Java 如何在 List<Object> 中找到最大日期？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/20995664/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
How to find Max Date in List<Object>?
提问by user2783484
Consider a class User
考虑一个类 User
public class User{
int userId;
String name;
Date date;
}
Now I have a List<User>of size 20, how can I find the max datein the list without using manual iterator?
现在我List<User>的大小为 20，如何在不使用手动迭代器的情况下找到列表中的最大日期？
采纳答案by assylias
Since you are asking for lambdas, you can use the following syntax with Java 8:
由于您要求使用 lambda，您可以在 Java 8 中使用以下语法：
Date maxDate = list.stream().map(u -> u.date).max(Date::compareTo).get();
or, if you have a getter for the date:
或者，如果您有日期的吸气剂：
Date maxDate = list.stream().map(User::getDate).max(Date::compareTo).get();
回答by giannis christofakis
Comparator<User> cmp = new Comparator<User>() {
@Override
public int compare(User user1, User user2) {
return user1.date.compareTo(user2.date);
}
};
Collections.max(list, cmp);
回答by Jajikanth pydimarla
A small improvement from the accepted answer is to do the null check and also get full object.
接受的答案的一个小改进是进行空检查并获得完整的对象。
public class DateComparator {
public static void main(String[] args) throws ParseException {
SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd");
List<Employee> employees = new ArrayList<>();
employees.add(new Employee(1, "name1", addDays(new Date(), 1)));
employees.add(new Employee(2, "name2", addDays(new Date(), 3)));
employees.add(new Employee(3, "name3", addDays(new Date(), 6)));
employees.add(new Employee(4, "name4", null));
employees.add(new Employee(5, "name5", addDays(new Date(), 4)));
employees.add(new Employee(6, "name6", addDays(new Date(), 5)));
System.out.println(employees);
Date maxDate = employees.stream().filter(emp -> emp.getJoiningDate() != null).map(Employee::getJoiningDate).max(Date::compareTo).get();
System.out.println(format.format(maxDate));
//Comparator<Employee> comparator = (p1, p2) -> p1.getJoiningDate().compareTo(p2.getJoiningDate());
Comparator<Employee> comparator = Comparator.comparing(Employee::getJoiningDate);
Employee maxDatedEmploye = employees.stream().filter(emp -> emp.getJoiningDate() != null).max(comparator).get();
System.out.println(" maxDatedEmploye : " + maxDatedEmploye);
Employee minDatedEmployee = employees.stream().filter(emp -> emp.getJoiningDate() != null).min(comparator).get();
System.out.println(" minDatedEmployee : " + minDatedEmployee);
}
public static Date addDays(Date date, int days) {
Calendar cal = Calendar.getInstance();
cal.setTime(date);
cal.add(Calendar.DATE, days); // minus number would decrement the days
return cal.getTime();
}
}
You would get below results :
你会得到以下结果：
[Employee [empId=1, empName=name1, joiningDate=Wed Mar 21 13:33:09 EDT 2018],
Employee [empId=2, empName=name2, joiningDate=Fri Mar 23 13:33:09 EDT 2018],
Employee [empId=3, empName=name3, joiningDate=Mon Mar 26 13:33:09 EDT 2018],
Employee [empId=4, empName=name4, joiningDate=null],
Employee [empId=5, empName=name5, joiningDate=Sat Mar 24 13:33:09 EDT 2018],
Employee [empId=6, empName=name6, joiningDate=Sun Mar 25 13:33:09 EDT 2018]
]
2018-03-26
maxDatedEmploye : Employee [empId=3, empName=name3, joiningDate=Mon Mar 26 13:33:09 EDT 2018]
minDatedEmployee : Employee [empId=1, empName=name1, joiningDate=Wed Mar 21 13:33:09 EDT 2018]
Update : What if list itself is empty ?
更新：如果列表本身为空怎么办？
Date maxDate = employees.stream().filter(emp -> emp.getJoiningDate() != null).map(Employee::getJoiningDate).max(Date::compareTo).orElse(new Date());
System.out.println(format.format(maxDate));
Comparator<Employee> comparator = Comparator.comparing(Employee::getJoiningDate);
Employee maxDatedEmploye = employees.stream().filter(emp -> emp.getJoiningDate() != null).max(comparator).orElse(null);
System.out.println(" maxDatedEmploye : " + maxDatedEmploye);
回答by Mike Moreira
LocalDate maxDate = dates.stream()
.max( Comparator.comparing( LocalDate::toEpochDay ) )
.get();
LocalDate minDate = dates.stream()
.min( Comparator.comparing( LocalDate::toEpochDay ) )
.get();
回答by epox
You should not call .get() directly. Optional<>, that Stream::maxreturns, was designed to benefit from .orElse... inline handling.
你不应该直接调用 .get() 。Stream::max返回的Optional<>旨在从 .orElse... 内联处理中受益。
If you are sure your arguments have their size of 20:
如果您确定您的参数大小为 20：
list.stream()
.map(u -> u.date)
.max(Date::compareTo)
.orElseThrow(() -> new IllegalArgumentException("Expected a list of size: 20. Was: 0"));
If you support empty lists, then return some default value, for example:
如果您支持空列表，则返回一些默认值，例如：
list.stream()
.map(u -> u.date)
.max(Date::compareTo)
.orElse(new Date(Long.MIN_VALUE));
CREDITS to: @JimmyGeers, @assylias from the accepted answer.
信用：@JimmyGeers，@assylias 来自已接受的答案。
回答by Kirill Groshkov
Just use Kotlin!
只需使用 Kotlin！
val list = listOf(user1, user2, user3)
val maxDate = list.maxBy { it.date }?.date
