相关疑难解决方法(0)

如何先分组,然后使用Java流应用过滤？

示例:考虑此类Employee:我希望按部门分组,其中包含薪水大于2000的员工列表.

public class Employee {
    private String department;
    private Integer salary;
    private String name;

    //getter and setter

    public Employee(String department, Integer salary, String name) {
        this.department = department;
        this.salary = salary;
        this.name = name;
    }
}   

这就是我如何做到这一点

List<Employee> list   = new ArrayList<>();
list.add(new Employee("A", 5000, "A1"));
list.add(new Employee("B", 1000, "B1"));
list.add(new Employee("C", 6000, "C1"));
list.add(new Employee("C", 7000, "C2"));

Map<String, List<Employee>> collect = list.stream()
    .filter(e -> e.getSalary() > 2000)
    .collect(Collectors.groupingBy(Employee::getDepartment));  

产量

{A=[Employee [department=A, salary=5000, name=A1]],
 C=[Employee [department=C, …

我有几个类,如下

class Pojo {
    List<Item> items;
}

class Item {
    T key1;
    List<SubItem> subItems;
}

class SubItem {
    V key2;
    Object otherAttribute1;
}

我想根据key1每个聚合聚合项目,子项目应按key2以下方式聚合:

Map<T, Map<V, List<Subitem>>

Java 8 Collectors.groupingBy嵌套有什么可能吗？

我正在尝试一些东西并且中途停留

pojo.getItems()
    .stream()
    .collect(
        Collectors.groupingBy(Item::getKey1, /* How to group by here SubItem::getKey2*/)
    );

注意:这与级联不同groupingBy,后者根据此处讨论的同一对象中的字段进行多级聚合

我想确定显示点集合所需的最小区域.简单的方法是循环遍历集合,如下所示:

int minX = Integer.MAX_VALUE;
int maxX = Integer.MIN_VALUE;
int minY = Integer.MAX_VALUE;
int maxY = Integer.MIN_VALUE;
for (Point point: points) {
    if (point.x < minX) {
        minX = point.x;
    }
    if (point.x > maxX) {
        maxX = point.x;
    }
    if (point.y < minY) {
        minY = point.y;
    }
    if (point.y > maxY) {
        maxY = point.y;
    }
}

我开始了解溪流.要做到这一点,您可以执行以下操作:

int minX = points.stream().mapToInt(point -> point.x).min().orElse(-1);
int maxX = points.stream().mapToInt(point -> point.x).max().orElse(-1);
int minY = points.stream().mapToInt(point -> point.y).min().orElse(-1);
int maxY …