Detect scheduling anomalies

Previously, events could overlap, leading to scheduling conflicts.

Overlapping events reduce the clarity of the schedule and make task
management confusing.

Let's,
* add a method to detect overlapping events
* throw an exception if a new event clashes with another event

src/main/java/morgana/commands/AddCommand.java

@@ -7,7 +7,7 @@
 
 /**
  * Represents an abstract command to add a task to the task list.
- * Concrete subclasses should implement the {@link #createTask(String)} method
+ * Concrete subclasses should implement the {@link #createTask(String, TaskList)} method
  * to define how the task is created based on the user's input.
  */
 public abstract class AddCommand extends Command {
@@ -30,7 +30,7 @@ public AddCommand(String args) {
 
     @Override
     public String execute(TaskList tasks, Storage storage) throws MorganaException {
-        Task task = createTask(args);
+        Task task = createTask(args, tasks);
         tasks.add(task);
         storage.save(tasks);
         return MESSAGE_SUCCESS.formatted(task, tasks.size(), tasks.size() > 1 ? "s" : "");
@@ -40,10 +40,11 @@ public String execute(TaskList tasks, Storage storage) throws MorganaException {
      * Creates a task based on the provided arguments.
      *
      * @param args The string containing the task details.
+     * @param tasks TODO
      * @return The created {@code Task}.
      * @throws MorganaException If an error occurs while creating the task.
      */
-    abstract Task createTask(String args) throws MorganaException;
+    abstract Task createTask(String args, TaskList tasks) throws MorganaException;
 
     @Override
     public String getStyleClass() {

src/main/java/morgana/commands/DeadlineCommand.java

@@ -6,6 +6,7 @@
 import morgana.exceptions.MorganaException;
 import morgana.task.Deadline;
 import morgana.task.Task;
+import morgana.task.TaskList;
 
 /**
  * Represents a command to add a {@link Deadline} to the task list.
@@ -29,7 +30,7 @@ public DeadlineCommand(String args) {
     }
 
     @Override
-    Task createTask(String args) throws MorganaException {
+    Task createTask(String args, TaskList tasks) throws MorganaException {
         String[] fields = args.split(" /by ");
         if (fields.length != 2) {
             throw new MorganaException(MESSAGE_INVALID_COMMAND_FORMAT);

src/main/java/morgana/commands/EventCommand.java

@@ -3,9 +3,12 @@
 import static morgana.util.DateTimeUtil.COMPACT_PATTERN;
 import static morgana.util.DateTimeUtil.parseDateTime;
 
+import java.time.LocalDateTime;
+
 import morgana.exceptions.MorganaException;
 import morgana.task.Event;
 import morgana.task.Task;
+import morgana.task.TaskList;
 
 /**
  * Represents a command to add an {@link Event} to the task list.
@@ -22,18 +25,27 @@ public class EventCommand extends AddCommand {
     /**
      * Constructs an {@code EventCommand} with the specified arguments.
      *
-     * @param args The string containing the task description, start date/time, and end date/time.
+     * @param args The string containing the task description and time period.
      */
     public EventCommand(String args) {
         super(args);
     }
 
     @Override
-    Task createTask(String args) throws MorganaException {
+    Task createTask(String args, TaskList tasks) throws MorganaException {
         String[] fields = args.split(" /from | /to ");
         if (fields.length != 3) {
             throw new MorganaException(MESSAGE_INVALID_COMMAND_FORMAT);
         }
-        return new Event(fields[0], parseDateTime(fields[1]), parseDateTime(fields[2]));
+
+        LocalDateTime start = parseDateTime(fields[1]);
+        LocalDateTime end = parseDateTime(fields[2]);
+        assert start.isBefore(end) : "Event start time must be before end time";
+
+        Event event = tasks.findClashingEvent(start, end);
+        if (event != null) {
+            throw new MorganaException("Event clashes with another event:\n" + event);
+        }
+        return new Event(fields[0], start, end);
     }
 }

src/main/java/morgana/commands/FindCommand.java

@@ -5,7 +5,7 @@
 import morgana.task.TaskList;
 
 /**
- * Represents a command to find tasks that match a given keyword in their description.
+ * Represents a command to find tasks that contain a given keyword in their description.
  */
 public class FindCommand extends Command {
     public static final String COMMAND_WORD = "find";

src/main/java/morgana/commands/TodoCommand.java

@@ -2,6 +2,7 @@
 
 import morgana.exceptions.MorganaException;
 import morgana.task.Task;
+import morgana.task.TaskList;
 import morgana.task.Todo;
 
 /**
@@ -23,7 +24,7 @@ public TodoCommand(String args) {
     }
 
     @Override
-    Task createTask(String description) throws MorganaException {
+    Task createTask(String description, TaskList tasks) throws MorganaException {
         if (description.isEmpty()) {
             throw new MorganaException(MESSAGE_EMPTY_DESCRIPTION);
         }

src/main/java/morgana/parser/Parser.java

@@ -25,7 +25,7 @@
 /**
  * The {@code Parser} class is responsible for interpreting user input and
  * creating the corresponding {@link Command} objects. It also provides utility
- * methods for parsing task indices and date/time strings.
+ * methods for parsing task indices and task strings into {@link Task} objects.
  */
 public class Parser {
     /**
@@ -73,12 +73,12 @@ public static int parseTaskIndex(String args) throws MorganaException {
     }
 
     /**
-     * Parses a string representation of a task from a file and converts it into a {@link Task} object.
+     * Parses a task string from a file and converts it into a {@link Task} object.
      *
      * @param line The input string representing a task, as stored in a file.
      * @return The {@code Task} object created from the input string.
      */
-    public static Task parseTaskFromString(String line) throws MorganaException {
+    public static Task parseTask(String line) throws MorganaException {
         String[] fields = line.split(" \\| ");
         assert fields.length >= 3 : "Expected at least 3 fields for a task, got: " + fields.length;
 

src/main/java/morgana/storage/Storage.java

@@ -49,7 +49,7 @@ public List<Task> load() throws MorganaException {
         List<Task> tasks = new ArrayList<>();
         try (Scanner sc = new Scanner(file)) {
             while (sc.hasNextLine()) {
-                tasks.add(Parser.parseTaskFromString(sc.nextLine()));
+                tasks.add(Parser.parseTask(sc.nextLine()));
             }
         } catch (FileNotFoundException e) {
             throw new MorganaException("Failed to load tasks from file: " + e.getMessage());

src/main/java/morgana/task/Deadline.java

@@ -6,29 +6,26 @@
 import java.time.LocalDateTime;
 
 /**
- * Represents a task with a description and a deadline.
+ * Represents a task with a description and a due date.
  */
 public class Deadline extends Task {
-    private final LocalDateTime by;
+    private final LocalDateTime dueDate;
 
     /**
-     * Constructs a {@code Deadline} task with the specified description and deadline.
-     *
-     * @param description The description of the task.
-     * @param by The deadline of the task.
+     * Constructs a {@code Deadline} with the specified description and due date.
      */
-    public Deadline(String description, LocalDateTime by) {
+    public Deadline(String description, LocalDateTime dueDate) {
         super(TaskType.DEADLINE, description);
-        this.by = by;
+        this.dueDate = dueDate;
     }
 
     @Override
     public String toFileFormat() {
-        return "%s | %s".formatted(super.toFileFormat(), by.format(COMPACT_FORMATTER));
+        return "%s | %s".formatted(super.toFileFormat(), dueDate.format(COMPACT_FORMATTER));
     }
 
     @Override
     public String toString() {
-        return "%s (by: %s)".formatted(super.toString(), by.format(VERBOSE_FORMATTER));
+        return "%s (by: %s)".formatted(super.toString(), dueDate.format(VERBOSE_FORMATTER));
     }
 }

src/main/java/morgana/task/Event.java

@@ -13,19 +13,22 @@ public class Event extends Task {
     private final LocalDateTime end;
 
     /**
-     * Constructs an {@code Event} task with the specified description,
-     * start date/time, and end date/time.
-     *
-     * @param description The description of the event.
-     * @param start The start date and time of the event.
-     * @param end The end date and time of the event.
+     * Constructs an {@code Event} with the specified description and time period.
      */
     public Event(String description, LocalDateTime start, LocalDateTime end) {
         super(TaskType.EVENT, description);
         this.start = start;
         this.end = end;
     }
 
+    public LocalDateTime getStart() {
+        return start;
+    }
+
+    public LocalDateTime getEnd() {
+        return end;
+    }
+
     @Override
     public String toFileFormat() {
         return "%s | %s | %s".formatted(

src/main/java/morgana/task/Task.java

@@ -34,9 +34,7 @@ public void markAsDone(boolean isDone) {
     }
 
     /**
-     * Converts the task to a format suitable for saving to a file.
-     *
-     * @return A string representing the task in a format suitable for file storage.
+     * Returns a string representing the task in a format suitable for file storage.
      */
     public String toFileFormat() {
         return "%s | %s | %s".formatted(type, getStatusIcon(), description);

src/main/java/morgana/task/TaskList.java

@@ -2,6 +2,7 @@
 
 import static morgana.common.Messages.MESSAGE_INVALID_TASK_NUMBER;
 
+import java.time.LocalDateTime;
 import java.util.ArrayList;
 import java.util.Iterator;
 import java.util.List;
@@ -24,8 +25,6 @@ public TaskList() {
 
     /**
      * Constructs a {@code TaskList} with an initial list of tasks.
-     *
-     * @param tasks The initial list of tasks.
      */
     public TaskList(List<Task> tasks) {
         this.tasks = tasks;
@@ -40,18 +39,13 @@ public int size() {
 
     /**
      * Adds a task to the end of the list.
-     *
-     * @param task The task to add.
      */
     public void add(Task task) {
         tasks.add(task);
     }
 
     /**
      * Removes the task at the specified index from the list and returns it.
-     *
-     * @param index The index of the task to remove.
-     * @return The task that was removed.
      */
     public Task remove(int index) throws MorganaException {
         validateIndex(index);
@@ -82,7 +76,6 @@ private void validateIndex(int index) throws MorganaException {
     /**
      * Finds tasks in the list that contain the given keyword in their description.
      *
-     * @param keyword The keyword to search for in the task description.
      * @return A formatted string listing the matching tasks.
      */
     public String find(String keyword) {
@@ -96,6 +89,22 @@ public String find(String keyword) {
         return sb.toString();
     }
 
+    /**
+     * Finds and returns the first {@link Event} that clashes with the given time period.
+     *
+     * @return The clashing {@code event} or {@code null} if there is no clash.
+     */
+    public Event findClashingEvent(LocalDateTime start, LocalDateTime end) {
+        for (Task task : tasks) {
+            if (task instanceof Event event
+                    && start.isBefore(event.getEnd())
+                    && end.isAfter(event.getStart())) {
+                return event;
+            }
+        }
+        return null;
+    }
+
     @Override
     public String toString() {
         StringBuilder sb = new StringBuilder("Here are the tasks in your list:\n");

src/main/java/morgana/task/Todo.java

@@ -5,9 +5,7 @@
  */
 public class Todo extends Task {
     /**
-     * Constructs a {@code Todo} task with the specified description.
-     *
-     * @param description The description of the task.
+     * Constructs a {@code Todo} with the specified description.
      */
     public Todo(String description) {
         super(TaskType.TODO, description);