Java 8:尽管避免了终端操作,但请参阅"流已经被操作或关闭"

Question

以下java 8流没有任何终端操作.下面的块不应该是懒惰的,因为我只有中间操作并且还没有通过终端操作进行操作.当我运行这个块时,我得到"流已经被操作或关闭".请参阅https://ideone.com/naR7GB

Stream<String> s = Stream.of("A", "B");
s.map(String::toUpperCase);
s.map(String::toLowerCase);

堆栈跟踪:

java.lang.IllegalStateException: stream has already been operated upon or closed
at java.util.stream.AbstractPipeline.<init>(AbstractPipeline.java:203)
at java.util.stream.ReferencePipeline.<init>(ReferencePipeline.java:94)
at java.util.stream.ReferencePipeline$StatelessOp.<init>(ReferencePipeline.java:618)
at java.util.stream.ReferencePipeline$3.<init>(ReferencePipeline.java:187)
at java.util.stream.ReferencePipeline.map(ReferencePipeline.java:186)

Answer 1

您需要将第二个map()应用于映射的实例:

s.map(String::toUpperCase).map(String::toLowerCase);

要么

Stream<String> s = Stream.of("A", "B");
Stream<String> s2 = s.map(String::toUpperCase);
Stream<String> s3 = s2.map(String::toLowerCase);

因为您只能对同一个流实例执行1次操作.

但请记住,你只能消耗s一次!所以无论是消费s2还是消费s3,都不能消费.这就是我们通常编写链式调用的原因,因为保留中间对象没有意义.