Java stream operation invocations

up vote
7
down vote

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Can anyone point to a official Java documentation which describes how many times Stream will invoke each "non-interfering and stateless" intermediate operation for each element.

For example:

 Arrays.asList("1", "2", "3", "4").stream()
 .filter( s -> check(s) )
 .forEach( s -> System.out.println(s) );

 public boolean check(Object o) 
 return true;

The above currently will invoke check method 4 times.

Is it possible that in the current or future versions of JDKs the check method gets executed more or less times than the number of elements in the stream created from List or any other standard Java API?

asked 1 hour ago

tsolakp

4,37911218

4

Is there some motive behind this question? Like, you want to add unit test like verify(times(4)).check(anyBool) such that it doesn't fail in future?
– Aayush Kumar Singha
1 hour ago

I do not see a reason why that stream would ever return more than 4 values?
– Mark
1 hour ago

The motive is that if check also does important processing for each element then I want to be sure that it does only once for each element in the stream.
– tsolakp
59 mins ago

The filter predicate is supposed to be stateless.
– Radiodef
38 mins ago

add a comment |

up vote
7
down vote

favorite

Can anyone point to a official Java documentation which describes how many times Stream will invoke each "non-interfering and stateless" intermediate operation for each element.

For example:

 Arrays.asList("1", "2", "3", "4").stream()
 .filter( s -> check(s) )
 .forEach( s -> System.out.println(s) );

 public boolean check(Object o) 
 return true;

The above currently will invoke check method 4 times.

asked 1 hour ago

tsolakp

4,37911218

4

Is there some motive behind this question? Like, you want to add unit test like verify(times(4)).check(anyBool) such that it doesn't fail in future?
– Aayush Kumar Singha
1 hour ago

I do not see a reason why that stream would ever return more than 4 values?
– Mark
1 hour ago

The motive is that if check also does important processing for each element then I want to be sure that it does only once for each element in the stream.
– tsolakp
59 mins ago

The filter predicate is supposed to be stateless.
– Radiodef
38 mins ago

add a comment |

up vote
7
down vote

favorite

Can anyone point to a official Java documentation which describes how many times Stream will invoke each "non-interfering and stateless" intermediate operation for each element.

For example:

 Arrays.asList("1", "2", "3", "4").stream()
 .filter( s -> check(s) )
 .forEach( s -> System.out.println(s) );

 public boolean check(Object o) 
 return true;

The above currently will invoke check method 4 times.

asked 1 hour ago

tsolakp

4,37911218

Can anyone point to a official Java documentation which describes how many times Stream will invoke each "non-interfering and stateless" intermediate operation for each element.

For example:

 Arrays.asList("1", "2", "3", "4").stream()
 .filter( s -> check(s) )
 .forEach( s -> System.out.println(s) );

 public boolean check(Object o) 
 return true;

The above currently will invoke check method 4 times.

java java-8 java-stream

asked 1 hour ago

tsolakp

4,37911218

asked 1 hour ago

tsolakp

4,37911218

asked 1 hour ago

tsolakp

4,37911218

asked 1 hour ago

tsolakp

4,37911218

asked 1 hour ago

tsolakp

4,37911218

4

Is there some motive behind this question? Like, you want to add unit test like verify(times(4)).check(anyBool) such that it doesn't fail in future?
– Aayush Kumar Singha
1 hour ago

I do not see a reason why that stream would ever return more than 4 values?
– Mark
1 hour ago

The motive is that if check also does important processing for each element then I want to be sure that it does only once for each element in the stream.
– tsolakp
59 mins ago

The filter predicate is supposed to be stateless.
– Radiodef
38 mins ago

add a comment |

4

Is there some motive behind this question? Like, you want to add unit test like verify(times(4)).check(anyBool) such that it doesn't fail in future?
– Aayush Kumar Singha
1 hour ago

I do not see a reason why that stream would ever return more than 4 values?
– Mark
1 hour ago

The motive is that if check also does important processing for each element then I want to be sure that it does only once for each element in the stream.
– tsolakp
59 mins ago

The filter predicate is supposed to be stateless.
– Radiodef
38 mins ago

Is there some motive behind this question? Like, you want to add unit test like verify(times(4)).check(anyBool) such that it doesn't fail in future?
– Aayush Kumar Singha
1 hour ago

I do not see a reason why that stream would ever return more than 4 values?
– Mark
1 hour ago

The motive is that if check also does important processing for each element then I want to be sure that it does only once for each element in the stream.
– tsolakp
59 mins ago

The filter predicate is supposed to be stateless.
– Radiodef
38 mins ago

add a comment |

2 Answers
2

active

oldest

votes

up vote
10
down vote

This does not have to do with the source of the stream, but rather the terminal operation and optimization done in the stream implementation itself. For example:

Stream.of(1,2,3,4)
 .map(x -> x + 1)
 .count();

Since java-9, map will not get executed a single time.

Or:

 someTreeSet.stream()
 .sorted()
 .findFirst();

sorted might not get executed at all, since the source is a TreeSet and getting the first element is trivial, but if this is implemented inside stream API or not, is a different question.

So the real answer here - it depends, but I can't imagine one operation that would get executed more that the numbers of elements in the source.

answered 59 mins ago

Eugene

65.9k992158

1

"but I can't imagine one operation that would get executed more that the numbers of elements in the source" - Well, if you specify a Comparator for sorting, its compare will definitely be invoked more times than the number of elements, but that's just nitpicking. +1
– Fureeish
53 mins ago

1

@Fureeish no no, very good nitpick, indeed you are right; but it seems sorted is the only one that could do that. distinct does not for example
– Eugene
46 mins ago

Some time ago I asked this question about jdk8 flatMap (also see this related question). In short, in jdk8&9 flatMap is not completely lazy and elements can be visited more than once, while in jdk10 this behavior has been fixed.
– Federico Peralta Schaffner
26 mins ago

@FedericoPeraltaSchaffner I think that one is about laziness only, not how many times an element can be visited
– Eugene
25 mins ago

Yes, but the thing is that for older versions, an element might end up being visited more than once (as my question shows by means of the peek method)
– Federico Peralta Schaffner
23 mins ago

|
show 7 more comments

up vote
1
down vote

From the documentation:

Laziness-seeking. Many stream operations, such as filtering, mapping, or duplicate removal, can be implemented lazily, exposing opportunities for optimization. For example, "find the first String with three consecutive vowels" need not examine all the input strings. Stream operations are divided into intermediate (Stream-producing) operations and terminal (value- or side-effect-producing) operations. Intermediate operations are always lazy.

By that virtue, because filter is an intermediate operation which creates a new Stream as part of its operation, due to its laziness, it will only ever invoke the filter predicate once per element as part of its rebuilding of the stream.

The only way that your method would possibly have a different number of invocations against it in the stream is if the stream were somehow mutated between states, which given the fact that nothing in a stream actually runs until the terminal operation, would only realistically be possible due to a bug upstream.

answered 51 mins ago

Makoto

78.9k15122164

So what does "Laziness" mean in Java streams? To me it means that the actual invocation of intermediate operation will happen after invocation of terminal operation. But it does not guarantee that intermediate operation will be invoked only once per element (or not at all depending on terminal operation).
– tsolakp
7 mins ago

add a comment |

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2 Answers
2

active

oldest

votes

2 Answers
2

active

oldest

votes

up vote
10
down vote

This does not have to do with the source of the stream, but rather the terminal operation and optimization done in the stream implementation itself. For example:

Stream.of(1,2,3,4)
 .map(x -> x + 1)
 .count();

Since java-9, map will not get executed a single time.

Or:

 someTreeSet.stream()
 .sorted()
 .findFirst();

sorted might not get executed at all, since the source is a TreeSet and getting the first element is trivial, but if this is implemented inside stream API or not, is a different question.

So the real answer here - it depends, but I can't imagine one operation that would get executed more that the numbers of elements in the source.

answered 59 mins ago

Eugene

65.9k992158

1

"but I can't imagine one operation that would get executed more that the numbers of elements in the source" - Well, if you specify a Comparator for sorting, its compare will definitely be invoked more times than the number of elements, but that's just nitpicking. +1
– Fureeish
53 mins ago

1

@Fureeish no no, very good nitpick, indeed you are right; but it seems sorted is the only one that could do that. distinct does not for example
– Eugene
46 mins ago

Some time ago I asked this question about jdk8 flatMap (also see this related question). In short, in jdk8&9 flatMap is not completely lazy and elements can be visited more than once, while in jdk10 this behavior has been fixed.
– Federico Peralta Schaffner
26 mins ago

@FedericoPeraltaSchaffner I think that one is about laziness only, not how many times an element can be visited
– Eugene
25 mins ago

Yes, but the thing is that for older versions, an element might end up being visited more than once (as my question shows by means of the peek method)
– Federico Peralta Schaffner
23 mins ago

|
show 7 more comments

up vote
10
down vote

This does not have to do with the source of the stream, but rather the terminal operation and optimization done in the stream implementation itself. For example:

Stream.of(1,2,3,4)
 .map(x -> x + 1)
 .count();

Since java-9, map will not get executed a single time.

Or:

 someTreeSet.stream()
 .sorted()
 .findFirst();

sorted might not get executed at all, since the source is a TreeSet and getting the first element is trivial, but if this is implemented inside stream API or not, is a different question.

So the real answer here - it depends, but I can't imagine one operation that would get executed more that the numbers of elements in the source.

answered 59 mins ago

Eugene

65.9k992158

1

"but I can't imagine one operation that would get executed more that the numbers of elements in the source" - Well, if you specify a Comparator for sorting, its compare will definitely be invoked more times than the number of elements, but that's just nitpicking. +1
– Fureeish
53 mins ago

1

@Fureeish no no, very good nitpick, indeed you are right; but it seems sorted is the only one that could do that. distinct does not for example
– Eugene
46 mins ago

Some time ago I asked this question about jdk8 flatMap (also see this related question). In short, in jdk8&9 flatMap is not completely lazy and elements can be visited more than once, while in jdk10 this behavior has been fixed.
– Federico Peralta Schaffner
26 mins ago

@FedericoPeraltaSchaffner I think that one is about laziness only, not how many times an element can be visited
– Eugene
25 mins ago

Yes, but the thing is that for older versions, an element might end up being visited more than once (as my question shows by means of the peek method)
– Federico Peralta Schaffner
23 mins ago

|
show 7 more comments

up vote
10
down vote

This does not have to do with the source of the stream, but rather the terminal operation and optimization done in the stream implementation itself. For example:

Stream.of(1,2,3,4)
 .map(x -> x + 1)
 .count();

Since java-9, map will not get executed a single time.

Or:

 someTreeSet.stream()
 .sorted()
 .findFirst();

sorted might not get executed at all, since the source is a TreeSet and getting the first element is trivial, but if this is implemented inside stream API or not, is a different question.

So the real answer here - it depends, but I can't imagine one operation that would get executed more that the numbers of elements in the source.

answered 59 mins ago

Eugene

65.9k992158

This does not have to do with the source of the stream, but rather the terminal operation and optimization done in the stream implementation itself. For example:

Stream.of(1,2,3,4)
 .map(x -> x + 1)
 .count();

Since java-9, map will not get executed a single time.

Or:

 someTreeSet.stream()
 .sorted()
 .findFirst();

sorted might not get executed at all, since the source is a TreeSet and getting the first element is trivial, but if this is implemented inside stream API or not, is a different question.

So the real answer here - it depends, but I can't imagine one operation that would get executed more that the numbers of elements in the source.

answered 59 mins ago

Eugene

65.9k992158

answered 59 mins ago

Eugene

65.9k992158

answered 59 mins ago

Eugene

65.9k992158

answered 59 mins ago

Eugene

65.9k992158

1

"but I can't imagine one operation that would get executed more that the numbers of elements in the source" - Well, if you specify a Comparator for sorting, its compare will definitely be invoked more times than the number of elements, but that's just nitpicking. +1
– Fureeish
53 mins ago

1

@Fureeish no no, very good nitpick, indeed you are right; but it seems sorted is the only one that could do that. distinct does not for example
– Eugene
46 mins ago

Some time ago I asked this question about jdk8 flatMap (also see this related question). In short, in jdk8&9 flatMap is not completely lazy and elements can be visited more than once, while in jdk10 this behavior has been fixed.
– Federico Peralta Schaffner
26 mins ago

@FedericoPeraltaSchaffner I think that one is about laziness only, not how many times an element can be visited
– Eugene
25 mins ago

Yes, but the thing is that for older versions, an element might end up being visited more than once (as my question shows by means of the peek method)
– Federico Peralta Schaffner
23 mins ago

|
show 7 more comments

1

"but I can't imagine one operation that would get executed more that the numbers of elements in the source" - Well, if you specify a Comparator for sorting, its compare will definitely be invoked more times than the number of elements, but that's just nitpicking. +1
– Fureeish
53 mins ago

1

@Fureeish no no, very good nitpick, indeed you are right; but it seems sorted is the only one that could do that. distinct does not for example
– Eugene
46 mins ago

Some time ago I asked this question about jdk8 flatMap (also see this related question). In short, in jdk8&9 flatMap is not completely lazy and elements can be visited more than once, while in jdk10 this behavior has been fixed.
– Federico Peralta Schaffner
26 mins ago

@FedericoPeraltaSchaffner I think that one is about laziness only, not how many times an element can be visited
– Eugene
25 mins ago

Yes, but the thing is that for older versions, an element might end up being visited more than once (as my question shows by means of the peek method)
– Federico Peralta Schaffner
23 mins ago

"but I can't imagine one operation that would get executed more that the numbers of elements in the source" - Well, if you specify a Comparator for sorting, its compare will definitely be invoked more times than the number of elements, but that's just nitpicking. +1
– Fureeish
53 mins ago

@Fureeish no no, very good nitpick, indeed you are right; but it seems sorted is the only one that could do that. distinct does not for example
– Eugene
46 mins ago

Some time ago I asked this question about jdk8 flatMap (also see this related question). In short, in jdk8&9 flatMap is not completely lazy and elements can be visited more than once, while in jdk10 this behavior has been fixed.
– Federico Peralta Schaffner
26 mins ago

@FedericoPeraltaSchaffner I think that one is about laziness only, not how many times an element can be visited
– Eugene
25 mins ago

Yes, but the thing is that for older versions, an element might end up being visited more than once (as my question shows by means of the peek method)
– Federico Peralta Schaffner
23 mins ago

|
show 7 more comments

up vote
1
down vote

From the documentation:

Laziness-seeking. Many stream operations, such as filtering, mapping, or duplicate removal, can be implemented lazily, exposing opportunities for optimization. For example, "find the first String with three consecutive vowels" need not examine all the input strings. Stream operations are divided into intermediate (Stream-producing) operations and terminal (value- or side-effect-producing) operations. Intermediate operations are always lazy.

answered 51 mins ago

Makoto

78.9k15122164

So what does "Laziness" mean in Java streams? To me it means that the actual invocation of intermediate operation will happen after invocation of terminal operation. But it does not guarantee that intermediate operation will be invoked only once per element (or not at all depending on terminal operation).
– tsolakp
7 mins ago

add a comment |

up vote
1
down vote

From the documentation:

Laziness-seeking. Many stream operations, such as filtering, mapping, or duplicate removal, can be implemented lazily, exposing opportunities for optimization. For example, "find the first String with three consecutive vowels" need not examine all the input strings. Stream operations are divided into intermediate (Stream-producing) operations and terminal (value- or side-effect-producing) operations. Intermediate operations are always lazy.

answered 51 mins ago

Makoto

78.9k15122164

So what does "Laziness" mean in Java streams? To me it means that the actual invocation of intermediate operation will happen after invocation of terminal operation. But it does not guarantee that intermediate operation will be invoked only once per element (or not at all depending on terminal operation).
– tsolakp
7 mins ago

add a comment |

up vote
1
down vote

From the documentation:

Laziness-seeking. Many stream operations, such as filtering, mapping, or duplicate removal, can be implemented lazily, exposing opportunities for optimization. For example, "find the first String with three consecutive vowels" need not examine all the input strings. Stream operations are divided into intermediate (Stream-producing) operations and terminal (value- or side-effect-producing) operations. Intermediate operations are always lazy.

answered 51 mins ago

Makoto

78.9k15122164

From the documentation:

Laziness-seeking. Many stream operations, such as filtering, mapping, or duplicate removal, can be implemented lazily, exposing opportunities for optimization. For example, "find the first String with three consecutive vowels" need not examine all the input strings. Stream operations are divided into intermediate (Stream-producing) operations and terminal (value- or side-effect-producing) operations. Intermediate operations are always lazy.

answered 51 mins ago

Makoto

78.9k15122164

answered 51 mins ago

Makoto

78.9k15122164

answered 51 mins ago

Makoto

78.9k15122164

answered 51 mins ago

Makoto

78.9k15122164

So what does "Laziness" mean in Java streams? To me it means that the actual invocation of intermediate operation will happen after invocation of terminal operation. But it does not guarantee that intermediate operation will be invoked only once per element (or not at all depending on terminal operation).
– tsolakp
7 mins ago

add a comment |

So what does "Laziness" mean in Java streams? To me it means that the actual invocation of intermediate operation will happen after invocation of terminal operation. But it does not guarantee that intermediate operation will be invoked only once per element (or not at all depending on terminal operation).
– tsolakp
7 mins ago

So what does "Laziness" mean in Java streams? To me it means that the actual invocation of intermediate operation will happen after invocation of terminal operation. But it does not guarantee that intermediate operation will be invoked only once per element (or not at all depending on terminal operation).
– tsolakp
7 mins ago

add a comment |

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