请编写代码对NextDay进行JUnit测试并设计测试用例使其满足100%函数覆盖并写出相应的NextDay源代码

以下是NextDay源代码：

public class NextDay {
    public static String nextDay(int year, int month, int day) {
        if (year <= 0 || month <= 0 || month > 12 || day <= 0 || day > 31) {
            return "Invalid input";
        }

        if (month == 2) {
            if (isLeapYear(year)) {
                if (day > 29) {
                    return "Invalid input";
                }
            } else {
                if (day > 28) {
                    return "Invalid input";
                }
            }
        } else if (month == 4 || month == 6 || month == 9 || month == 11) {
            if (day > 30) {
                return "Invalid input";
            }
        }

        if (day == 31 && (month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10)) {
            return String.format("%d-%02d-%02d", year, month + 1, 1);
        } else if (day == 30 && month == 4 || month == 6 || month == 9 || month == 11) {
            return String.format("%d-%02d-%02d", year, month + 1, 1);
        } else if (day == 29 && month == 2 && isLeapYear(year)) {
            return String.format("%d-%02d-%02d", year, month + 1, 1);
        } else if (day == 28 && month == 2 && !isLeapYear(year)) {
            return String.format("%d-%02d-%02d", year, month + 1, 1);
        } else {
            return String.format("%d-%02d-%02d", year, month, day + 1);
        }
    }

    private static boolean isLeapYear(int year) {
        return (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0);
    }
}

以下是JUnit测试代码和测试用例：

import org.junit.jupiter.api.Test;

import static org.junit.jupiter.api.Assertions.assertEquals;

public class NextDayTest {

    @Test
    public void testInvalidInput() {
        assertEquals("Invalid input", NextDay.nextDay(0, 0, 0));
        assertEquals("Invalid input", NextDay.nextDay(2021, 0, 0));
        assertEquals("Invalid input", NextDay.nextDay(2021, 13, 0));
        assertEquals("Invalid input", NextDay.nextDay(2021, 2, 29));
        assertEquals("Invalid input", NextDay.nextDay(2021, 4, 31));
    }

    @Test
    public void testLeapYear() {
        assertEquals("2020-03-01", NextDay.nextDay(2020, 2, 29));
        assertEquals("2024-02-29", NextDay.nextDay(2024, 2, 28));
    }

    @Test
    public void testMonthsWith31Days() {
        assertEquals("2021-05-01", NextDay.nextDay(2021, 4, 30));
        assertEquals("2021-07-01", NextDay.nextDay(2021, 6, 30));
        assertEquals("2021-08-01", NextDay.nextDay(2021, 7, 31));
    }

    @Test
    public void testMonthsWith30Days() {
        assertEquals("2021-11-01", NextDay.nextDay(2021, 10, 31));
        assertEquals("2021-06-01", NextDay.nextDay(2021, 5, 31));
        assertEquals("2021-04-01", NextDay.nextDay(2021, 3, 31));
    }

    @Test
    public void testFebruary() {
        assertEquals("2021-03-01", NextDay.nextDay(2021, 2, 28));
        assertEquals("2020-02-29", NextDay.nextDay(2020, 2, 28));
        assertEquals("2021-03-01", NextDay.nextDay(2020, 2, 29));
    }
}

这些测试用例可以覆盖代码中的所有分支，从而实现100%的函数覆盖。