Java 8 modify stream elements

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

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());