- 1.1. From Iterating to Stream Operations
- 1.2. Stream Creation
- 1.3. The filter, map, and flatMap Methods
- 1.4. Extracting Substreams and Combining Streams
- 1.5. Other Stream Transformations
- 1.6. Simple Reductions
- 1.7. The Optional Type
- 1.8. Collecting Results
- 1.9. Collectors
- 1.10. Reduction Operations
- 1.11. Gatherers
- 1.12. Primitive Type Streams
- 1.13. Parallel Streams
This chapter is from the book
This chapter is from the book
This chapter is from the book
1.8. Collecting Results
When you are done with a stream, you will often want to look at the results. You can call the iterator method, which yields an old-fashioned iterator that you can use to visit the elements.
Alternatively, you can call the forEach method to apply a function to each element:
stream.forEach(IO::println);
On a parallel stream, the forEach method traverses elements in arbitrary order. If you want to process them in stream order, call forEachOrdered instead. Of course, you might then give up some or all of the benefits of parallelism.
But more often than not, you will want to collect the result in a data structure. You have already seen the toList method that yields a list of the stream elements.
Call toArray to get an array of the stream elements.
Since it is not possible to create a generic array at runtime, the expression stream.toArray() returns an Object[] array. If you want an array of the correct type, pass in the array constructor:
String[] result = stream.toArray(String[]::new);
// stream.toArray() has type Object[]
To collect results into other data structures, the Stream interface has a generic collect method. You can concatenate the results in a string, store them in any collection, and insert them into maps. By implementing your own collector, you can consume the stream data in arbitrary ways. You will see the details in the following sections.
