stream()

Learn to create streams from collections with stream().

The Stream<T> interface

Stream<T> is an interface that supports sequential or parallel operations on a collection. Calling list.stream() returns a Stream; it does not modify the collection itself.

Stream operations split into two: intermediate operations return a new Stream and run lazily; terminal operations consume the stream and produce a result. A Stream can be consumed only once; a second use throws IllegalStateException.

Laziness and single use

Intermediate operations do not run immediately; the pipeline starts only when a terminal operation is called. This is known as lazy evaluation. list.stream().filter(x -> x > 0) alone does nothing; adding a terminal like .count() triggers execution.

Once a Stream is consumed it cannot be reused. If a new stream is needed, you call stream() on the collection again. This condition holds regardless of the stream source.

Exercise Count with a Stream

Declare List<String> names = new ArrayList<>();; add "Ayşe", "Alex", "Mehmet". Compute the count with long count = names.stream().count();. Print it with System.out.println(count);.

Show example solution

import java.util.List;
import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>();
        names.add("Ayşe");
        names.add("Alex");
        names.add("Mehmet");
        long count = names.stream().count();
        System.out.println(count);
    }
}

Test yourself

What does list.stream() return?
- A new List
- An Iterator
- An empty array
- A Stream<T>
Show answer

A Stream<T> — stream() returns a Stream<T>; the original list is unchanged.

Complete the Stream call

Stream<String> s = names.____();

Show answer

Stream<String> s = names.stream();

Match the Stream concepts

stream()

Starts a stream
intermediate

Returns a new Stream
terminal

Consumes the stream
lazy

Lazy evaluation

Next lesson: filter

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