Android Calendar获取上一年前前月份异常问题求助

Hey there! Let's break down why your current code fails when crossing year boundaries, and how to fix it properly.

The Problem with Your Current Code #

Your approach manually subtracts 1 from the year and 2 from the month, but this doesn't account for cases where subtracting 2 months would push you into the previous year (or even earlier). For example:

• If today were January 2020 (month index 0), month = 0 - 2 = -2. Setting this directly on the calendar would mess up the year calculation, since Calendar doesn't automatically adjust the year when you set an invalid month value like this.
• Even when it works for non-crossing cases (like April 2020), it's fragile because it relies on manual date arithmetic instead of using Calendar's built-in handling for date adjustments.

The Correct Approach: Use Calendar.add() #

Instead of manually calculating year and month, let the Calendar class do the heavy lifting with its add() method. This method automatically handles all date boundary cases (like rolling back years when subtracting months) for you.

Here's the revised code:

Date dt = new Date();
Calendar cal = Calendar.getInstance();
cal.setTime(dt);

// Subtract 14 months total: 1 year (12 months) + 2 months = 14 months
cal.add(Calendar.MONTH, -14);

// Alternatively, split into two steps for clarity (same result):
// cal.add(Calendar.YEAR, -1);
// cal.add(Calendar.MONTH, -2);

String formatMonth = new SimpleDateFormat("MMM").format(cal.getTime());
String formatYear = new SimpleDateFormat("yy").format(cal.getTime());

Why This Works #

• The add() method adjusts the calendar date intelligently. For your test case (April 29, 2020), subtracting 14 months correctly lands on February 29, 2019 (or February 28 if it's not a leap year).
• For edge cases like January 2020, subtracting 14 months would give you November 2018, which is exactly the "previous year's two-months-ago" value you'd expect.

Bonus Tip #

If you need consistent results across time zones, consider setting the calendar's time zone explicitly:

cal.setTimeZone(TimeZone.getTimeZone("UTC"));

内容的提问来源于stack exchange，提问作者Narendra