我设法使用 Java 8 Streams API 编写了一个解决方案，该解决方案首先按对象 Route 的值对列表进行分组，然后计算每组中的对象数。它返回一个映射 Route -> Long。这是代码:

Map<Route, Long> routesCounted = routes.stream()
                .collect(Collectors.groupingBy(gr -> gr, Collectors.counting()));

还有 Route 类:

public class Route implements Comparable<Route> {
    private long lastUpdated;
    private Cell startCell;
    private Cell endCell;
    private int dropOffSize;

    public Route(Cell startCell, Cell endCell, long lastUpdated) {
        this.startCell = startCell;
        this.endCell = endCell;
        this.lastUpdated = lastUpdated;
    }

    public long getLastUpdated() {
        return this.lastUpdated;
    }

    public void setLastUpdated(long lastUpdated) {
        this.lastUpdated = lastUpdated;
    }

    public Cell getStartCell() {
        return startCell;
    }

    public void setStartCell(Cell startCell) {
        this.startCell = startCell;
    }

    public Cell getEndCell() {
        return endCell;
    }

    public void setEndCell(Cell endCell) {
        this.endCell = endCell;
    }

    public int getDropOffSize() {
        return this.dropOffSize;
    }

    public void setDropOffSize(int dropOffSize) {
        this.dropOffSize = dropOffSize;
    }

    @Override
    /**
     * Compute hash code by using Apache Commons Lang HashCodeBuilder.
     */
    public int hashCode() {
        return new HashCodeBuilder(43, 59)
                .append(this.startCell)
                .append(this.endCell)
                .toHashCode();
    }

    @Override
    /**
     * Compute equals by using Apache Commons Lang EqualsBuilder.
     */
    public boolean equals(Object obj) {
        if (!(obj instanceof Route))
            return false;
        if (obj == this)
            return true;

        Route route = (Route) obj;
        return new EqualsBuilder()
                .append(this.startCell, route.startCell)
                .append(this.endCell, route.endCell)
                .isEquals();
    }

    @Override
    public int compareTo(Route route) {
        if (this.dropOffSize < route.dropOffSize)
            return -1;
        else if (this.dropOffSize > route.dropOffSize)
            return 1;
        else {
                // if contains drop off timestamps, order by last timestamp in drop off
                // the highest timestamp has preceding
            if (this.lastUpdated < route.lastUpdated)
                return -1;
            else if (this.lastUpdated > route.lastUpdated)
                return 1;
            else
                return 0;
        }
    }
}

我还想实现的是，每个组的键都是具有最大 lastUpdated 值的键。我已经在看 this solution但我不知道如何结合计数和按值分组以及路由最大 lastUpdated 值。这是我想要实现的示例数据:

示例:

List<Route> routes = new ArrayList<>();
routes.add(new Route(new Cell(1, 2), new Cell(2, 1), 1200L));
routes.add(new Route(new Cell(3, 2), new Cell(2, 5), 1800L));
routes.add(new Route(new Cell(1, 2), new Cell(2, 1), 1700L));

应转换为:

Map<Route, Long> routesCounted = new HashMap<>();
routesCounted.put(new Route(new Cell(1, 2), new Cell(2, 1), 1700L), 2);
routesCounted.put(new Route(new Cell(3, 2), new Cell(2, 5), 1800L), 1);

请注意，计算 2 个路由的映射键是具有最大 lastUpdated 值的键。

最佳答案

这是一种方法。首先分组到列表中，然后将列表处理成您真正想要的值:

import static java.util.Comparator.comparingLong;
import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.toMap;


Map<Route,Integer> routeCounts = routes.stream()
        .collect(groupingBy(x -> x))
        .values().stream()
        .collect(toMap(
            lst -> lst.stream().max(comparingLong(Route::getLastUpdated)).get(),
            List::size
        ));

关于java - 按对象值分组，计数，然后按最大对象属性设置组键，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/30210547/