获取第一个匹配条件的元素

How to get first element that matches a criteria in a stream? I've tried this but doesn't work

如何获得与流中的标准匹配的第一个元素?我试过了，但是没用

this.stops.stream().filter(Stop s-> s.getStation().getName().equals(name));

That criteria is not working, the filter method is invoked in an other class than Stop.

该条件无效，筛选器方法在停止之外的另一个类中调用。

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;
}

}

2 个解决方案

#1

115

This might be what you are looking for:

这可能是你正在寻找的:

yourStream
    .filter(/* your criteria */)
    .findFirst()
    .get();

An example:

一个例子:

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()
            .get();

    System.out.printf("At the first stop at Station1 there were %d passengers in the train.", firstStopAtStation1.passengerCount);
}

Output is:

输出是:

At the first stop at Station1 there were 250 passengers in the train.

#2

When you write a lambda expression, the argument list to the left of -> can be either a parenthesized argument list (possibly empty), or a single identifier without any parentheses. But in the second form, the identifier cannot be declared with a type name. Thus:

当您编写lambda表达式时，在->左边的参数列表可以是一个圆括号的参数列表(可能是空的)，或者一个没有任何括号的单一标识符。但是在第二种形式中，不能用类型名声明标识符。因此:

this.stops.stream().filter(Stop s-> s.getStation().getName().equals(name));

is incorrect syntax; but

是不正确的语法;但

this.stops.stream().filter((Stop s)-> s.getStation().getName().equals(name));

is correct. Or:

是正确的。或者:

this.stops.stream().filter(s -> s.getStation().getName().equals(name));

is also correct if the compiler has enough information to figure out the types.

如果编译器有足够的信息来计算类型，则也是正确的。

#1

115