I've a a stream which I want to partition into smaller parts based on matching Id and then apply some proccessing logic on each of the part/element.
class BigRequest{
String bId;
List<Parts> parts;
//getters and setter here
}
Class Parts{
String pId;
String partId;
//getters and setter here
}
I want to segregate and create a list of Parts of size 10 when the partId of different parts are same.
How to use the filter or reduce or groupingBy function to compare the two elements and put them to a list?
I've tried filter like below, doesn't take p1 variable:
big.stream().filter( p -> p.getPartId() == p1.getPartId()) //error
Tried groupingBy like this
big.stream().collect(Collectors.groupingBy(Parts::getPartId) //error
I want to iterate over the filtered/reduced list a another and call another function called abc(). How can I do it using Java Streams?
pseudo:
big.getParts().stream.
//dividing logic logic
for(i < parts.size)
abc(p)
Thanks
You might use something like this:
Map<String,List<Parts>> commonId = big.getParts().
stream().
collect(
Collectors.groupingBy(
Parts::getPartId,
Collectors.mapping(
Function.identity(),
Collectors.toList()
)
)
);
and after it, you will just need to iterate over the map and apply your function.
commonId.entrySet().stream().map(entry -> apply(entry))...
Updated
We can omit Collectors.mapping(Function.identity(),Collectors.toList()) part, since it is a default behaviour of groupingBy
Map<String,List<Parts>> commonId = big.getParts().
stream().
collect(
Collectors.groupingBy(
Parts::getPartId
)
);
Related
Model:
public class AgencyMapping {
private Integer agencyId;
private String scoreKey;
}
public class AgencyInfo {
private Integer agencyId;
private Set<String> scoreKeys;
}
My code:
List<AgencyMapping> agencyMappings;
Map<Integer, AgencyInfo> agencyInfoByAgencyId = agencyMappings.stream()
.collect(groupingBy(AgencyMapping::getAgencyId,
collectingAndThen(toSet(), e -> e.stream().map(AgencyMapping::getScoreKey).collect(toSet()))))
.entrySet().stream().map(e -> new AgencyInfo(e.getKey(), e.getValue()))
.collect(Collectors.toMap(AgencyInfo::getAgencyId, identity()));
Is there a way to get the same result and use more simpler code and faster?
You can simplify the call to collectingAndThen(toSet(), e -> e.stream().map(AgencyMapping::getScoreKey).collect(toSet())))) with a call to mapping(AgencyMapping::getScoreKey, toSet()).
Map<Integer, AgencyInfo> resultSet = agencyMappings.stream()
.collect(groupingBy(AgencyMapping::getAgencyId,
mapping(AgencyMapping::getScoreKey, toSet())))
.entrySet()
.stream()
.map(e -> new AgencyInfo(e.getKey(), e.getValue()))
.collect(toMap(AgencyInfo::getAgencyId, identity()));
A different way to see it using a toMap collector:
Map<Integer, AgencyInfo> resultSet = agencyMappings.stream()
.collect(toMap(AgencyMapping::getAgencyId, // key extractor
e -> new HashSet<>(singleton(e.getScoreKey())), // value extractor
(left, right) -> { // a merge function, used to resolve collisions between values associated with the same key
left.addAll(right);
return left;
}))
.entrySet()
.stream()
.map(e -> new AgencyInfo(e.getKey(), e.getValue()))
.collect(toMap(AgencyInfo::getAgencyId, identity()));
The latter example is arguably more complicated than the former. Nevertheless, your approach is pretty much the way to go apart from using mapping as opposed to collectingAndThen as mentioned above.
Apart from that, I don't see anything else you can simplify with the code shown.
As for faster code, if you're suggesting that your current approach is slow in performance then you may want to read the answers here that speak about when you should consider going parallel.
You are collecting to an intermediate map, then streaming the entries of this map to create AgencyInfo instances, which are finally collected to another map.
Instead of all this, you could use Collectors.toMap to collect directly to a map, mapping each AgencyMapping object to the desired AgencyInfo and merging the scoreKeys as needed:
Map<Integer, AgencyInfo> agencyInfoByAgencyId = agencyMappings.stream()
.collect(Collectors.toMap(
AgencyMapping::getAgencyId,
mapping -> new AgencyInfo(
mapping.getAgencyId(),
new HashSet<>(Set.of(mapping.getScoreKey()))),
(left, right) -> {
left.getScoreKeys().addAll(right.getScoreKeys());
return left;
}));
This works by grouping the AgencyMapping elements of the stream by AgencyMapping::getAgencyId, but storing AgencyInfo objects in the map instead. We get these AgencyInfo instances from manually mapping each original AgencyMapping object. Finally, we're merging AgencyInfo instances that are already in the map by means of a merge function that folds left scoreKeys from one AgencyInfo to another.
I'm using Java 9's Set.of to create a singleton set. If you don't have Java 9, you can replace it with Collections.singleton.
I am learning Java Streams and want to replace the below code with java 8 features.
i was able to use stream.filter() and stream.map features , but i could not replace the below code with java 8 features.
List<Subject> subjects= null;
Set<SubjectData> subjectData= new SubjectData();
for (String name: studentNames)
{
//subjects = student.getSubjects(name);
// consider instead of above line , which returns a collection of <Subject>
for (Subject subject : subjects)
{
subjectData.add(new SubjectData(subject.syllabus(), subject.code()));
}
}
any pointers would be appreciated
I imagine something like this is what you intend:
Set<SubjectData> subjectData = studentNames.stream()
.flatMap(name -> student.getSubjects(name).stream())
.map(subject -> new SubjectData(subject.syllabus(), subject.code()))
.collect(Collectors.toSet());
This streams the student names, maps them to their subjects while concatenating those streams, and then creates SubjectData objects for each. Lastly, those objects are collected into a set.
I am trying to extract a filtered list on top of the original list based on some condition. I am using backport version of Java 8 and am not pretty sure how to do this.I get the Set from ccarReport.getCcarReportWorkflowInstances() call. I need to iterate and filter this set based on a condition match( I am comparing the date attribute in each object with the request date being passed. Below is the code
Set<CcarReportWorkflowInstance> ccarReportWorkflowInstanceSet = ccarReport.getCcarReportWorkflowInstances();
List<CcarReportWorkflowInstance> ccarReportWorkflowInstances = StreamSupport.stream(ccarReportWorkflowInstanceSet).filter(ccarReportWorkflowInstance -> DateUtils.isSameDay(cobDate, ccarReportWorkflowInstance.getCobDate()));
The routine which is doing the job
public List<CcarRepWfInstDTO> fetchReportInstances(Long reportId, Date cobDate) {
List<CcarRepWfInstDTO> ccarRepWfInstDTOs = null;
CcarReport ccarReport = validateInstanceSearchParams(reportId, cobDate);
Set<CcarReportWorkflowInstance> ccarReportWorkflowInstanceSet = ccarReport.getCcarReportWorkflowInstances();
List<CcarReportWorkflowInstance> ccarReportWorkflowInstances = StreamSupport.stream(ccarReportWorkflowInstanceSet).filter(ccarReportWorkflowInstance -> DateUtils.isSameDay(cobDate, ccarReportWorkflowInstance.getCobDate()));
ccarRepWfInstDTOs = ccarRepWfInstMapper.ccarRepWfInstsToCcarRepWfInstDTOs(ccarReportWorkflowInstances);
return ccarRepWfInstDTOs;
}
Error I get when I tried to use streams.
Assuming I understood what you are trying to do, you can replace your method body with a single line :
return
validateInstanceSearchParams(reportId, cobDate).getCcarReportWorkflowInstances()
.stream()
.filter(c -> DateUtils.isSameDay(cobDate, c.getCobDate()))
.collect(Collectors.toList());
You can obtain a Stream from the Set by using the stream() method. No need for StreamSupport.stream().
After filtering the Stream, you should collect it into the output List.
I'd use shorter variable and method names. Your code is painful to read.
I want to sort a list of strings (with possibly duplicate entries) by using as ordering reference the order of the entries in another list. So, the following list is the list I want to sort
List<String> list = ['apple','pear','apple','x','x','orange','x','pear'];
And the list that specifies the order is
List<String> order = ['orange','apple','x','pear'];
And the output should be
List<String> result = ['orange','apple','apple','x','x','x','pear','pear'];
Is there a clean way of doing this?
I don't understand if I can use list's sort and compare with the following problem. I tried using map, iterable, intersection, etc.
There might be a more efficient way but at least you get the desired result:
main() {
List<String> list = ['apple','pear','apple','x','x','orange','x','pear'];
List<String> order = ['orange','apple','x','pear'];
list.sort((a, b) => order.indexOf(a).compareTo(order.indexOf(b)));
print(list);
}
Try it on DartPad
The closure passed to list.sort(...) is a custom comparer which instead of comparing the passed item, compares their position in order and returns the result.
Using a map for better lookup performance:
main() {
List<String> list = ['apple','pear','apple','x','x','orange','x','pear'];
List<String> orderList = ['orange','apple','x','pear'];
Map<String,int> order = new Map.fromIterable(
orderList, key: (key) => key, value: (key) => orderList.indexOf(key));
list.sort((a, b) => order[a].compareTo(order[b]));
print(list);
}
Try it on DartPad
I'm trying to use LINQ to return a list of ids given a list of objects where the id is a property. I'd like to be able to do this without looping through each object and pulling out the unique ids that I find.
I have a list of objects of type MyClass and one of the properties of this class is an ID.
public class MyClass
{
public int ID { get; set; }
}
I want to write a LINQ query to return me a list of those Ids.
How do I do that, given an IList<MyClass> such that it returns an IEnumerable<int> of the ids?
I'm sure it must be possible to do it in one or two lines using LINQ rather than looping through each item in the MyClass list and adding the unique values into a list.
IEnumerable<int> ids = list.Select(x=>x.ID).Distinct();
Use the Distinct operator:
var idList = yourList.Select(x=> x.ID).Distinct();
Using straight LINQ, with the Distinct() extension:
var idList = (from x in yourList select x.ID).Distinct();
When taking Distinct, we have to cast into IEnumerable too. If the list is <T> model, it means you need to write code like this:
IEnumerable<T> ids = list.Select(x => x).Distinct();
int[] numbers = {1,2,3,4,5,3,6,4,7,8,9,1,0 };
var nonRepeats = (from n in numbers select n).Distinct();
foreach (var d in nonRepeats)
{
Response.Write(d);
}
Output
1234567890