java.lang.NullPointerException: element cannot be mapped to a null key
import java.util.ArrayList;
import java.util.stream.Collectors;
import lombok.Builder;
import lombok.Data;
@Data
@Builder
public class S {
private String a;
private String b;
public static void main(String[] args) {
ArrayList<S> list = new ArrayList<>();
list.add(S.builder().a(null).b("1").build()); // groupingBy的元素为空
list.add(S.builder().a(null).b("2").build());// groupingBy的元素为空
list.add(S.builder().a("a").b("1").build());
Map<String, List<S>> result = list.stream().collect(Collectors.groupingBy(S::getA)); // 此行代码报错
}
}
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.stream.Collectors;
import lombok.Builder;
import lombok.Data;
@Data
@Builder
public class S {
private String a;
private String b;
public static void main(String[] args) {
ArrayList<S> list = new ArrayList<>();
list.add(S.builder().a(null).b("1").build());
list.add(S.builder().a(null).b("2").build());
list.add(S.builder().a("a").b("1").build());
Map<Optional<String>, List<S>> aOptionalMap = list.stream().collect(Collectors.groupingBy(s -> Optional.ofNullable(s.getA())));
Map<String, List<S>> result = new HashMap<>();
aOptionalMap.keySet().parallelStream().forEach(aOptional ->
result.put(aOptional.orElse(null), aOptionalMap.get(aOptional)));
}
}