Add more tests

src/test/java/seedu/address/logic/parser/AddCommandParserTest.java

@@ -223,4 +223,18 @@ public void parse_invalidValue_failure() {
                         + EMAIL_DESC_BOB + ADDRESS_DESC_BOB + TAG_DESC_HUSBAND + TAG_DESC_FRIEND,
                 String.format(MESSAGE_INVALID_COMMAND_FORMAT, AddCommand.MESSAGE_USAGE));
     }
+
+    @Test
+    public void parse_duplicatePrefixes_failure() {
+        assertParseFailure(parser, COMPANY_NAME_DESC_AMY + COMPANY_NAME_DESC_BOB + NAME_DESC_BOB + PHONE_DESC_BOB
+                + EMAIL_DESC_BOB + ADDRESS_DESC_BOB, Messages.getErrorMessageForDuplicatePrefixes(PREFIX_COMPANY_NAME));
+
+    }
+
+    @Test
+    public void parse_nonEmptyPreamble_failure() {
+        assertParseFailure(parser, PREAMBLE_NON_EMPTY + COMPANY_NAME_DESC_BOB + NAME_DESC_BOB + PHONE_DESC_BOB
+                + EMAIL_DESC_BOB + ADDRESS_DESC_BOB, String.format(MESSAGE_INVALID_COMMAND_FORMAT,
+                AddCommand.MESSAGE_USAGE));
+    }
 }

src/test/java/seedu/address/logic/parser/EditCommandParserTest.java

@@ -259,4 +259,15 @@ public void parse_resetTags_success() {
 
         assertParseSuccess(parser, userInput, expectedCommand);
     }
+
+    @Test
+    public void parse_resetProgrammingLanguages_success() {
+        Index targetIndex = INDEX_THIRD_PERSON;
+        String userInput = targetIndex.getOneBased() + TAG_EMPTY;
+
+        EditPersonDescriptor descriptor = new EditPersonDescriptorBuilder().withProgrammingLanguages().build();
+        EditCommand expectedCommand = new EditCommand(targetIndex, descriptor);
+
+        assertParseSuccess(parser, userInput, expectedCommand);
+    }
 }

src/test/java/seedu/address/model/ModelManagerTest.java

@@ -129,4 +129,14 @@ public void equals() {
         differentUserPrefs.setAddressBookFilePath(Paths.get("differentFilePath"));
         assertFalse(modelManager.equals(new ModelManager(addressBook, differentUserPrefs)));
     }
+
+    @Test
+    public void toString_validModelManager_returnsStringRepresentation() {
+        AddressBook addressBook = new AddressBookBuilder().withPerson(ALICE).withPerson(BENSON).build();
+        UserPrefs userPrefs = new UserPrefs();
+        ModelManager modelManager = new ModelManager(addressBook, userPrefs);
+
+        String expectedString = String.format("%s, %s, %s", addressBook, userPrefs, modelManager.getFilteredPersonList());
+        assertEquals(expectedString, modelManager.toString());
+    }
 }

src/test/java/seedu/address/model/language/ProgrammingLanguageTest.java

@@ -1,5 +1,7 @@
 package seedu.address.model.language;
 
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 import static seedu.address.testutil.Assert.assertThrows;
 
 import org.junit.jupiter.api.Test;