闰年判断Java代码解析：==0的含义与作用问询

Hey there! Let's break down that confusing == 0 part and the leap year logic step by step so everything clicks.

First: What does the % operator do? #

In Java, % is the modulus operator—it calculates the remainder when you divide the first number by the second. For example:

• 10 % 4 gives 2 (since 4×2=8, 10-8=2 left over)
• 12 % 4 gives 0 (4×3=12, no remainder at all)

Why do we use == 0? #

When we write year % 4 == 0, we're checking if dividing the year by 4 leaves no remainder. That's exactly what it means for a number to be perfectly divisible by another number—this is the core of our leap year rules.

Let's map this to the leap year conditions you mentioned:

• Rule 1: A year is a leap year if it's divisible by 4 but not divisible by 100.
  • In code: year % 4 == 0 && year % 100 != 0
  • The != 0 here does the opposite: it checks if dividing by 100 does leave a remainder (so the year isn't a century year like 1900).
• Rule 2: If a year is divisible by 400, it's a leap year (even if it's a century year).
  • In code: year % 400 == 0
  • This covers exceptions like 2000 (2000%400=0, so it's a leap year) but not 1900 (1900%400=300≠0, so it's not).

Putting it all together #

The line boolean isLeapYear = (year % 4 == 0 && year % 100 != 0) || year % 400 == 0; is just translating our two leap year rules into Java logic:

• The && means "both conditions must be true"
• The || means "either condition can be true"

Let's test some examples to make it concrete:

• 2020: 2020%4=0 and 2020%100=20≠0 → returns true
• 1900: 1900%4=0 but 1900%100=0 (so first condition fails), and 1900%400=300≠0 → returns false
• 2000: 2000%400=0 → returns true

内容的提问来源于stack exchange，提问作者Tomas Bernatonis