Add tests

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

@@ -0,0 +1,35 @@
+package seedu.address.model;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+
+public class AddressBookListTest {
+    private final AddressBookList addressBookList = new AddressBookList();
+
+    @Test
+    public void undo_success() {
+        addressBookList.add(new AddressBook());
+        addressBookList.add(new AddressBook());
+        assertEquals(addressBookList.undo(), new AddressBook());
+    }
+
+    @Test
+    public void undo_failure() {
+        assertThrows(IllegalArgumentException.class, () -> addressBookList.undo());
+    }
+
+    @Test
+    public void redo_success() {
+        addressBookList.add(new AddressBook());
+        addressBookList.add(new AddressBook());
+        addressBookList.undo();
+        assertEquals(addressBookList.redo(), new AddressBook());
+    }
+
+    @Test
+    public void redo_failure() {
+        assertThrows(IllegalArgumentException.class, () -> addressBookList.redo());
+    }
+}

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

@@ -85,7 +85,10 @@ public void hasPerson_personNotInAddressBook_returnsFalse() {
     @Test
     public void hasPerson_personInAddressBook_returnsTrue() {
         modelManager.addPerson(ALICE);
+        modelManager.addCommandText("add Alice");
         assertTrue(modelManager.hasPerson(ALICE));
+        assertTrue(modelManager.undo().equals("add Alice"));
+        assertTrue(modelManager.redo().equals("add Alice"));
     }
 
     @Test