Overview

• Typically used to perform a common operation on all items in a Collection or Stream, or on the value which may be contained in an Optional. (It can be useful to view Collection.forEach(Consumer) as an “inside-out” variation on the enhanced-for.)

• Parameterized by the type contained in the Collection, Stream, Optional, etc.

• Method to be implemented is void accept(T t). Thus, a lambda that implements Consumer must take a single parameter and return no result. (If an expression lambda is used, the evaluated result—if any—will be ignored.)

Examples

Print all items in a Collectionusing Collection.forEach(Consumer):

List.of(1, 1, 2, 3, 5, 8, 13)
    .forEach(System.out::println); // Print each value.

Print all items in a Stream using Stream.forEach(Consumer):

Stream.of(1, 1, 2, 3, 5, 8, 13)
    .forEach(System.out::println); // Print each value.

Output

Both of above examples produce this output:

1
1
2
3
5
8
13