setUp() {
         resultDisplay = new ResultDisplay();
         uiPartRule.setUiPart(resultDisplay);
 
         resultDisplayHandle = new ResultDisplayHandle(getChildNode(resultDisplay.getRoot(),
                 ResultDisplayHandle.RESULT_DISPLAY_ID));
+
+        defaultStyleOfResultDisplay = new ArrayList<>(resultDisplayHandle.getStyleClass());
+
+        errorStyleOfResultDisplay = new ArrayList<>(defaultStyleOfResultDisplay);
+        errorStyleOfResultDisplay.add(CommandBox.ERROR_STYLE_CLASS);
     }
 
     @Test
-    public void display() {
+    public void defaultDisplay_emptyDisplay() {
         // default result text
         guiRobot.pauseForHuman();
         assertEquals("", resultDisplayHandle.getText());
+    }
+
+    @Test
+    public void emptyFeedback_emptyDisplay() {
+        guiRobot.interact(() -> resultDisplay.setFeedbackSuccessToUser(""));
+        guiRobot.pauseForHuman();
+        assertEquals("", resultDisplayHandle.getText());
+        assertEquals(defaultStyleOfResultDisplay, resultDisplayHandle.getStyleClass());
+    }
+    @Test
+    public void successfulFeedback() {
+        guiRobot.interact(() -> resultDisplay.setFeedbackSuccessToUser("Dummy feedback to user"));
+        guiRobot.pauseForHuman();
+        assertEquals("Dummy feedback to user", resultDisplayHandle.getText());
+        assertStyleForSuccessfulFeedback(resultDisplayHandle);
+    }
+
+    @Test
+    public void errorFeedback() {
+        guiRobot.interact(() -> resultDisplay.setFeedbackErrorToUser("Dummy feedback to user"));
+        guiRobot.pauseForHuman();
+        assertEquals("Dummy feedback to user", resultDisplayHandle.getText());
+        assertStyleForErrorFeedback(resultDisplayHandle);
+    }
+
+    @Test
+    public void consecutiveFeedBack() {
+        // new error result received
+        guiRobot.interact(() -> resultDisplay.setFeedbackErrorToUser("Dummy feedback to user"));
+        guiRobot.pauseForHuman();
+        assertEquals("Dummy feedback to user", resultDisplayHandle.getText());
+        assertStyleForErrorFeedback(resultDisplayHandle);
+
+        // new error result received
+        guiRobot.interact(() -> resultDisplay.setFeedbackErrorToUser("Dummy feedback to user"));
+        guiRobot.pauseForHuman();
+        assertEquals("Dummy feedback to user", resultDisplayHandle.getText());
+        assertStyleForErrorFeedback(resultDisplayHandle);
+
+        // new success result received
+        guiRobot.interact(() -> resultDisplay.setFeedbackSuccessToUser("Dummy feedback to user"));
+        guiRobot.pauseForHuman();
+        assertEquals("Dummy feedback to user", resultDisplayHandle.getText());
+        assertStyleForSuccessfulFeedback(resultDisplayHandle);
+
+        // new success result received
+        guiRobot.interact(() -> resultDisplay.setFeedbackSuccessToUser("Dummy feedback to user"));
+        guiRobot.pauseForHuman();
+        assertEquals("Dummy feedback to user", resultDisplayHandle.getText());
+        assertStyleForSuccessfulFeedback(resultDisplayHandle);
 
-        // new result received
-        guiRobot.interact(() -> resultDisplay.setFeedbackToUser("Dummy feedback to user"));
+        // new error result received
+        guiRobot.interact(() -> resultDisplay.setFeedbackErrorToUser("Dummy feedback to user"));
         guiRobot.pauseForHuman();
         assertEquals("Dummy feedback to user", resultDisplayHandle.getText());
+        assertStyleForErrorFeedback(resultDisplayHandle);
+    }
+
+    private void assertStyleForSuccessfulFeedback(ResultDisplayHandle resultDisplayHandle) {
+        assertEquals(defaultStyleOfResultDisplay, resultDisplayHandle.getStyleClass());
+    }
+
+    private void assertStyleForErrorFeedback(ResultDisplayHandle resultDisplayHandle) {
+        assertEquals(errorStyleOfResultDisplay, resultDisplayHandle.getStyleClass());
     }
 }