java通过一个有效的键访问Map的Map给出了NPE

Question

我有一张地图：

Map<Long, Map<Integer, Integer>> employeeYearWiseLeaveAppCount=new HashMap<>();

此地图在其他位置填充了值。现在，我需要像这样访问内部地图：

for (AnnualLeaveBalance balance: annualLeaveBalanceList) {
    Map<Integer, Integer> yearToCountMap = employeeYearWiseLeaveAppCount.get(
        balance.getEmployeeId()
    );
    int year = Integer.valueOf(balance.getYear());
    int takenLeave = yearToCountMap.get(year); //year is valid value. NPE here!
}

我传递给此映射的键employeeYearWiseLeaveAppCount.get(balance.getEmployeeId());在映射中确实存在，但yearToCountMap显示为null。 请帮帮我这里怎么了

Answer 1

需要两个空检查来防止NPE，但是在这两种情况下都可以通过^{}方法解决

此外，应该简化year的类型和赋值^仅当balance.getYear()返回字符串值时才需要{}，否则可以将其作为Integer year = balance.getYear();

for (AnnualLeaveBalance balance: annualLeaveBalanceList) {
    Map<Integer, Integer> yearToCountMap = employeeYearWiseLeaveAppCount.getOrDefault(
        balance.getEmployeeId(), Collections.emptyMap()
    ); // empty map is returned if no yearToCount map is found for a user

    Integer year = balance.getYear();
    int takenLeave = yearToCountMap.getOrDefault(year, 0); // no leave taken
}