Streams: Collecting Results in Java

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(System.out::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. 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[]

For collecting stream elements to another target, there is a convenient collect method that takes an instance of the Collector interface. A collector is an object that accumulates elements and produces a result. The Collectors class provides a large number of factory methods for common collectors. To collect stream elements into a list, use the collector produced by Collectors.toList():

List<String> result = stream.collect(Collectors.toListO);

Similarly, here is how you can collect stream elements into a set:

Set<String> result = stream.collect(Collectors.toSet());

If you want to control which kind of set you get, use the following call instead:

TreeSet<String> result = stream.collect(Collectors.toCollection(TreeSet::new));

Suppose you want to collect all strings in a stream by concatenating them. You can call

String result = stream.collect(Col1ectors.joining());

If you want a delimiter between elements, pass it to the joining method:

String result = stream.collect(Collectors.joining(“, “));

If your stream contains objects other than strings, you need to first convert them to strings, like this:

String result = stream.map(Object::toString).collect(Collectors.joining(“, “));

If you want to reduce the stream results to a sum, count, average, maximum, or minimum, use one of the summarizing(Int|Long|Double) methods. These methods take a function that maps the stream objects to numbers and yield a result of type (Int|Long|Double)SummaryStatistics, simultaneously computing the sum, count, average, maximum, and minimum.

IntSummaryStatistics summary = stream.collect(

Collectors.summarizingInt(String::length));

double averageWordLength = summary.getAverage();

double maxWordLength = summary.getMax();

The example program in Listing 1.4 shows how to collect elements from a stream.