Add JUnit for NextDayCommand

src/test/java/seedu/cafectrl/command/NextDayCommandTest.java

@@ -0,0 +1,34 @@
+package seedu.cafectrl.command;
+
+import org.junit.jupiter.api.Test;
+import seedu.cafectrl.data.CurrentDate;
+import seedu.cafectrl.data.Sales;
+import seedu.cafectrl.ui.Ui;
+
+class NextDayCommandTest {
+    @Test
+    public void execute_oneNextDay_expectDayTwo() {
+        Ui ui = new Ui();
+        Sales sales = new Sales();
+        CurrentDate currentDate = new CurrentDate(sales);
+        Command nextDayCommand = new NextDayCommand(ui, sales, currentDate);
+        nextDayCommand.execute();
+        int actualDay = currentDate.getCurrentDay() + 1;
+        int expectedDay = 2;
+        assert actualDay == expectedDay;
+    }
+
+    @Test
+    public void execute_nineNextDay_expectDayTen() {
+        Ui ui = new Ui();
+        Sales sales = new Sales();
+        CurrentDate currentDate = new CurrentDate(sales);
+        for (int i = 0; i < 9; i ++) {
+            Command nextDayCommand = new NextDayCommand(ui, sales, currentDate);
+            nextDayCommand.execute();
+        }
+        int actualDay = currentDate.getCurrentDay() + 1;
+        int expectedDay = 10;
+        assert actualDay == expectedDay;
+    }
+}