How do you set the time and only the time in a calendar in Java?

Use set(int field, int value). The first parameter is the field number for HOUR/MINUTE/SECOND.

From here:

calendar.set(Calendar.HOUR_OF_DAY, hour);
calendar.set(Calendar.MINUTE, minute);

In 2018 no one should use the Calendar class anymore. It it long outmoded, and java.time the modern Java date and time API is so much nicer to work with. Instead use, depending on you exact requirements, a ZonedDateTime or perhaps an OffsetDateTime or LocalDateTime. In either case, set the time of day this way:

dateTime = dateTime.with(LocalTime.of(hour, minute));

Link: Oracle Tutorial Date Time