Jasper Jasper - 4 years ago 1952
Java Question

Java 8 NullPointerException in Collectors.toMap

The Java 8

throws a
if one of the values is 'null'. I don't understand this behaviour, maps can contain null pointers as value without any problems. Is there a good reason why values cannot be null for

Also, is there a nice Java 8 way of fixing this, or should I revert to plain old for loop?

An example of my problem:

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

class Answer {
private int id;

private Boolean answer;

Answer() {

Answer(int id, Boolean answer) {
this.id = id;
this.answer = answer;

public int getId() {
return id;

public void setId(int id) {
this.id = id;

public Boolean getAnswer() {
return answer;

public void setAnswer(Boolean answer) {
this.answer = answer;

public class Main {
public static void main(String[] args) {
List<Answer> answerList = new ArrayList<>();

answerList.add(new Answer(1, true));
answerList.add(new Answer(2, true));
answerList.add(new Answer(3, null));

Map<Integer, Boolean> answerMap =
.collect(Collectors.toMap(Answer::getId, Answer::getAnswer));


Exception in thread "main" java.lang.NullPointerException
at java.util.HashMap.merge(HashMap.java:1216)
at java.util.stream.Collectors.lambda$toMap$168(Collectors.java:1320)
at java.util.stream.Collectors$$Lambda$5/1528902577.accept(Unknown Source)
at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1359)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:512)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:502)
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 Main.main(Main.java:48)
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:483)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)

Answer Source

It is not possible with the static methods of Collectors. The javadoc of toMap explains that toMap is based on Map.merge:

@param mergeFunction a merge function, used to resolve collisions between values associated with the same key, as supplied to Map#merge(Object, Object, BiFunction)}

and the javadoc of Map.merge says:

@throws NullPointerException if the specified key is null and this map does not support null keys or the value or remappingFunction is null

You can avoid the for loop by using the forEach method of your list.

Map<Integer,  Boolean> answerMap = new HashMap<>();
answerList.forEach((answer) -> answerMap.put(answer.getId(), answer.getAnswer()));

but it is not really simple than the old way:

Map<Integer, Boolean> answerMap = new HashMap<>();
for (Answer answer : answerList) {
    answerMap.put(answer.getId(), answer.getAnswer());
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