java.lang.NullPointerException: null
        at java.util.HashMap.merge(HashMap.java:1225)
        at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
        at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
        at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1380)
        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 com.xxx.web.controller.TaskController.getOmsAccidCloudCCUserIdMap(TaskController.java:648)
        at com.xxx.web.controller.TaskController.executePushNewRegisterLead(TaskController.java:400)
        at com.xxx.web.controller.TaskController.pushNewRegister(TaskController.java:145)

omsAccidCloudCCUserIdMap = administratorList.stream()
.collect(Collectors.toMap(Administrator::getAccid,administrator
-> cloudccAccidUserIdMap.get(administrator.getCloudccAccid())));

administrator -> cloudccAccidUserIdMap.get(administrator.getCloudccAccid())

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

/**
     * If the specified key is not already associated with a value or is
     * associated with null, associates it with the given non-null value.
     * Otherwise, replaces the associated value with the results of the given
     * remapping function, or removes if the result is {@code null}. This
     * method may be of use when combining multiple mapped values for a key.
     * For example, to either create or append a {@code String msg} to a
     * value mapping:
     *
     * <pre> {@code
     * map.merge(key, msg, String::concat)
     * }</pre>
     *
     * <p>If the function returns {@code null} the mapping is removed.  If the
     * function itself throws an (unchecked) exception, the exception is
     * rethrown, and the current mapping is left unchanged.
     *
     * @implSpec
     * The default implementation is equivalent to performing the following
     * steps for this {@code map}, then returning the current value or
     * {@code null} if absent:
     *
     * <pre> {@code
     * V oldValue = map.get(key);
     * V newValue = (oldValue == null) ? value :
     *              remappingFunction.apply(oldValue, value);
     * if (newValue == null)
     *     map.remove(key);
     * else
     *     map.put(key, newValue);
     * }</pre>
     *
     * <p>The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties. In particular, all implementations of
     * subinterface {@link java.util.concurrent.ConcurrentMap} must document
     * whether the function is applied once atomically only if the value is not
     * present.
     *
     * @param key key with which the resulting value is to be associated
     * @param value the non-null value to be merged with the existing value
     *        associated with the key or, if no existing value or a null value
     *        is associated with the key, to be associated with the key
     * @param remappingFunction the function to recompute a value if present
     * @return the new value associated with the specified key, or null if no
     *         value is associated with the key
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified key is null and this map
     *         does not support null keys or the value or remappingFunction is
     *         null
     * @since 1.8
     */
default V merge(K key, V value,
                BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
    Objects.requireNonNull(remappingFunction);
    Objects.requireNonNull(value);
    V oldValue = get(key);
    V newValue = (oldValue == null) ? value :
    remappingFunction.apply(oldValue, value);
    if(newValue == null) {
        remove(key);
    } else {
        put(key, newValue);
    }
    return newValue;
}