如何获取流中与条件匹配的第一个元素?我已经尝试过这个但不起作用
this.stops.stream().filter(Stop s-> s.getStation().getName().equals(name));
该标准不起作用,过滤器方法是在 Stop 之外的其他类中调用的。
public class Train {
private final String name;
private final SortedSet<Stop> stops;
public Train(String name) {
this.name = name;
this.stops = new TreeSet<Stop>();
}
public void addStop(Stop stop) {
this.stops.add(stop);
}
public Stop getFirstStation() {
return this.getStops().first();
}
public Stop getLastStation() {
return this.getStops().last();
}
public SortedSet<Stop> getStops() {
return stops;
}
public SortedSet<Stop> getStopsAfter(String name) {
// return this.stops.subSet(, toElement);
return null;
}
}
import java.util.ArrayList;
import java.util.List;
public class Station {
private final String name;
private final List<Stop> stops;
public Station(String name) {
this.name = name;
this.stops = new ArrayList<Stop>();
}
public String getName() {
return name;
}
}
这可能就是您正在寻找的:
yourStream
.filter(/* your criteria */)
.findFirst()
.get();
更好的是,如果有可能不匹配任何元素,在这种情况下get()
会抛出一个NPE。所以使用:
yourStream
.filter(/* your criteria */)
.findFirst()
.orElse(null); /* You could also create a default object here */
An example:
public static void main(String[] args) {
class Stop {
private final String stationName;
private final int passengerCount;
Stop(final String stationName, final int passengerCount) {
this.stationName = stationName;
this.passengerCount = passengerCount;
}
}
List<Stop> stops = new LinkedList<>();
stops.add(new Stop("Station1", 250));
stops.add(new Stop("Station2", 275));
stops.add(new Stop("Station3", 390));
stops.add(new Stop("Station2", 210));
stops.add(new Stop("Station1", 190));
Stop firstStopAtStation1 = stops.stream()
.filter(e -> e.stationName.equals("Station1"))
.findFirst()
.orElse(null);
System.out.printf("At the first stop at Station1 there were %d passengers in the train.", firstStopAtStation1.passengerCount);
}
输出是:
At the first stop at Station1 there were 250 passengers in the train.
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