Java代码实现周几判断：根据首字母判断，并使用JUnit测试覆盖

下面是一个可能的解决方案：

public class WeekdayChecker {
    public String getWeekday(char firstLetter, char secondLetter) {
        if (firstLetter == 'M' || firstLetter == 'm') {
            return "Monday";
        } else if (firstLetter == 'T' || firstLetter == 't') {
            if (secondLetter == 'U' || secondLetter == 'u') {
                return "Tuesday";
            } else if (secondLetter == 'H' || secondLetter == 'h') {
                return "Thursday";
            }
        } else if (firstLetter == 'W' || firstLetter == 'w') {
            return "Wednesday";
        } else if (firstLetter == 'F' || firstLetter == 'f') {
            return "Friday";
        } else if (firstLetter == 'S' || firstLetter == 's') {
            if (secondLetter == 'A' || secondLetter == 'a') {
                return "Saturday";
            } else if (secondLetter == 'U' || secondLetter == 'u') {
                return "Sunday";
            }
        }
        return "Invalid input";
    }
}

接下来是针对上述代码的JUnit测试类：

import org.junit.jupiter.api.Test;

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

public class WeekdayCheckerTest {
    private final WeekdayChecker weekdayChecker = new WeekdayChecker();

    @Test
    public void testMonday() {
        assertEquals("Monday", weekdayChecker.getWeekday('M', 'a'));
    }

    @Test
    public void testTuesday() {
        assertEquals("Tuesday", weekdayChecker.getWeekday('T', 'u'));
    }

    @Test
    public void testThursday() {
        assertEquals("Thursday", weekdayChecker.getWeekday('T', 'h'));
    }

    @Test
    public void testWednesday() {
        assertEquals("Wednesday", weekdayChecker.getWeekday('W', 'e'));
    }

    @Test
    public void testFriday() {
        assertEquals("Friday", weekdayChecker.getWeekday('F', 'r'));
    }

    @Test
    public void testSaturday() {
        assertEquals("Saturday", weekdayChecker.getWeekday('S', 'a'));
    }

    @Test
    public void testSunday() {
        assertEquals("Sunday", weekdayChecker.getWeekday('S', 'u'));
    }

    @Test
    public void testInvalidInput() {
        assertEquals("Invalid input", weekdayChecker.getWeekday('X', 'y'));
    }
}

以上测试类中包含了最少的测试数据，以满足语句覆盖测试。每个测试方法测试了一个不同的输入情况，并使用assertEquals方法来判断期望结果值和实际返回值是否一致。