Java Collectors.toMap（）在内部提供NPE

我试图在流执行中收集数据作为映射

为了确保我没有任何重复，我使用合并功能，但最终有NPE

请参阅代码片段

streamableList().stream()
    .filter(Objects::nonNull)
    .filter(it-> nonNull(it.getKeyHere()))
    .collect(toMap(it -> it.getKeyHere(),
            it -> it.getValueHere(), (a1, a2) -> a1));

请参见下面的例外情况

java.lang.NullPointerException: null
        at java.util.HashMap.merge(HashMap.java:1216)
        at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
        at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
        at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:175)
        at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:175)
        at java.util.ArrayList$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)....
.....and further calls of written code....

有人能提出收集时失败的原因和解决方法吗？

Edit 我只是从Java1.8中查找HashMap代码。下面是片段。当要合并的值作为null传递时，它抛出一个NPE

如果任何值为空，则合并将抛出

@Override
public V merge(K key, V value,
               BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
    if (value == null)
        throw new NullPointerException(); //here

---

什么是StreamableList？您可以将一些数据发布到simulatestreamableList吗？另外，正如@YCF_L所问的，返回的列表类型。是否有可能it.getValueHere返回null？只要您不添加最小的、可验证的示例，我们就无法帮助或.filterit->nonNullit.getKeyHere&&nonNullit.getValueHere

public class NPECollectorsToMap{

    public static void main( String[] args ){
        Map<String, String> map = doThis();
        System.out.println( map );
    }

    private static Map<String, String> doThis() {
        return streamableList().stream()
        .filter(Objects::nonNull)
        .filter(it-> nonNull(it.getKeyHere()))
        .collect(toMap(it -> it.getKeyHere(),
                it -> it.getValueHere(), (a1, a2) -> a1));
    }

    private static List<It> streamableList(){
        List<It> list = new ArrayList<>();
        list.add( new It( "a", null ) );
        return list;
    }

    private static class It{
        private String keyHere;
        public It( String keyHere, String valueHere ){
            super();
            this.keyHere = keyHere;
            this.valueHere = valueHere;
        }
        private String valueHere;
        public String getKeyHere(){ return keyHere; }
        public void setKeyHere( String keyHere ){ this.keyHere = keyHere; }
        public String getValueHere(){ return valueHere; }
        public void setValueHere( String valueHere ){ this.valueHere = valueHere; }

    }
}

@Override
public V merge(K key, V value,
               BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
    if (value == null)
        throw new NullPointerException(); //here

.filter(it -> Objects.nonNull(it.getValueHere()))