java - Cannot make filter->forEach->collect in one stream?

Question

I want to achieve something like this:

items.stream()
    .filter(s-> s.contains("B"))
    .forEach(s-> s.setState("ok"))
.collect(Collectors.toList());

filter, then change a property from the filtered result, then collect the result to a list. However, the debugger says:

Cannot invoke collect(Collectors.toList()) on the primitive type void.

Do I need 2 streams for that?

Answer 1

The forEach is designed to be a terminal operation and yes - you can't do anything after you call it.

The idiomatic way would be to apply a transformation first and then collect() everything to the desired data structure.

The transformation can be performed using map which is designed for non-mutating operations.

If you are performing a non-mutating operation:

 items.stream()
   .filter(s -> s.contains("B"))
   .map(s -> s.withState("ok"))
   .collect(Collectors.toList());

where withState is a method that returns a copy of the original object including the provided change.

---

If you are performing a side effect:

items.stream()
  .filter(s -> s.contains("B"))
  .collect(Collectors.toList());

items.forEach(s -> s.setState("ok"))