Java8 Adding Hours To LocalDateTime Not Working

问题:

I tried like below, but in both the cases it is showing same time? What i am doing wrong.

    LocalDateTime currentTime = LocalDateTime.now(ZoneId.of("UTC"));
    Instant instant = currentTime.toInstant(ZoneOffset.UTC);
    Date currentDate = Date.from(instant);
    System.out.println("Current Date = " + currentDate);
    currentTime.plusHours(12);
    Instant instant2 = currentTime.toInstant(ZoneOffset.UTC);
    Date expiryDate = Date.from(instant2);
    System.out.println("After 12 Hours = " + expiryDate);

"Current Date" Time is showing Same as "After 12 Hours"...

回答1:

The documentation of LocalDateTime specifies the instance of LocalDateTime is immutable, for example plusHours

public LocalDateTime plusHours(long hours)

Returns a copy of this LocalDateTime with the specified number of hours added.

This instance is immutable and unaffected by this method call.

Parameters:
hours - the hours to add, may be negative
Returns:
a LocalDateTime based on this date-time with the hours added, not null
Throws:
DateTimeException - if the result exceeds the supported date range

So, you create a new instance of LocalDateTime when you execute plus operation, you need to assign this value as follows:

LocalDateTime nextTime = currentTime.plusHours(12);
Instant instant2 = nextTime.toInstant(ZoneOffset.UTC);
Date expiryDate = Date.from(instant2);
System.out.println("After 12 Hours = " + expiryDate);

I hope it can be helpful for you.

回答2:

From the java.time package Javadoc (emphasis mine):

The classes defined here represent the principal date-time concepts, including instants, durations, dates, times, time-zones and periods. They are based on the ISO calendar system, which is the de facto world calendar following the proleptic Gregorian rules. All the classes are immutable and thread-safe.

Since every class in the java.time package is immutable, you need to capture the result:

LocalDateTime after = currentTime.plusHours(12);
...