LocalDate endDate = startDate.withDayOfMonth(1).plusMonths(1).minusDays(1);

LocalDate endDate = startDate.withDayOfMonth(1).plusMonths(1).minusDays(1);

or

    LocalDate startDate = LocalDate.now();
    System.out.println("startDate: "+startDate);
    
    LocalDate firstDayOfMonth_of_startDate = startDate.withDayOfMonth(1); 
    System.out.println("firstDayOfMonth_of_startDate: "+firstDayOfMonth_of_startDate);
    
    LocalDate firstDayOfNextMonth_of_startDate = firstDayOfMonth_of_startDate.plusMonths(1); 
    System.out.println("firstDayOfNextMonth_of_startDate: "+firstDayOfNextMonth_of_startDate);
    
    LocalDate lastDayOfTheMonth_of_startDate = firstDayOfNextMonth_of_startDate.minusDays(1); 
    System.out.println("lastDayOfTheMonth_of_startDate: "+lastDayOfTheMonth_of_startDate);

    // or everything in one line
    LocalDate endDate = startDate.withDayOfMonth(1).plusMonths(1).minusDays(1);
    System.out.println("endDate: "+endDate);

and the printouts

startDate: 2021-11-05
firstDayOfMonth_of_startDate: 2021-11-01
firstDayOfNextMonth_of_startDate: 2021-12-01
lastDayOfTheMonth_of_startDate: 2021-11-30
endDate: 2021-11-30