Java 8 Streams reduce remove duplicates keeping the most recent entry
2
I have a Java bean, like
class EmployeeContract {
Long id;
Date date;
getter/setter
}
If a have a long list of these, in which we have duplicates by id but with different date, such as:
1, 2015/07/07
1, 2018/07/08
2, 2015/07/08
2, 2018/07/09
How can I reduce such a list keeping only the entries with the most recent date, such as:
1, 2018/07/08
2, 2018/07/09
?
Preferably using Java 8...
I've started with something like:
contract.stream()
.collect(Collectors.groupingBy(EmployeeContract::getId, Collectors.mapping(EmployeeContract::getId, Collectors.toList())))
.entrySet().stream().findFirst();
That gives me the mapping within individual groups, but I'm stuck as to how to collect that into a result list - my streams are not too strong I'm afraid...
java java-8 java-stream reduction
edited Nov 27 '18 at 17:54
nullpointer
43.8k10102201
asked Nov 27 '18 at 17:12
Nestor MilyaevNestor Milyaev
1,1191216
|
show 2 more comments
2
I have a Java bean, like
class EmployeeContract {
Long id;
Date date;
getter/setter
}
If a have a long list of these, in which we have duplicates by id but with different date, such as:
1, 2015/07/07
1, 2018/07/08
2, 2015/07/08
2, 2018/07/09
How can I reduce such a list keeping only the entries with the most recent date, such as:
1, 2018/07/08
2, 2018/07/09
?
Preferably using Java 8...
I've started with something like:
contract.stream()
.collect(Collectors.groupingBy(EmployeeContract::getId, Collectors.mapping(EmployeeContract::getId, Collectors.toList())))
.entrySet().stream().findFirst();
That gives me the mapping within individual groups, but I'm stuck as to how to collect that into a result list - my streams are not too strong I'm afraid...
java java-8 java-stream reduction
edited Nov 27 '18 at 17:54
nullpointer
43.8k10102201
asked Nov 27 '18 at 17:12
Nestor MilyaevNestor Milyaev
1,1191216
2
I wanted to post an answer but this one was closed too fast...yourList.stream() .collect(Collectors.toMap( EmployeeContract::getId, Function.identity(), BinaryOperator.maxBy(Comparator.comparing(EmployeeContract::getDate).reversed())) ) .values();
– Eugene
Nov 27 '18 at 17:22
3
@Eugene instead ofBinaryOperator.maxBy( … .reversed()), you can useBinaryOperator.minBy(…). Though in this case, it looks like the OP wantsmaxBy, without.reversed().
– Holger
Nov 27 '18 at 17:43
2
@Holger and given that this(values()) would return aCollection<EmployeeContract>and not precisely aList<EmployeeContract>, is there a concise way to resolve that?
– nullpointer
Nov 27 '18 at 17:51
1
Given there is a valid discussion and it's a bona fide quiestion, perhaps it's worth to un-hold this question?
– Nestor Milyaev
Nov 27 '18 at 17:53
4
@nullpointer if it really needs to be aList, you can a) wrap the entire expression in anew ArrayList<>( … )or b) wrap the collector in aCollectors.collectingAndThen( …, m -> new ArrayList<>(m.values())).
– Holger
Nov 27 '18 at 17:57
|
show 2 more comments
2
2
2
1
I have a Java bean, like
class EmployeeContract {
Long id;
Date date;
getter/setter
}
If a have a long list of these, in which we have duplicates by id but with different date, such as:
1, 2015/07/07
1, 2018/07/08
2, 2015/07/08
2, 2018/07/09
How can I reduce such a list keeping only the entries with the most recent date, such as:
1, 2018/07/08
2, 2018/07/09
?
Preferably using Java 8...
I've started with something like:
contract.stream()
.collect(Collectors.groupingBy(EmployeeContract::getId, Collectors.mapping(EmployeeContract::getId, Collectors.toList())))
.entrySet().stream().findFirst();
That gives me the mapping within individual groups, but I'm stuck as to how to collect that into a result list - my streams are not too strong I'm afraid...
java java-8 java-stream reduction
edited Nov 27 '18 at 17:54
nullpointer
43.8k10102201
asked Nov 27 '18 at 17:12
Nestor MilyaevNestor Milyaev
1,1191216
I have a Java bean, like
class EmployeeContract {
Long id;
Date date;
getter/setter
}
If a have a long list of these, in which we have duplicates by id but with different date, such as:
1, 2015/07/07
1, 2018/07/08
2, 2015/07/08
2, 2018/07/09
How can I reduce such a list keeping only the entries with the most recent date, such as:
1, 2018/07/08
2, 2018/07/09
?
Preferably using Java 8...
I've started with something like:
contract.stream()
.collect(Collectors.groupingBy(EmployeeContract::getId, Collectors.mapping(EmployeeContract::getId, Collectors.toList())))
.entrySet().stream().findFirst();
That gives me the mapping within individual groups, but I'm stuck as to how to collect that into a result list - my streams are not too strong I'm afraid...
java java-8 java-stream reduction
java java-8 java-stream reduction
edited Nov 27 '18 at 17:54
nullpointer
43.8k10102201
asked Nov 27 '18 at 17:12
Nestor MilyaevNestor Milyaev
1,1191216
edited Nov 27 '18 at 17:54
nullpointer
43.8k10102201
asked Nov 27 '18 at 17:12
Nestor MilyaevNestor Milyaev
1,1191216
edited Nov 27 '18 at 17:54
nullpointer
43.8k10102201
edited Nov 27 '18 at 17:54
nullpointer
43.8k10102201
edited Nov 27 '18 at 17:54
nullpointer
43.8k10102201
43.8k10102201
asked Nov 27 '18 at 17:12
Nestor MilyaevNestor Milyaev
1,1191216
asked Nov 27 '18 at 17:12
Nestor MilyaevNestor Milyaev
1,1191216
asked Nov 27 '18 at 17:12
Nestor MilyaevNestor Milyaev
1,1191216
1,1191216
2
I wanted to post an answer but this one was closed too fast...yourList.stream() .collect(Collectors.toMap( EmployeeContract::getId, Function.identity(), BinaryOperator.maxBy(Comparator.comparing(EmployeeContract::getDate).reversed())) ) .values();
– Eugene
Nov 27 '18 at 17:22
3
@Eugene instead ofBinaryOperator.maxBy( … .reversed()), you can useBinaryOperator.minBy(…). Though in this case, it looks like the OP wantsmaxBy, without.reversed().
– Holger
Nov 27 '18 at 17:43
2
@Holger and given that this(values()) would return aCollection<EmployeeContract>and not precisely aList<EmployeeContract>, is there a concise way to resolve that?
– nullpointer
Nov 27 '18 at 17:51
1
Given there is a valid discussion and it's a bona fide quiestion, perhaps it's worth to un-hold this question?
– Nestor Milyaev
Nov 27 '18 at 17:53
4
@nullpointer if it really needs to be aList, you can a) wrap the entire expression in anew ArrayList<>( … )or b) wrap the collector in aCollectors.collectingAndThen( …, m -> new ArrayList<>(m.values())).
– Holger
Nov 27 '18 at 17:57
|
show 2 more comments
2
I wanted to post an answer but this one was closed too fast...yourList.stream() .collect(Collectors.toMap( EmployeeContract::getId, Function.identity(), BinaryOperator.maxBy(Comparator.comparing(EmployeeContract::getDate).reversed())) ) .values();
– Eugene
Nov 27 '18 at 17:22
3
@Eugene instead ofBinaryOperator.maxBy( … .reversed()), you can useBinaryOperator.minBy(…). Though in this case, it looks like the OP wantsmaxBy, without.reversed().
– Holger
Nov 27 '18 at 17:43
2
@Holger and given that this(values()) would return aCollection<EmployeeContract>and not precisely aList<EmployeeContract>, is there a concise way to resolve that?
– nullpointer
Nov 27 '18 at 17:51
1
Given there is a valid discussion and it's a bona fide quiestion, perhaps it's worth to un-hold this question?
– Nestor Milyaev
Nov 27 '18 at 17:53
4
@nullpointer if it really needs to be aList, you can a) wrap the entire expression in anew ArrayList<>( … )or b) wrap the collector in aCollectors.collectingAndThen( …, m -> new ArrayList<>(m.values())).
– Holger
Nov 27 '18 at 17:57
2
2
I wanted to post an answer but this one was closed too fast...
yourList.stream() .collect(Collectors.toMap( EmployeeContract::getId, Function.identity(), BinaryOperator.maxBy(Comparator.comparing(EmployeeContract::getDate).reversed())) ) .values();– Eugene
Nov 27 '18 at 17:22
I wanted to post an answer but this one was closed too fast...
yourList.stream() .collect(Collectors.toMap( EmployeeContract::getId, Function.identity(), BinaryOperator.maxBy(Comparator.comparing(EmployeeContract::getDate).reversed())) ) .values();– Eugene
Nov 27 '18 at 17:22
3
3
@Eugene instead of
BinaryOperator.maxBy( … .reversed()), you can use BinaryOperator.minBy(…). Though in this case, it looks like the OP wants maxBy, without .reversed().– Holger
Nov 27 '18 at 17:43
@Eugene instead of
BinaryOperator.maxBy( … .reversed()), you can use BinaryOperator.minBy(…). Though in this case, it looks like the OP wants maxBy, without .reversed().– Holger
Nov 27 '18 at 17:43
2
2
@Holger and given that this(
values()) would return a Collection<EmployeeContract> and not precisely a List<EmployeeContract>, is there a concise way to resolve that?– nullpointer
Nov 27 '18 at 17:51
@Holger and given that this(
values()) would return a Collection<EmployeeContract> and not precisely a List<EmployeeContract>, is there a concise way to resolve that?– nullpointer
Nov 27 '18 at 17:51
1
1
Given there is a valid discussion and it's a bona fide quiestion, perhaps it's worth to un-hold this question?
– Nestor Milyaev
Nov 27 '18 at 17:53
Given there is a valid discussion and it's a bona fide quiestion, perhaps it's worth to un-hold this question?
– Nestor Milyaev
Nov 27 '18 at 17:53
4
4
@nullpointer if it really needs to be a
List, you can a) wrap the entire expression in a new ArrayList<>( … ) or b) wrap the collector in a Collectors.collectingAndThen( …, m -> new ArrayList<>(m.values())).– Holger
Nov 27 '18 at 17:57
@nullpointer if it really needs to be a
List, you can a) wrap the entire expression in a new ArrayList<>( … ) or b) wrap the collector in a Collectors.collectingAndThen( …, m -> new ArrayList<>(m.values())).– Holger
Nov 27 '18 at 17:57
|
show 2 more comments
3 Answers
3
active
oldest
votes
5
Well, I am just going to put my comment here in the shape of an answer:
yourList.stream()
.collect(Collectors.toMap(
EmployeeContract::getId,
Function.identity(),
BinaryOperator.maxBy(Comparator.comparing(EmployeeContract::getDate)))
)
.values();
This will give you a Collection instead of a List, if you really care about this.
answered Nov 27 '18 at 19:33
EugeneEugene
70.8k9102169
add a comment |
1
You can do it in two steps as follows :
List<EmployeeContract> finalContract = contract.stream() // Stream<EmployeeContract>
.collect(Collectors.toMap(EmployeeContract::getId,
EmployeeContract::getDate, (a, b) -> a.after(b) ? a : b)) // Map<Long, Date> (Step 1)
.entrySet().stream() // Stream<Entry<Long, Date>>
.map(a -> new EmployeeContract(a.getKey(), a.getValue())) // Stream<EmployeeContract>
.collect(Collectors.toList()); // Step 2
First step: ensures the comparison of dates with the most recent one mapped to an id.
Second step: maps these key, value pairs to a final List<EmployeeContract> as a result.
answered Nov 27 '18 at 18:50
nullpointernullpointer
43.8k10102201
why(a, b) -> a.after(b) ? a : b)whenDatealready implementComparable?
– Eugene
Nov 27 '18 at 19:34
no specific reason @Eugene, I was just looking into theDateAPI for comparison and found usingaftera little better in terms of readability.
– nullpointer
Nov 28 '18 at 1:37
1
Good answer, and with step-by step explanation, much appreciated!
– Nestor Milyaev
Nov 28 '18 at 17:36
add a comment |
0
With vavr.io you can do it like this:
var finalContract = Stream.ofAll(contract) //create io.vavr.collection.Stream
.groupBy(EmployeeContract::getId)
.map(tuple -> tuple._2.maxBy(EmployeeContract::getDate))
.collect(Collectors.toList()); //result is list from java.util package
answered Nov 27 '18 at 19:56
jkerjker
15010
Not sure to understand the difference with a classic Stream from java.util.stream ?
– azro
Nov 27 '18 at 22:35
in io.vavr collections objects are immutable and they have map(), filter() etc methods
– jker
Nov 27 '18 at 22:39
add a comment |
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
5
Well, I am just going to put my comment here in the shape of an answer:
yourList.stream()
.collect(Collectors.toMap(
EmployeeContract::getId,
Function.identity(),
BinaryOperator.maxBy(Comparator.comparing(EmployeeContract::getDate)))
)
.values();
This will give you a Collection instead of a List, if you really care about this.
answered Nov 27 '18 at 19:33
EugeneEugene
70.8k9102169
add a comment |
5
Well, I am just going to put my comment here in the shape of an answer:
yourList.stream()
.collect(Collectors.toMap(
EmployeeContract::getId,
Function.identity(),
BinaryOperator.maxBy(Comparator.comparing(EmployeeContract::getDate)))
)
.values();
This will give you a Collection instead of a List, if you really care about this.
answered Nov 27 '18 at 19:33
EugeneEugene
70.8k9102169
add a comment |
5
5
5
Well, I am just going to put my comment here in the shape of an answer:
yourList.stream()
.collect(Collectors.toMap(
EmployeeContract::getId,
Function.identity(),
BinaryOperator.maxBy(Comparator.comparing(EmployeeContract::getDate)))
)
.values();
This will give you a Collection instead of a List, if you really care about this.
answered Nov 27 '18 at 19:33
EugeneEugene
70.8k9102169
Well, I am just going to put my comment here in the shape of an answer:
yourList.stream()
.collect(Collectors.toMap(
EmployeeContract::getId,
Function.identity(),
BinaryOperator.maxBy(Comparator.comparing(EmployeeContract::getDate)))
)
.values();
This will give you a Collection instead of a List, if you really care about this.
answered Nov 27 '18 at 19:33
EugeneEugene
70.8k9102169
answered Nov 27 '18 at 19:33
EugeneEugene
70.8k9102169
answered Nov 27 '18 at 19:33
EugeneEugene
70.8k9102169
answered Nov 27 '18 at 19:33
EugeneEugene
70.8k9102169
70.8k9102169
add a comment |
add a comment |
1
You can do it in two steps as follows :
List<EmployeeContract> finalContract = contract.stream() // Stream<EmployeeContract>
.collect(Collectors.toMap(EmployeeContract::getId,
EmployeeContract::getDate, (a, b) -> a.after(b) ? a : b)) // Map<Long, Date> (Step 1)
.entrySet().stream() // Stream<Entry<Long, Date>>
.map(a -> new EmployeeContract(a.getKey(), a.getValue())) // Stream<EmployeeContract>
.collect(Collectors.toList()); // Step 2
First step: ensures the comparison of dates with the most recent one mapped to an id.
Second step: maps these key, value pairs to a final List<EmployeeContract> as a result.
answered Nov 27 '18 at 18:50
nullpointernullpointer
43.8k10102201
why(a, b) -> a.after(b) ? a : b)whenDatealready implementComparable?
– Eugene
Nov 27 '18 at 19:34
no specific reason @Eugene, I was just looking into theDateAPI for comparison and found usingaftera little better in terms of readability.
– nullpointer
Nov 28 '18 at 1:37
1
Good answer, and with step-by step explanation, much appreciated!
– Nestor Milyaev
Nov 28 '18 at 17:36
add a comment |
1
You can do it in two steps as follows :
List<EmployeeContract> finalContract = contract.stream() // Stream<EmployeeContract>
.collect(Collectors.toMap(EmployeeContract::getId,
EmployeeContract::getDate, (a, b) -> a.after(b) ? a : b)) // Map<Long, Date> (Step 1)
.entrySet().stream() // Stream<Entry<Long, Date>>
.map(a -> new EmployeeContract(a.getKey(), a.getValue())) // Stream<EmployeeContract>
.collect(Collectors.toList()); // Step 2
First step: ensures the comparison of dates with the most recent one mapped to an id.
Second step: maps these key, value pairs to a final List<EmployeeContract> as a result.
answered Nov 27 '18 at 18:50
nullpointernullpointer
43.8k10102201
why(a, b) -> a.after(b) ? a : b)whenDatealready implementComparable?
– Eugene
Nov 27 '18 at 19:34
no specific reason @Eugene, I was just looking into theDateAPI for comparison and found usingaftera little better in terms of readability.
– nullpointer
Nov 28 '18 at 1:37
1
Good answer, and with step-by step explanation, much appreciated!
– Nestor Milyaev
Nov 28 '18 at 17:36
add a comment |
1
1
1
You can do it in two steps as follows :
List<EmployeeContract> finalContract = contract.stream() // Stream<EmployeeContract>
.collect(Collectors.toMap(EmployeeContract::getId,
EmployeeContract::getDate, (a, b) -> a.after(b) ? a : b)) // Map<Long, Date> (Step 1)
.entrySet().stream() // Stream<Entry<Long, Date>>
.map(a -> new EmployeeContract(a.getKey(), a.getValue())) // Stream<EmployeeContract>
.collect(Collectors.toList()); // Step 2
First step: ensures the comparison of dates with the most recent one mapped to an id.
Second step: maps these key, value pairs to a final List<EmployeeContract> as a result.
answered Nov 27 '18 at 18:50
nullpointernullpointer
43.8k10102201
You can do it in two steps as follows :
List<EmployeeContract> finalContract = contract.stream() // Stream<EmployeeContract>
.collect(Collectors.toMap(EmployeeContract::getId,
EmployeeContract::getDate, (a, b) -> a.after(b) ? a : b)) // Map<Long, Date> (Step 1)
.entrySet().stream() // Stream<Entry<Long, Date>>
.map(a -> new EmployeeContract(a.getKey(), a.getValue())) // Stream<EmployeeContract>
.collect(Collectors.toList()); // Step 2
First step: ensures the comparison of dates with the most recent one mapped to an id.
Second step: maps these key, value pairs to a final List<EmployeeContract> as a result.
answered Nov 27 '18 at 18:50
nullpointernullpointer
43.8k10102201
edited Nov 28 '18 at 8:40
answered Nov 27 '18 at 18:50
nullpointernullpointer
43.8k10102201
answered Nov 27 '18 at 18:50
nullpointernullpointer
43.8k10102201
answered Nov 27 '18 at 18:50
nullpointernullpointer
43.8k10102201
43.8k10102201
why(a, b) -> a.after(b) ? a : b)whenDatealready implementComparable?
– Eugene
Nov 27 '18 at 19:34
no specific reason @Eugene, I was just looking into theDateAPI for comparison and found usingaftera little better in terms of readability.
– nullpointer
Nov 28 '18 at 1:37
1
Good answer, and with step-by step explanation, much appreciated!
– Nestor Milyaev
Nov 28 '18 at 17:36
add a comment |
why(a, b) -> a.after(b) ? a : b)whenDatealready implementComparable?
– Eugene
Nov 27 '18 at 19:34
no specific reason @Eugene, I was just looking into theDateAPI for comparison and found usingaftera little better in terms of readability.
– nullpointer
Nov 28 '18 at 1:37
1
Good answer, and with step-by step explanation, much appreciated!
– Nestor Milyaev
Nov 28 '18 at 17:36
why
(a, b) -> a.after(b) ? a : b) when Date already implement Comparable?– Eugene
Nov 27 '18 at 19:34
why
(a, b) -> a.after(b) ? a : b) when Date already implement Comparable?– Eugene
Nov 27 '18 at 19:34
no specific reason @Eugene, I was just looking into the
Date API for comparison and found using after a little better in terms of readability.– nullpointer
Nov 28 '18 at 1:37
no specific reason @Eugene, I was just looking into the
Date API for comparison and found using after a little better in terms of readability.– nullpointer
Nov 28 '18 at 1:37
1
1
Good answer, and with step-by step explanation, much appreciated!
– Nestor Milyaev
Nov 28 '18 at 17:36
Good answer, and with step-by step explanation, much appreciated!
– Nestor Milyaev
Nov 28 '18 at 17:36
add a comment |
0
With vavr.io you can do it like this:
var finalContract = Stream.ofAll(contract) //create io.vavr.collection.Stream
.groupBy(EmployeeContract::getId)
.map(tuple -> tuple._2.maxBy(EmployeeContract::getDate))
.collect(Collectors.toList()); //result is list from java.util package
answered Nov 27 '18 at 19:56
jkerjker
15010
Not sure to understand the difference with a classic Stream from java.util.stream ?
– azro
Nov 27 '18 at 22:35
in io.vavr collections objects are immutable and they have map(), filter() etc methods
– jker
Nov 27 '18 at 22:39
add a comment |
0
With vavr.io you can do it like this:
var finalContract = Stream.ofAll(contract) //create io.vavr.collection.Stream
.groupBy(EmployeeContract::getId)
.map(tuple -> tuple._2.maxBy(EmployeeContract::getDate))
.collect(Collectors.toList()); //result is list from java.util package
answered Nov 27 '18 at 19:56
jkerjker
15010
Not sure to understand the difference with a classic Stream from java.util.stream ?
– azro
Nov 27 '18 at 22:35
in io.vavr collections objects are immutable and they have map(), filter() etc methods
– jker
Nov 27 '18 at 22:39
add a comment |
0
0
0
With vavr.io you can do it like this:
var finalContract = Stream.ofAll(contract) //create io.vavr.collection.Stream
.groupBy(EmployeeContract::getId)
.map(tuple -> tuple._2.maxBy(EmployeeContract::getDate))
.collect(Collectors.toList()); //result is list from java.util package
answered Nov 27 '18 at 19:56
jkerjker
15010
With vavr.io you can do it like this:
var finalContract = Stream.ofAll(contract) //create io.vavr.collection.Stream
.groupBy(EmployeeContract::getId)
.map(tuple -> tuple._2.maxBy(EmployeeContract::getDate))
.collect(Collectors.toList()); //result is list from java.util package
answered Nov 27 '18 at 19:56
jkerjker
15010
answered Nov 27 '18 at 19:56
jkerjker
15010
answered Nov 27 '18 at 19:56
jkerjker
15010
answered Nov 27 '18 at 19:56
jkerjker
15010
15010
Not sure to understand the difference with a classic Stream from java.util.stream ?
– azro
Nov 27 '18 at 22:35
in io.vavr collections objects are immutable and they have map(), filter() etc methods
– jker
Nov 27 '18 at 22:39
add a comment |
Not sure to understand the difference with a classic Stream from java.util.stream ?
– azro
Nov 27 '18 at 22:35
in io.vavr collections objects are immutable and they have map(), filter() etc methods
– jker
Nov 27 '18 at 22:39
Not sure to understand the difference with a classic Stream from java.util.stream ?
– azro
Nov 27 '18 at 22:35
Not sure to understand the difference with a classic Stream from java.util.stream ?
– azro
Nov 27 '18 at 22:35
in io.vavr collections objects are immutable and they have map(), filter() etc methods
– jker
Nov 27 '18 at 22:39
in io.vavr collections objects are immutable and they have map(), filter() etc methods
– jker
Nov 27 '18 at 22:39
add a comment |
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2
I wanted to post an answer but this one was closed too fast...
yourList.stream() .collect(Collectors.toMap( EmployeeContract::getId, Function.identity(), BinaryOperator.maxBy(Comparator.comparing(EmployeeContract::getDate).reversed())) ) .values();– Eugene
Nov 27 '18 at 17:22
3
@Eugene instead of
BinaryOperator.maxBy( … .reversed()), you can useBinaryOperator.minBy(…). Though in this case, it looks like the OP wantsmaxBy, without.reversed().– Holger
Nov 27 '18 at 17:43
2
@Holger and given that this(
values()) would return aCollection<EmployeeContract>and not precisely aList<EmployeeContract>, is there a concise way to resolve that?– nullpointer
Nov 27 '18 at 17:51
1
Given there is a valid discussion and it's a bona fide quiestion, perhaps it's worth to un-hold this question?
– Nestor Milyaev
Nov 27 '18 at 17:53
4
@nullpointer if it really needs to be a
List, you can a) wrap the entire expression in anew ArrayList<>( … )or b) wrap the collector in aCollectors.collectingAndThen( …, m -> new ArrayList<>(m.values())).– Holger
Nov 27 '18 at 17:57