为什么我的代码会进入else-if语句，其条件应为false？

Question

I have a method that is supposed to return the seasons of the year depending upon the month and day. The first three times I call the method it works fine. However, the fourth method returns spring when it should return fall.

It should bypass the else-if statement for spring and continue down to fall but when I debug it, that is not what it does. it goes into that else-if and returns it.

I tried moving the months and dates around but no matter where I put it, it returns spring. I cannot figure out why since the arguments do not meet the criteria for that particular statement.

Here is my code:

public static void main(String[] args) {
    System.out.println(season(1, 5)); 
    System.out.println(season(4, 12));
    System.out.println(season(7, 1));
    System.out.println(season(10, 31));
}

public static String season(int month, int day) {
    if(month >= 12 && day >= 16 || month <= 3 && day <= 15) {
        return "winter";
    } else if(month >= 3 && day >= 16 || month <= 6 && day <= 15) {
        return "spring";
    } else if(month >= 6 && day >= 16 || month <= 9 && day <= 15) {
        return "summer";
    } else {
        return "fall";
    }
}

Answer 1

Your program is doing exactly what you told it to. When you call season(10, 31) , you get to the test:

if (month >= 3 && day >= 16 || ...)

which is true because the month is 10 and the day is 31 (both greater than their targets in this test). Clearly, this is not what you want. So you have to make a slightly more complicated test:

if (month == 3 && day >= 16 || month > 3 && month < 6 || month == 6 && day <= 15)

This tests the day only if the month is not the starting or ending month for spring. Your other tests will need similar adjustments.

You may find that you might like to add parentheses to your tests to help group them as suggested by other answers here, but due to the Java operator precedence, it is not strictly necessary in this case.

Answer 2

If you remove first half of your conditions it will work fine.

  public static String season(int month, int day)
  {
    if(month <= 3 && day <= 15)
    {
        return "winter";
    }
    else if(month <= 6 && day <= 15)
    {
        return "spring";
    }
    else if(month <= 9 && day <= 15)
    {
        return "summer";
    }
    else
    {
        return "fall";
    }
  }