Implement command to delete tag

src/main/java/seedu/address/logic/Messages.java

@@ -18,7 +18,8 @@ public class Messages {
     public static final String MESSAGE_PERSONS_LISTED_OVERVIEW = "%1$d persons listed!";
     public static final String MESSAGE_DUPLICATE_FIELDS =
                 "Multiple values specified for the following single-valued field(s): ";
-
+    public static final String MESSAGE_NO_TAG = "The person does not contain the tag";
+    public static final String MESSAGE_DELETE_TAG_SUCCESS = "Tag %1$s is deleted from student %2$s";
     /**
      * Returns an error message indicating the duplicate prefixes.
      */

src/main/java/seedu/address/logic/commands/DeleteTagCommand.java

@@ -0,0 +1,68 @@
+package seedu.address.logic.commands;
+
+import static java.util.Objects.requireNonNull;
+
+import java.util.List;
+import java.util.Set;
+
+import seedu.address.commons.core.index.Index;
+import seedu.address.logic.Messages;
+import seedu.address.logic.commands.exceptions.CommandException;
+import seedu.address.model.Model;
+import seedu.address.model.person.Person;
+import seedu.address.model.tag.Tag;
+
+/**
+ * Implements command to delete a tag from a person
+ * format: deletetag [INDEX] [TAG NAME]
+ */
+public class DeleteTagCommand extends Command {
+    public static final String COMMAND_WORD = "deletetag";
+
+    public static final String MESSAGE_USAGE = COMMAND_WORD
+            + ": Deletes tag from the person identified by the index number used in the displayed person list.\n"
+            + "Parameters: INDEX (must be a positive integer) [TAG NAME]\n"
+            + "Example: " + COMMAND_WORD + " 1 " + " hello_world";
+
+    private final Index targetIndex;
+    private final Tag targetTag;
+
+    public DeleteTagCommand(Index targetIndex, Tag target) {
+        this.targetIndex = targetIndex;
+        this.targetTag = target;
+    }
+
+    @Override
+    public CommandResult execute(Model model) throws CommandException {
+        requireNonNull(model);
+        List<Person> lastShownList = model.getFilteredPersonList();
+        if (targetIndex.getZeroBased() >= lastShownList.size()) {
+            throw new CommandException(Messages.MESSAGE_INVALID_PERSON_DISPLAYED_INDEX);
+        }
+
+        Person personToEdit = lastShownList.get(targetIndex.getZeroBased());
+        if (!personToEdit.hasTag(this.targetTag)) {
+            throw new CommandException(Messages.MESSAGE_NO_TAG);
+        }
+
+        personToEdit.removeTag(this.targetTag);
+        return new CommandResult(String.format(Messages.MESSAGE_DELETE_TAG_SUCCESS,
+                this.targetTag, personToEdit.getName()));
+    }
+
+    @Override
+    public boolean equals(Object other) {
+        if (other == this) {
+            return true;
+        }
+
+        // instanceof handles nulls
+        if (!(other instanceof DeleteTagCommand)) {
+            return false;
+        }
+
+        DeleteTagCommand otherDeleteTagCommand = (DeleteTagCommand) other;
+        return targetIndex.equals(otherDeleteTagCommand.targetIndex)
+                && targetTag.equals(otherDeleteTagCommand.targetTag);
+    }
+}

src/main/java/seedu/address/logic/commands/EditCommand.java

@@ -76,9 +76,7 @@ public CommandResult execute(Model model) throws CommandException {
         Person personToEdit = lastShownList.get(index.getZeroBased());
         Person editedPerson = createEditedPerson(personToEdit, editPersonDescriptor);
 
-        // if (!personToEdit.isSamePerson(editedPerson) && model.hasPerson(editedPerson)) {
-        // throw new CommandException(MESSAGE_DUPLICATE_PERSON);
-        // }
+
         model.deletePerson(personToEdit);
         if (model.hasPerson(editedPerson)) {
             model.insertPerson(personToEdit, index.getZeroBased());

src/main/java/seedu/address/model/person/Person.java

@@ -99,6 +99,20 @@ public boolean hasDuplicateInfo(Person otherPerson) {
                 || this.hasSamePhoneNumber(otherPerson);
     }
 
+    /**
+     * Checks whether the person has the tags
+     */
+    public boolean hasTag(Tag t) {
+        return this.getTags().contains(t);
+    }
+
+    /**
+     * Remove a tag from a person
+     */
+    public void removeTag(Tag t) {
+        this.getTags().remove(t);
+    }
+
     /**
      * Returns true if both persons have the same identity and data fields.
      * This defines a stronger notion of equality between two persons.