I am new to Java 8 and looking to understand the difference between the two scenarios. I know that once a stream is operated and consumed then stream cant be reused again it will give an error.
List<String> title = Arrays.asList("Java8", "In", "Action"); Stream<String> s = title.stream(); s.forEach(System.out::println); s.forEach(System.out::println); // THIS WILL GIVE ERROR - streams has been already operated and closed.
When I run this, I get below error... which is fair.
Java8 In Action Exception in thread "main" java.lang.IllegalStateException: stream has already been operated upon or closed at java.util.stream.AbstractPipeline.sourceStageSpliterator(Unknown Source) at java.util.stream.ReferencePipeline$Head.forEach(Unknown Source) at com.test.Java8InAction.CH4.TraversableOnlyOnce.main(TraversableOnlyOnce.java:12)
// Filtering unique elements List<Integer> numbers = Arrays.asList(1, 2, 1, 3, 3, 2, 4); numbers.stream().forEach(System.out::println); numbers.stream().filter(n -> n % 2 == 0).distinct().forEach(System.out::println); numbers.stream().filter(n -> n % 2 == 0).forEach(System.out::println);
Here also I have operated stream and closed the terminal operation, then why I did not get any error?
The two calls to
s.forEach utilise the same stream hence the exception in the first example whereas the call to the
stream() method -->
numbers.stream() generates (yields) a new stream each time hence doesnt throw a "java.lang.IllegalStateException: stream has already been operated upon or closed".