Add J-Unit Tests for Deadline.

Add JUnit tests to test Deadline. Check two methods for correctness, the toString method,
and the method for generating the saved String format used for storing the task in the file.

There could be instances where there is a typo that results in an incorrect String format.
This might lead to bugs when the tasks are being stored in another .txt file and fetched the next
time the app is open.

J-Unit tests help to automate the testing.

Let's add unit test to check for toString method correctness.
a unit test to check the getSavedString format correctness
and a unit test to check if Deadline throws Exception when
no date is able to be parsed from its by String.

Author: Maurice2n97

src/main/java/duke/CommandType.java

@@ -7,7 +7,7 @@
  * A Task with a deadline to complete.
  */
 public class Deadline extends Task {
-    protected String by;
+    private String by;
     private LocalDate dateOfDeadline;
 
     public Deadline(String description,String by) {

src/main/java/duke/Deadline.java

@@ -27,7 +27,7 @@ public String getUserCommand(){
         return input;
     }
 
-    
+
 
     /**
      * Prints all the tasks stored on the TaskList.

src/main/java/duke/Ui.java

@@ -0,0 +1,31 @@
+package duke;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.fail;
+
+public class DeadlineTest {
+    @Test
+    public void toStringFormat_dateIncluded_success() throws Exception {
+        Deadline d = new Deadline("Read book", "2020-03-04 2359");
+        assertEquals( "[D][ ] Read book (by: Mar 4 2020 2359)",d.toString());
+    }
+    @Test
+    public void savedStringFormat_checkFormat_success() throws Exception {
+        Deadline d = new Deadline("Do Homework", "2021-03-04");
+        assertEquals("D | 0 | Do Homework | 2021-03-04", d.getSavedStringFormat());
+    }
+    @Test
+    public void toStringFormat_checkFormat_throwsException() {
+        try {
+            Deadline d = new Deadline("do Something", "this date");
+            fail(); // test should not reach this line.
+        } catch (Exception e) {
+            assertEquals("Sorry Unable to Parse date for Deadline. "
+                    + "Did you try to do it in yyyy-mm-dd format?", e.getMessage());
+        }
+    }
+
+
+}

src/test/java/duke/DeadlineTest.java

@@ -0,0 +1,15 @@
+package duke;
+
+
+import org.junit.jupiter.api.DynamicTest;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class DukeTest {
+    @Test
+    public void dummyTest() {
+        assertEquals(2,2);
+    }
+
+}