Ilya Yevlampiev:
Tengo una corriente de mis objetos de cosecha propia StudentInGroup(ver abajo) y me gustaría colocarlo en el mapa con 3 formas casi idénticas. Por alguna razón, 2 de ellos tienen éxito mientras que el último ha planteado la excepción:
Exception in thread "main" java.lang.UnsupportedOperationException
at java.util.AbstractCollection.add(AbstractCollection.java:262)
at java.util.AbstractCollection.addAll(AbstractCollection.java:344)
at containers.streams.StreamsTest3.lambda$main$13(StreamsTest3.java:93)
at java.util.HashMap.merge(HashMap.java:1245)
at java.util.stream.Collectors.lambda$toMap$172(Collectors.java:1320)
at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
at containers.streams.StreamsTest3.main(StreamsTest3.java:91)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
por lo que el objeto es:
import java.util.Optional;
public class StudentInGroup {
private Optional<String> groupNumber;
private String name;
private boolean isGirl;
private int age;
@Override
public String toString() {
return "StudentInGroup{" +
"groupNumber=" + groupNumber +
", name='" + name + '\'' +
", isGirl=" + isGirl +
", age=" + age +
'}';
}
public StudentInGroup(String name, String groupNumber, int age, boolean isGirl)
{
this.name=name;
this.groupNumber=Optional.ofNullable(groupNumber);
this.age=age;
this.isGirl=isGirl;
}
public Optional<String> getGroupNumber() {
return groupNumber;
}
public void setGroupNumber(Optional<String> groupNumber) {
this.groupNumber = groupNumber;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public boolean isGirl() {
return isGirl;
}
public void setGirl(boolean girl) {
isGirl = girl;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
}
la creación de corriente si así:
Optional<StudentInGroup> petrov = Optional.of(new StudentInGroup("Petrov", null, 18, false));
Optional<StudentInGroup> petrova = Optional.of(new StudentInGroup("Petrova", "11-305", 18, true));
List<Optional<StudentInGroup>> studentInGroupList = new ArrayList<>();
studentInGroupList.add(Optional.of(new StudentInGroup("Ivanov", "11-307", 18, false)));
studentInGroupList.add(Optional.of(new StudentInGroup("Johns", "11-307", 19, false)));
studentInGroupList.add(Optional.of(new StudentInGroup("Walitov", "11-307", 17, false)));
studentInGroupList.add(Optional.of(new StudentInGroup("Kowalski", "11-302", 19, false)));
studentInGroupList.add(Optional.of(new StudentInGroup("Glauberová", "11-302", 19, true)));
studentInGroupList.add(Optional.of(new StudentInGroup("Dybenko", "11-307", 17, true)));
studentInGroupList.add(Optional.of(new StudentInGroup("Ishikawa", "11-305", 18, false)));
studentInGroupList.add(Optional.of(new StudentInGroup("Nguyen", "11-305", 18, false)));
studentInGroupList.add(Optional.of(new StudentInGroup("Chui", "11-305", 18, true)));
studentInGroupList.add(petrov);
studentInGroupList.add(Optional.empty());
studentInGroupList.add(petrova);
Supplier<Stream<StudentInGroup>> studentInGroupStreamSupplier = () -> studentInGroupList.stream().map((o) -> o.orElse(new StudentInGroup(null, null, -1, false)));
y dos métodos de trabajo son
Map<Integer, Set<StudentInGroup>> stringStudentInGroupMap3 = studentInGroupStreamSupplier.<StudentInGroup>get().collect(Collectors.toMap(StudentInGroup::getAge, Collections::<StudentInGroup>singleton,
(Set<StudentInGroup> a, Set<StudentInGroup> b) -> {
Set<StudentInGroup> studentInGroupSet = new HashSet<>();
studentInGroupSet.addAll(a);
studentInGroupSet.addAll(b);
return studentInGroupSet;
}));
System.out.println(stringStudentInGroupMap3);
Map<Integer, Set<StudentInGroup>> stringStudentInGroupMap4 = studentInGroupStreamSupplier.<StudentInGroup>get().collect(Collectors.toMap(StudentInGroup::getAge, x -> new HashSet<>(Arrays.asList(x)),
(x, y) -> {
x.addAll(y);
return x;
}));
System.out.println(stringStudentInGroupMap4);
con el ser valor de salida
{-1=[StudentInGroup{groupNumber=Optional.empty, name='null', isGirl=false, age=-1}], 17=[StudentInGroup{groupNumber=Optional[11-307], name='Dybenko', isGirl=true, age=17}, StudentInGroup{groupNumber=Optional[11-307], name='Walitov', isGirl=false, age=17}], 18=[StudentInGroup{groupNumber=Optional[11-307], name='Ivanov', isGirl=false, age=18}, StudentInGroup{groupNumber=Optional.empty, name='Petrov', isGirl=false, age=18}, StudentInGroup{groupNumber=Optional[11-305], name='Petrova', isGirl=true, age=18}, StudentInGroup{groupNumber=Optional[11-305], name='Nguyen', isGirl=false, age=18}, StudentInGroup{groupNumber=Optional[11-305], name='Ishikawa', isGirl=false, age=18}, StudentInGroup{groupNumber=Optional[11-305], name='Chui', isGirl=true, age=18}], 19=[StudentInGroup{groupNumber=Optional[11-302], name='Kowalski', isGirl=false, age=19}, StudentInGroup{groupNumber=Optional[11-302], name='Glauberová', isGirl=true, age=19}, StudentInGroup{groupNumber=Optional[11-307], name='Johns', isGirl=false, age=19}]}
{-1=[StudentInGroup{groupNumber=Optional.empty, name='null', isGirl=false, age=-1}], 17=[StudentInGroup{groupNumber=Optional[11-307], name='Dybenko', isGirl=true, age=17}, StudentInGroup{groupNumber=Optional[11-307], name='Walitov', isGirl=false, age=17}], 18=[StudentInGroup{groupNumber=Optional[11-307], name='Ivanov', isGirl=false, age=18}, StudentInGroup{groupNumber=Optional.empty, name='Petrov', isGirl=false, age=18}, StudentInGroup{groupNumber=Optional[11-305], name='Petrova', isGirl=true, age=18}, StudentInGroup{groupNumber=Optional[11-305], name='Nguyen', isGirl=false, age=18}, StudentInGroup{groupNumber=Optional[11-305], name='Ishikawa', isGirl=false, age=18}, StudentInGroup{groupNumber=Optional[11-305], name='Chui', isGirl=true, age=18}], 19=[StudentInGroup{groupNumber=Optional[11-302], name='Kowalski', isGirl=false, age=19}, StudentInGroup{groupNumber=Optional[11-302], name='Glauberová', isGirl=true, age=19}, StudentInGroup{groupNumber=Optional[11-307], name='Johns', isGirl=false, age=19}]}
mientras que esta llamada provocar el error anterior:
Map<Integer, Set<StudentInGroup>> stringStudentInGroupMap6 = studentInGroupStreamSupplier.<StudentInGroup>get().collect(Collectors.toMap(StudentInGroup::getAge, Collections::<StudentInGroup>singleton,
(x, y) -> {
x.addAll(y);
return x;
}));
System.out.println(stringStudentInGroupMap6);
así, ¿cuál es la razón de la UnsupportedOperationException lanzado en este caso particular?
PD. Yo sé sobre el groupingByes la mejor manera de hacerlo, la pregunta es ¿por qué se produce la excepción.
Lino:
Los Collections.singleton*()métodos devuelven implementaciones Collection inmutables.
Así que hay que reemplazar:
Collections::<StudentInGroup>singleton
Con:
HashSet::new
O la adecuada Setaplicación que se adapte a sus necesidades.