Java 8 modify stream elements

Question

I wanted to write pure function with Java 8 that would take a collection as an argument, apply some change to every object of that collection and return a new collection after the update. I want to follow FP principles so I dont want to update/modify the collection that was passed as an argument.

Is there any way of doing that with Stream API without creating a copy of the original collection first (and then using forEach or 'normal' for loop)?

Sample object below and lets assume that I want to append a text to one of the object property:

public class SampleDTO {     private String text; } 

So I want to do something similar to below, but without modifying the collection. Assuming "list" is a List<SampleDTO>.

list.forEach(s -> {     s.setText(s.getText()+"xxx"); }); 

Answer 1

You must have some method/constructor that generates a copy of an existing SampleDTO instance, such as a copy constructor.

Then you can map each original SampleDTO instance to a new SampleDTO instance, and collect them into a new List :

List<SampleDTO> output =      list.stream()         .map(s-> {                      SampleDTO n = new SampleDTO(s); // create new instance                      n.setText(n.getText()+"xxx"); // mutate its state                      return n; // return mutated instance                  })        .collect(Collectors.toList());