.collect(Collectors.toMap（））报错 NullPointerException: null at java.util.HashMap.merge

改进前

Map collect = clientEntities.stream()
        .collect(Collectors
        .toMap(ClientEntity::getClientId,ClientEntity::getUseLocation));

改进后：

Map collect = clientEntities.stream()
        .collect(HashMap::new
                ,(map,client)->map.put(client.getClientId(),client.getUseLocation())
                ,HashMap::putAll);