Variable declaration for examples:
Collection<String> abc = Arrays.asList("a", "b", "c");
Collection<String> digits = Arrays.asList("1", "2", "3");
Collection<String> greekAbc = Arrays.asList("alpha", "beta", "gamma");Example 1 - Concatenate two Streams
final Stream<String> concat1 = Stream.concat(abc.stream(), digits.stream());
concat1.forEach(System.out::print);
// prints: abc123Example 2 - Concatenate more than two Streams
final Stream<String> concat2 = Stream.concat(
Stream.concat(abc.stream(), digits.stream()),
greekAbc.stream());
System.out.println(concat2.collect(Collectors.joining(", ")));
// prints: a, b, c, 1, 2, 3, alpha, beta, gammaAlternatively to simplify the nested concat() syntax the Streams can also be concatenated with flatMap():
final Stream<String> concat3 = Stream.of(
abc.stream(), digits.stream(), greekAbc.stream())
.flatMap(s -> s);
// or `.flatMap(Function.identity());` (java.util.function.Function)
System.out.println(concat3.collect(Collectors.joining(", ")));
// prints: a, b, c, 1, 2, 3, alpha, beta, gammaBe careful when constructing Streams from repeated concatenation, because accessing an element of a deeply concatenated Stream can result in deep call chains or even a StackOverflowException.