[英]Create list of object from another using Java 8 Streams
我正在嘗試了解 Java 8 流。 我有兩個班級:
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
字段應指示是否為全天的卡路里總和。 該字段對於當天的所有條目都是相同的。
我嘗試從List<UserMeal> mealList
獲取對象,按天分組,計算一段時間的卡路里,並創建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( ????? );
}
但我不明白如何在forEach
創建新對象並返回集合。
我如何在偽代碼中看到:
.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
如果你想遍歷一個列表並創建一個帶有“轉換”對象的新列表,你應該使用 stream + collect()
的map()
函數。 在下面的示例中,我找到所有姓氏為“l1”的人,並將每個人“映射”到一個新的 Employee 實例。
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
}
您可能正在尋找的是map()
。 您可以通過以下方式將流中的對象“轉換”為另一個對象:
...
.map(userMeal -> new UserMealExceed(...))
...
@Rafael Teles 對解決方案的補充。 語法糖Collectors.mapping
一步完成相同的工作:
//...
List<Employee> employees = persons.stream()
.filter(p -> p.getLastName().equals("l1"))
.collect(
Collectors.mapping(
p -> new Employee(p.getName(), p.getLastName(), 1000),
Collectors.toList()));
可以在此處找到詳細示例
我更喜歡以經典方式解決這個問題,創建一個我想要的數據類型的新數組:
List<MyNewType> newArray = new ArrayList<>();
myOldArray.forEach(info -> newArray.add(objectMapper.convertValue(info, MyNewType.class)));
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