给定这个简单的 Person POJO:
public class Person {
private String id;
private String name;
public Person(String id, String name) {
super();
this.id = id;
this.name = name;
}
public String getId() {
return id;
}
public String getName() {
return name;
}
}
我想收集一个Map<String, Person>
关键是id
的人。
我尝试像这样实现它:
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;
public class CollectorMain {
public static void main(String[] args) {
List<Person> list = new ArrayList<>();
list.add(new Person("312", "John"));
list.add(new Person("454", "Alice"));
list.add(new Person("712", "Bob"));
Map<String, Person> map = list.stream()
.collect(
Collectors.toMap(Person::getId, Function::identity) // COMPILE ERROR
);
System.out.println(map.size());
}
}
但我得到一个编译错误:
CollectorMain.java:[14,41] no suitable method found for toMap(Person::getId,Function::identity)
method java.util.stream.Collectors.<T,K,U>toMap(java.util.function.Function<? super T,? extends K>,java.util.function.Function<? super T,? extends U>) is not applicable
(cannot infer type-variable(s) T,K,U
(argument mismatch; incompatible parameter types in method reference))
method java.util.stream.Collectors.<T,K,U>toMap(java.util.function.Function<? super T,? extends K>,java.util.function.Function<? super T,? extends U>,java.util.function.BinaryOperator<U>) is not applicable
(cannot infer type-variable(s) T,K,U
(actual and formal argument lists differ in length))
method java.util.stream.Collectors.<T,K,U,M>toMap(java.util.function.Function<? super T,? extends K>,java.util.function.Function<? super T,? extends U>,java.util.function.BinaryOperator<U>,java.util.function.Supplier<M>) is not applicable
(cannot infer type-variable(s) T,K,U,M
(actual and formal argument lists differ in length))
作为解决方案,我可以替换Function::identity
有了这个 lambdap -> p
像这样:
Map<String, Person> map = list.stream()
.collect(
Collectors.toMap(Person::getId, p -> p)
);
有没有可能使用Function::identity
?