我正在尝试理解Java 8流。我有两节课:
public class UserMeal {
protected final LocalDateTime dateTime;
protected final String description;
protected final int calories;
public UserMeal(LocalDateTime dateTime, String description, int calories) {
this.dateTime = dateTime;
this.description = description;
this.calories = calories;
}
public LocalDateTime getDateTime() {
return dateTime;
}
public String getDescription() {
return description;
}
public int getCalories() {
return calories;
}
}
和:
public class UserMealWithExceed {
protected final LocalDateTime dateTime;
protected final String description;
protected final int calories;
protected final boolean exceed;
public UserMealWithExceed(LocalDateTime dateTime, String description, int calories, boolean exceed) {
this.dateTime = dateTime;
this.description = description;
this.calories = calories;
this.exceed = exceed;
}
}
exceed
字段应指示一整天的卡路里总和。该天对于当天的所有条目都是相同的。
我尝试从List<UserMeal> mealList
中获取对象,按天分组,计算一段时间内的卡路里,并创建List<UserMealWithExceed>
:
public static List<UserMealWithExceed> getFilteredMealsWithExceeded(List<UserMeal> mealList, LocalTime startTime, LocalTime endTime, int caloriesPerDay) {
return mealList.stream()
.filter(userMeal -> userMeal.getDateTime().toLocalTime().isAfter(startTime)&&userMeal.getDateTime().toLocalTime().isBefore(endTime))
.collect(Collectors.groupingBy(userMeal -> userMeal.getDateTime().getDayOfMonth(),
Collectors.summingInt(userMeal -> userMeal.getCalories())))
.forEach( ????? );
}
但是我不明白如何在forEach
中创建新对象并返回集合。
我怎么看伪代码:
.foreach(
if (sumCalories>caloriesPerDay)
{return new UserMealWithExceed(userMeal.getdateTime, usermeal.getDescription, usermeal.getCalories, true);}
else
{return new UserMealWithExceed(userMeal.getdateTime, usermeal.getDescription, usermeal.getCalories, false)
}
)//foreach
如果要遍历列表并使用“已转换”的对象创建新列表,则应使用流+ map()
的collect()
功能。在下面的示例中,我找到所有姓为“ l1”的人,并且我正在“映射”到新的Employee实例的每个人。
public class Test {
public static void main(String[] args) {
List<Person> persons = Arrays.asList(
new Person("e1", "l1"),
new Person("e2", "l1"),
new Person("e3", "l2"),
new Person("e4", "l2")
);
List<Employee> employees = persons.stream()
.filter(p -> p.getLastName().equals("l1"))
.map(p -> new Employee(p.getName(), p.getLastName(), 1000))
.collect(Collectors.toList());
System.out.println(employees);
}
}
class Person {
private String name;
private String lastName;
public Person(String name, String lastName) {
this.name = name;
this.lastName = lastName;
}
// Getter & Setter
}
class Employee extends Person {
private double salary;
public Employee(String name, String lastName, double salary) {
super(name, lastName);
this.salary = salary;
}
// Getter & Setter
}
您可能正在寻找的是map()
。您可以通过以下方式映射将流中的对象“转换”为另一个对象:
...
.map(userMeal -> new UserMealExceed(...))
...
@ Rafael Teles解决方案的补充。语法糖Collectors.mapping
一步执行相同的操作:
//...
List<Employee> employees = persons.stream()
.filter(p -> p.getLastName().equals("l1"))
.collect(
Collectors.mapping(
p -> new Employee(p.getName(), p.getLastName(), 1000),
Collectors.toList()));
可以找到详细的例子here