Updating list of POJOs using Java Stream API

Question

I have the following POJO:

public class Order {
  private String name;
  private String status;
  private BigDecimal total;

  // getters, setters and ctors down here

}

I am looping through a List<Order> and trying to update all their status fields to a value of "ORDERED". The old (pre Streaming API) way of doing this was:

for (Order order : orders) {
  order.setStatus("ORDERED");
}

I'm trying to figure out the Java 8 ("Streaming") way of accomplishing the same thing. My best attempt thus far:

orders.stream().map(order -> order.setStatus("H"));

Produces a compiler error:

"Incompatible types. Required List but 'map' was inferred to Stream: no instance(s) of type variable(s) R exist so that Stream conforms to List"

Any ideas where I'm going awry?

Don't use streams for mutating the object while streaming the collection. — Naman
– Naman, Commented Jul 17, 2019 at 14:36
You want to perform an action for each element, so what made you decide for trying map instead of one of the other stream operations, e.g. forEach? — Holger
– Holger, Commented Jul 17, 2019 at 14:41

Jean Logeart · Accepted Answer · 2019-07-17 14:40:48Z

3

Use forEach:

orders.forEach(order -> order.setStatus("H"));

answered Jul 17, 2019 at 14:40

Jean Logeart

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Comments

Samuel Philipp · Accepted Answer · 2019-07-17 14:56:21Z

0

You do not want to use Stream.map() because it requires a return value which replaces the original value in the stream. You are also missing a terminal operation in your stream, so even if you fix that by returning the original value it wont work. Stream.forEach() is a terminal operation you can use for this.

To update each object in your list you can just use orders.forEach(). This is the same as orders.stream().forEach().

orders.forEach(o -> o.setStatus("H"));

If you want to update only some values of your List you can use Stream.filter() before:

orders.stream()
        .filter(o -> "ABC".equals(o.getName())
        .forEach(o -> o.setStatus("H"));

answered Jul 17, 2019 at 14:56

Samuel Philipp

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