Add Username class

Game does not have any username information.

Gamers usually associate themselves with a
username when playing a game.

Let's make a Game optionally have a Username.

src/main/java/seedu/address/model/game/Game.java

@@ -19,16 +19,30 @@ public class Game {
     public static final String VALIDATION_REGEX = "[\\p{Alnum}][\\p{Alnum} ]*";
 
     public final String gameName;
+    public final Username username;
 
     /**
-     * Constructs a {@code Game}.
+     * Constructs a {@code Game} without a username.
      *
      * @param gameName A valid Game name.
      */
     public Game(String gameName) {
         requireNonNull(gameName);
         checkArgument(isValidGameName(gameName), MESSAGE_CONSTRAINTS);
         this.gameName = gameName;
+        this.username = null;
+    }
+
+    /**
+     * Constructs a {@code Game} with a username.
+     *
+     * @param gameName A valid Game name.
+     */
+    public Game(String gameName, String username) {
+        requireNonNull(gameName);
+        checkArgument(isValidGameName(gameName), MESSAGE_CONSTRAINTS);
+        this.gameName = gameName;
+        this.username = new Username(username);
     }
 
     /**
@@ -62,7 +76,11 @@ public int hashCode() {
      * Format state as text for viewing.
      */
     public String toString() {
-        return '[' + gameName + ']';
+        if (username == null) {
+            return '[' + gameName + ']';
+        } else {
+            return '[' + gameName + "]: " + username.toString();
+        }
     }
 
 }

src/main/java/seedu/address/model/game/Username.java

@@ -0,0 +1,73 @@
+package seedu.address.model.game;
+
+import static java.util.Objects.requireNonNull;
+import static seedu.address.commons.util.AppUtil.checkArgument;
+
+/**
+ * Represents a username in a Game.
+ */
+public class Username {
+
+    private static final String MESSAGE_CONSTRAINTS =
+            "Username should not be blank";
+
+    /*
+     * Regex expression matches Strings that contain at least one non-whitespace character.
+     */
+    private static final String VALIDATION_REGEX = "^(?!\\s*$).+";
+
+    private final String username;
+
+    /**
+     * Constructs a {@code Username}.
+     *
+     * @param username a valid username.
+     */
+    public Username(String username) {
+        requireNonNull(username);
+        checkArgument(isValidGameName(username), MESSAGE_CONSTRAINTS);
+        this.username = username;
+    }
+
+    /**
+     * Returns true if a given string is a valid Username.
+     */
+    public static boolean isValidGameName(String test) {
+        return test.matches(VALIDATION_REGEX);
+    }
+
+    /**
+     * Getter for username field.
+     */
+    public String getUsername() {
+        return username;
+    }
+
+    @Override
+    public boolean equals(Object other) {
+        if (other == this) {
+            return true;
+        }
+
+        // instanceof handles nulls
+        if (!(other instanceof Username)) {
+            return false;
+        }
+
+        Username otherName = (Username) other;
+        return username.equals(otherName.username);
+    }
+
+    @Override
+    public int hashCode() {
+        return username.hashCode();
+    }
+
+    /**
+     * Format state as text for viewing.
+     */
+    public String toString() {
+        return username;
+    }
+
+}