Add JUnit tests

src/test/java/duke/ParserTest.java

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+package duke;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import duke.Parser;
+import duke.command.AddCommand;
+import duke.command.DeleteCommand;
+import duke.command.ExitCommand;
+import duke.command.ListCommand;
+import duke.command.MarkCommand;
+import duke.command.UnmarkCommand;
+import duke.command.Command;
+import duke.exception.InvalidArgumentException;
+import org.junit.jupiter.api.Test;
+
+public class ParserTest {
+
+    @Test
+    public void testParseListCommand() throws InvalidArgumentException {
+        Command command = Parser.parse("list");
+        assertTrue(command instanceof ListCommand);
+    }
+
+    @Test
+    public void testParseDeleteCommand() throws InvalidArgumentException {
+        Command command = Parser.parse("delete 1");
+        assertTrue(command instanceof DeleteCommand);
+    }
+
+    @Test
+    public void testParseMarkCommand() throws InvalidArgumentException {
+        Command command = Parser.parse("mark 1");
+        assertTrue(command instanceof MarkCommand);
+    }
+
+    @Test
+    public void testParseUnmarkCommand() throws InvalidArgumentException {
+        Command command = Parser.parse("unmark 1");
+        assertTrue(command instanceof UnmarkCommand);
+    }
+
+    @Test
+    public void testParseAddCommand() throws InvalidArgumentException {
+        Command command = Parser.parse("todo Test");
+        assertTrue(command instanceof AddCommand);
+    }
+
+    @Test
+    public void testParseExitCommand() throws InvalidArgumentException {
+        Command command = Parser.parse("bye");
+        assertTrue(command instanceof ExitCommand);
+    }
+
+    @Test
+    public void testInvalidCommand() {
+        assertThrows(InvalidArgumentException.class, () -> Parser.parse("invalid_command"));
+    }
+}

src/test/java/duke/UiTest.java

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+package duke;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+public class UiTest {
+    private final PrintStream standardOut = System.out;
+    private final ByteArrayOutputStream outputStreamCaptor = new ByteArrayOutputStream();
+
+    @BeforeEach
+    public void setUp() {
+        System.setOut(new PrintStream(outputStreamCaptor));
+    }
+
+    @Test
+    public void testShowWelcome() {
+        Ui ui = new Ui();
+        ui.showWelcome();
+        String expected = "__________________________________________________________________________" +
+                "    ___    _   ___   ________  __      ____      __________  ____  _   __\n"
+                + "   /   |  / | / / | / / __ \\ \\/ /     / __ \\    /_  __/ __ \\/ __ \\/ | / /\n"
+                + "  / /| | /  |/ /  |/ / / / /\\  /_____/ / / /_____/ / / /_/ / / / /  |/ / \n"
+                + " / ___ |/ /|  / /|  / /_/ / / /_____/ /_/ /_____/ / / _, _/ /_/ / /|  /  \n"
+                + "/_/  |_/_/ |_/_/ |_/\\____/ /_/      \\____/     /_/ /_/ |_|\\____/_/ |_/   \n"
+                + "Hello! I'm ANNOY-O-TRON!\nWhat can I do for you?\n"
+                + "__________________________________________________________________________";
+        assertEquals(expected, outputStreamCaptor.toString().trim());
+    }
+
+    @Test
+    public void testShowBye() {
+        Ui ui = new Ui();
+        ui.showBye();
+        String expected = "Bye. Hope to see you again soon!";
+        assertEquals(expected, outputStreamCaptor.toString().trim());
+    }
+
+
+    @AfterEach
+    public void tearDown() {
+        System.setOut(standardOut);
+    }
+}

src/test/java/duke/task/ToDoTest.java

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+package duke.task;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class ToDoTest {
+    @Test
+    public void testToString() {
+        Todo todo = new Todo("Sample Task");
+        String expected = "[T][ ] Sample Task";
+        assertEquals(expected, todo.toString());
+    }
+
+    @Test
+    public void testToFileString() {
+        Todo todo = new Todo("Sample Task");
+        String expected = "T | 0 | Sample Task";
+        assertEquals(expected, todo.toFileString());
+    }
+}