OCP: Doubt on re-using of stream

[Vicky Roy]

Stream can't be used again but OCP book (author Jeanne Boyarsky) states one question wrong (Chapter 4, Question 3) while other one is correct (Chapter 4, Question 14).
Both questions are using same stream though.

Question 3. What is the output of the following?
			Predicate<? super String> predicate = s -> s.length() > 3;
			Stream<String> stream = Stream.iterate("-", (s) -> s + s);
			boolean b1 = stream.noneMatch(predicate);
			boolean b2 = stream.anyMatch(predicate);
			System.out.println(b1 + " " + b2);
A. false true
B. false false
C. java.util.stream.ReferencePipeline$3@4517d9a3
D. The code does not compile.
E. An exception is thrown.
F. The code hangs.

Question 14. Which of the following is true?
			4: Stream<Integer> s = Stream.of(1);
			5: IntStream is = s.mapToInt(x -> x);
			6: DoubleStream ds = s.mapToDouble(x -> x);
			7: Stream<Integer> s2 = ds.mapToInt(x -> x);
			8: s2.forEach(System.out::print);
A. Line 4 does not compile.
B. Line 5 does not compile.
C. Line 6 does not compile.
D. Line 7 does not compile.
E. Line 8 does not compile.
F. The code throws an exception.
G. The code compiles and prints 1.

Thanks.

[Paweł Baczyński]

In the second example there is no reusing of streams. Each stream is used only once (or would be used if the code compiled).

By "used" I refer to a terminal operation being invoked.

[Vicky Roy]

Paweł Baczyński wrote:In the second example there is no reusing of streams. Each stream is used only once (or would be used if the code compiled).

By "used" I refer to a terminal operation being invoked.

In 2nd example line 5 and 6 is using same s variable means same stream. And as per description given for first example, once stream is used can't be re-used.

[Paweł Baczyński]

Vicky Roy wrote:In 2nd example line 5 and 6 is using same s variable means same stream. And as per description given for first example, once stream is used can't be re-used.

There is only one terminal operation invoked on s2 stream. This means the stream is not being re-used.

[Vicky Roy]

Paweł Baczyński wrote:

Vicky Roy wrote:In 2nd example line 5 and 6 is using same s variable means same stream. And as per description given for first example, once stream is used can't be re-used.

There is only one terminal operation invoked on s2 stream. This means the stream is not being re-used.

That seems to be new concept for me "if terminal operation is not involved then stream can be reused". I don't think it was discussed in the book.

Anyways, it means "nonMatch" and "anyMatch" is terminal operations and "mapToInt" not? Right?

[Campbell Ritchie]

It isn't nonMatch, but noneMatch. The xxxMatch methods are equivalent to quantifications:

allMatch ≡ ∀

anyMatch ≡ ∃

noneMatch ≡ ∀•¬ ≡ ¬∃

Have you read the documentation for Stream? It explains there what the difference is between terminal and internediate operations on Streams, and each method has a comment saying something like, “this is a stateful intermediate operation.”

What did you think the correct answers to the questions you quoted were, and what were the answers shown in the book?

[Jeanne Boyarsky]

Campbell Ritchie wrote:It isn't nonMatch, but noneMatch. The xxxMatch methods are equivalent to quantifications:

allMatch ≡ ∀

anyMatch ≡ ∃

noneMatch ≡ ∀•¬ ≡ ¬∃

Have you read the documentation for Stream? It explains there what the difference is between terminal and internediate operations on Streams, and each method has a comment saying something like, “this is a stateful intermediate operation.”

What did you think the correct answers to the questions you quoted were, and what were the answers shown in the book?

#3 - E
#14 - D

$14 doesn't compile (hence D). So it isn't in conflict with #3 in any way!

[Campbell Ritchie]

I tried it and the first one threw an exception because the Stream had been used and cannot be rewound to its beginning. I had guessed it would throw an exception, but got the wrong exception.
The second one is easier; you have a conversion of a double to an int without an explicit cast, and I thought the compiler wouldn't be happy with that.