Java 获取符合条件的第一个元素
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Fetch first element which matches criteria
提问by user2147674
How to get first element that matches a criteria in a stream? I've tried this but doesn't work
如何获取与流中的条件匹配的第一个元素？我试过这个但不起作用
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.
该标准不起作用，过滤器方法是在 Stop 以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;
}
}
采纳答案by ifloop
This might be what you are looking for:
这可能是您正在寻找的：
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.
回答by ajb
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 identifierwithout any parentheses. But in the second form, the identifier cannot be declared with a type name. Thus:
编写 lambda 表达式时，左侧的参数列表->可以是带括号的参数列表（可能为空），也可以是不带任何括号的单个标识符。但在第二种形式中，标识符不能用类型名称声明。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确定类型也是正确的。
回答by Martin Volek
I think this is best way:
我认为这是最好的方法：
this.stops.stream().filter(s -> Objects.equals(s.getStation().getName(), this.name)).findFirst().orElse(null);
