Java8 Stream上的Call Filter方法

Question

Is it possible to update Stream data and return it directly? I would like to update stream data on request params send by the consumer, kindly see the code below.

Supplier<Stream<User>> userStream = users::stream;

userStream.get().filter(user -> user.getRole().getType().getName().equalsIgnoreCase(value));
userStream.get().filter(user -> user.getDownloadCertificate() == check );

Answer 1

You can't update Stream because it is not a data structure, but a sequence of elements supporting sequential and parallel aggregate operations (source: https://docs.oracle.com/javase/8/docs/api/?java/util/stream/Stream.html ).

Java 8 documentation explains the main difference between Streams and Collections:

Collections and streams , while bearing some superficial similarities, have different goals. Collections are primarily concerned with the efficient management of, and access to, their elements. By contrast, streams do not provide a means to directly access or manipulate their elements, and are instead concerned with declaratively describing their source and the computational operations which will be performed in aggregate on that source. However, if the provided stream operations do not offer the desired functionality, the BaseStream.iterator() and BaseStream.spliterator() operations can be used to perform a controlled traversal.

A stream pipeline, like the "widgets" example above, can be viewed as a query on the stream source. Unless the source was explicitly designed for concurrent modification (such as a ConcurrentHashMap), unpredictable or erroneous behavior may result from modifying the stream source while it is being queried.

Source: https://docs.oracle.com/javase/8/docs/api/?java/util/stream/Stream.html

The desired usage of a Stream in your case may look like this:

List<User> users = getUsers(); // let's assume this method returns a list of users
String name = "some name";
boolean check = true;

//Now let's create a new list of filtered users
List<User> filteredUsers = users.stream()
        .filter(user -> user.getRole().getType().getName().equalsIgnoreCase(name))
        .filter(user -> user.getDownloadCertificate() == check)
        .collect(Collectors.toList());

Keep in mind that initial list of users was not modified. Stream iterated once when the terminal operation ( .collect ) was called.

Answer 2

A Stream will iterate over a data source to perform the operation it represents, but never modify the source.

If you want to modify the source collection in-place, just do it directly without any stream:

users.removeIf(user -> !user.getRole().getType().getName().equalsIgnoreCase(value));
users.removeIf(user -> user.getDownloadCertificate() != check );

Of course, this requires that users is a mutable collection. If that's not the case, you have to create a new collection anyway and in that case, you may use the Stream API to create a new collection, as shown in this answer .