grouping and sum with nested lists

You have to use two flatMap so you can group by assetId

Map<String, BigDecimal> collect = accounts.stream()
        .flatMap(account -> account.getList().stream())
        .flatMap(subAccount -> subAccount.getList().stream())
        .collect(Collectors.groupingBy(Assets::getAssetId,
                Collectors.reducing(
                        BigDecimal.ZERO, 
                        Assets::getQuantity,
                        BigDecimal::add)
        ));

from your code assetId is a String so the key of map should be a String, or you have to convert it, or change it in your class, like so :

Map<Long, BigDecimal> collect = accounts.stream()
        .flatMap(account -> account.getList().stream())
        .flatMap(subAccount -> subAccount.getList().stream())
        .collect(Collectors.groupingBy(asset -> Long.valueOf(asset.getAssetId()),
                Collectors.reducing(
                        BigDecimal.ZERO,
                        Assets::getQuantity,
                        BigDecimal::add
                )
        ));

Notes

• don't use plurals in the name of classes;
• don't name variable as list, use another significant name;
• don't use assetId as a name of attribute instead use just use id;
• don't use List in the name of variables assetList, instead use s in the end for example assets or accounts.