I was confused about the difference between map() and forEach() method in java8 stream. For instance,
List<String> strings = Lists.newArrayList("1", "2");
Map<String, String> map = Maps.newHashMap();
strings.stream().map(s->map.put(s, s));
System.out.println(map);
I got empty output here, but if I change map to forEach() just like
List<String> strings = Lists.newArrayList("1", "2");
Map<String, String> map = Maps.newHashMap();
strings.stream().forEach(s->map.put(s, s));
System.out.println(map);
I can get
{1=1, 2=2}
Why it just didn't run map() method? What's difference between them?
strings.stream().map(s->map.put(s, s));
does nothing, since the stream pipeline is not processed until you execute a terminal operation. Therefore the Map remains empty.
Adding a terminal operation to the stream pipeline will cause map.put(s, s) to be executed for each element of the Stream required by the terminal operation (some terminal operations require just one element, while others require all elements of the Stream).
On the other hand, the second stream pipeline:
strings.stream().forEach(s->map.put(s, s));
ends with a terminal operation - forEach - which is executed for each element of the Stream.
That said, both snippets are misusing Streams. In order to populate a Collection or a Map based on the contents of the Stream, you should use collect(), which can create a Map or a Collection and populate it however you like. forEach and map have different purposes.
For example, to create a Map:
List<String> strings = Lists.newArrayList("1", "2");
Map<String, String> map = strings.stream()
.collect(Collectors.toMap(Function.identity(),
Function.identity()));
System.out.println(map);