How to retrieve nested list from object using Stream API?

问题

Do you have any idea how to retrieve all SimpleProperty from TopComplexity object? I need to change that for loop into stream "kind" piece of code.

@Data
public class TopComplexity {
   List<SuperComplexProperty> superComplexProperties;
}
@Data
public class SuperComplexProperty {
   List<SimpleProperty> simpleProperties;
   ComplexProperty complexProperty;
}

@Data
public class ComplexProperty {
   List<SimpleProperty> simpleProperties;
}

public class MainClass {
   public static void main(String[] args) {

       TopComplexity top = null;
       List<SimpleProperty> result = new ArrayList<>();
      
       for(SuperComplexProperty prop : top.getSuperComplexProperties) {
          result.addAll(prop.getSimpleProperties());
      
          if(Objects.nonNull(prop.getComplexProperty()) {
              result.addAll(prop.getComplexProperty().getSimpleProperties());
         }
      }
   }
}

Really appreciate any kind of help

回答1:

You can mix up flatMap with a concatenation and ternary operator involving Streams such as:

List<SimpleProperty> result = top.getSuperComplexProperties().stream()
        .flatMap(scp -> Stream.concat(
                scp.getSimpleProperties().stream(),
                scp.getComplexProperty() == null ?
                        Stream.empty() :
                        scp.getComplexProperty().getSimpleProperties().stream()))
        .collect(Collectors.toList());

来源：https://stackoverflow.com/questions/63189341/how-to-retrieve-nested-list-from-object-using-stream-api