Java use streams to modify nested list and return parent list

April 25, 2023

I have following dto

public class StepsDto {
    
    private String step;
    private String status;
    private List<StepsDto> sub_steps;
    private String previousStep;
    private String nextStep;
}

Using streams I need to change the status of Substeps if name of a particular step and its substep and its sub sub step matches.
And then I need to return the complete Parent list of Steps.

I have tried using flatmap but it returns sublist and not the main list.

So for example I have parent step P1,child step C1 and in the child I have another child CC1. I want to change the status of CC1 and return the Parent list.

>Solution :

You are not specifying what kind of stream usage you are looking for. You don’t need flatMap because you are not going to update the structure of the list but only modify the values in it.

You can forcibly use the stream() to iterate over your lists and update the values in place.

    List<StepsDto> l1steps = new ArrayList<>();

    l1steps.stream().forEach(l1step -> {
        l1step.sub_steps.stream().forEach(l2step -> {
            l2step.sub_steps.stream().forEach(l3step -> {
                if(l3step.step.equals(l2step.step) && l3step.step.equals(l1step.step)){
                    l3step.status = "newStatus";
                }
            });
        });
    });

Alternatively, if you don’t want to modify the values in place, instead of foreach you could use map while creating new instances of steps each time.