Streaming question: Best way to get property out of a List of Lists

user1660256 :

I have created a list of say, 100 configurations. I have partitioned that list into groups of 10, so now I have List<List<Config>>. All of these configs have a common attribute value, the domainCode. What's the best way for me to get to that attribute? Say the config domain class looks like this:

public class Config {

    private String configId;
    private String portfolioId;
    private String domainCode;

    public Config(String configId, String portfolioId, String domainCode) {
        this.configId = configId;
        this.portfolioId = portfolioId;
        this.domainCode = domainCode;
    }

    public String getConfigId() { return configId;}
    public String getPortfolioSymbolCode() { return portfolioId;}
    public String getDomainCode() { return domainCode; }
}

I build 100 of them, then group them:

List<Config> configs = getAllConfigs();
List<List<Config>> partitionedConfigs = new ArrayList<>();
if (configs != null) {
    AtomicInteger counter = new AtomicInteger();
    partitionedConfigs.addAll(configs.stream()
            .collect(Collectors.groupingBy(it -> counter.getAndIncrement() / 10))
            .values());
}

I pass partitionedConfigs to a method. That method needs domainCode. What's an efficient way to get the single domainCode out of partitionedConfigs? Even if there are 100 Config objects, the domain code is unique, there will only be one domain code. That is the one that my method needs to find.

Deadpool :

you can get it from configs list it self

List<String> domains = configs.stream()
                              .map(Config::getDomainCode)
                              .collect(Collectors.toList());

Or from partitionedConfigs list also

 List<String> domains = partitionedConfigs.stream()
                                          .flatMap(List::stream)
                                          .map(Config::getDomainCode)
                                          .collect(Collectors.toList());

If every object in list has same domain name then you can get using findFirst

String domain = configs.stream()
                       .findFirst()
                       .map(Config::getDomainCode)
                       .orElse(null); // if list is empty give some default value

From partitionedConfigs list

String domain = partitionedConfigs.stream()
                                  .flatMap(List::stream)
                                  .findFirst()
                                  .map(Config::getDomainCode)
                                  .orElse(null); // if list is empty give some