Fix bug where user can enter a negative duration for DurationTask

Author: w2vgd

src/main/java/popo/parser/Parser.java

@@ -2,6 +2,7 @@
 
 import static popo.utils.Messages.MESSAGE_EMPTY_DESCRIPTION;
 import static popo.utils.Messages.MESSAGE_ENTER_COMMAND;
+import static popo.utils.Messages.MESSAGE_ENTER_POSITIVE_DURATION;
 import static popo.utils.Messages.MESSAGE_FOLLOW_USAGE;
 import static popo.utils.Messages.MESSAGE_INDICATE_TASK;
 import static popo.utils.Messages.MESSAGE_INVALID_COMMAND;
@@ -211,6 +212,9 @@ private Command parseArgumentsForDuration(String arguments) throws NoDescription
             String durationTaskName = durationInputArr[0].strip();
             String durationString = durationInputArr[1].strip();
             long amount = Long.parseLong(durationString);
+            if (amount < 0) {
+                throw new InvalidDescriptionException(MESSAGE_ENTER_POSITIVE_DURATION);
+            }
             Duration duration = Duration.of(amount, unit);
             return new DurationCommand(durationTaskName, duration);
         } catch (NumberFormatException ex) {

src/main/java/popo/utils/Messages.java

@@ -7,6 +7,7 @@ public class Messages {
     // Messages to show to user upon a parse error.
     public static final String MESSAGE_EMPTY_DESCRIPTION = "The description of a task cannot be empty.";
     public static final String MESSAGE_ENTER_COMMAND = "Please enter a command.";
+    public static final String MESSAGE_ENTER_POSITIVE_DURATION = "Please enter a positive duration!";
     public static final String MESSAGE_FOLLOW_USAGE = "Please follow the usage as shown below:";
     public static final String MESSAGE_INDICATE_TASK = "Please indicate a task number.";
     public static final String MESSAGE_INVALID_COMMAND = "OOPS!!! I'm sorry, but I don't know what that means :-(";