Java Stream API – Best way to transform a list: map or forEach?

Don’t worry about any performance differences, they’re going to be minimal in this case normally.

Method 2 is preferable because

1. it doesn’t require mutating a collection that exists outside the lambda expression.

2. it’s more readable because the different steps that are performed in the collection pipeline are written sequentially: first a filter operation, then a map operation, then collecting the result (for more info on the benefits of collection pipelines, see Martin Fowler’s excellent article.)

3. you can easily change the way values are collected by replacing the Collector that is used. In some cases you may need to write your own Collector, but then the benefit is that you can easily reuse that.