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How do I turn this expression into a lambda expression?

I'd like to turn what I'm doing into lambda, in which case it would be I scroll through a list (listRegistrationTypeWork) within the other, check if the child list (getRegistrationTypeWorkAuthors) is != null, if it is, scroll through it looking for an authorCoautor = type, and increment a count, to find out how many records within the lists have this same type.
public int qtyMaximumWorksByAuthorCoauthor(AuthorCoauthor type) {
int count = 0;
for (RegistrationTypeWork tab : listRegistrationTypeWork) {
if (CollectionUtils.isNotEmpty(tab.getRegistrationTypeWorkAuthors())) {
for (RegistrationTypeWorkAuthors author : tab.getRegistrationTypeWorkAuthors()) {
if (author.getAuthorCoauthor().equals(type))
count++;
}
}
}
return count;
}

Although your statement is not clear enough on what transforming to lambda expression would mean, but I am assuming you would like to turn your imperative looping step to a functional stream and lambda based one.
This should be straightforward using:
filter to filter out the unwanted values from both of your collections
flatMap to flatten all inner collections into a single stream so that you can operate your count on it as a single source
public int qtyMaximumWorksByAuthorCoauthor(AuthorCoauthor type) {
return listRegistrationTypeWork.stream()
.filter(tab -> tab.getRegistrationTypeWorkAuthors() != null)
.flatMap(tab -> tab.getRegistrationTypeWorkAuthors().stream())
.filter(author -> type.equals(author.getAuthorCoauthor()))
.count();
}

In addition to Thomas fine comment I think you would want to write your stream something like this.
long count = listRegistrationTypeWork.stream()
// to make sure no lists that are actual null are mapped.
// map all RegistrationTypeWork into optionals of lists of RegistrationTypeWorkAuthors
.map(registrationTypeWork -> Optional.ofNullable(registrationTypeWork.getRegistrationTypeWorkAuthors()))
// this removes all empty Optionals from the stream
.flatMap(Optional::stream)
// this turns the stream of lists of RegistrationTypeWorkAuthors into a stream of plain RegistrationTypeWorkAuthors
.flatMap(Collection::stream)
// this filters out RegistrationTypeWorkAuthors which are of a different type
.filter(registrationTypeWorkAuthors -> type.equals(registrationTypeWorkAuthors.getAuthorCoauthor()))
.count();
// count returns a long so you either need to return a long in your method signature or cast the long to an integer.
return (int) count;

Convert search in list using simple for loop to stream approach

I want to get an integer value out of list of objects (Labels) and one of the fields inside this object is integer which I would like to retrieve.
I was able to do it using simple foreach loop and I wonder how it can be done using java 8 Stream approach.
public int getLabelIdByLabelName(String labelName) throws ApiException {
List<Labels> labelsList = labelsAPI.listAllLabels();
for(Labels labels : labelsList){
if(labels.getName().equals(labelName)){
return labels.getId();
}
}
return 0;
}

The equivalent stream can be :
public int getLabelIdByLabelName(String labelName) throws ApiException {
return labelsAPI.listAllLabels().stream()
.filter(label -> label.getName().equals(labelName))
.findFirst()
.map(Labels::getId)
.orElse(0);
}

Is it possible to replace nested loops on different collections with Stream API

I was wondering if it's possible to rewrite nested for loops using java.utils.stream in Java 8?
Here is a sample data type I'm working with:
class Folder {
private final String name;
private final Integer itemCound;
Folder(String name, Integer itemCount) {
this.name = name;
this.itemCount = itemCount;
}
public String getName() { return this.name; }
public Integer getItemCount() { return this.itemCount; }
}
Here's code in action:
List<Folder> oldFolders = new ArrayList<>();
List<Folder> newFolders = new ArrayList<>();
// Fill folder collections with sample data...
oldFolders.add(new Folder("folder1", 2));
oldFolders.add(new Folder("folder2", 4));
newFolders.add(new Folder("folder1", 0));
newFolders.add(new Folder("folder2", 100));
// This part should be rewrited using streams
for (Folder newFolder : newFolders) {
for (Folder oldFolder : oldFolders) {
if (newFolder.getName().equals(oldFolder.getName())
&& !newFolder.getItemCount().equals(oldFolder.getItemCount())) {
// do stuff...
}
}
}
P.S: I've seen other questions on SO, but all of them had 1 collection or a collection with it's own nested collection instead of two different collections like in my example.
Thanks in advance!

That not much of an improvement to be fair unless if you can parallelize the first iteration (commented in this example)
List<String> oldList = new ArrayList<>();
List<String> newList = new ArrayList<>();
oldList
//.stream()
//.parallel()
.forEach(s1 ->
newList
.stream()
.filter(s2 -> s1.equals(s2)) //could become a parameter Predicate
.forEach(System.out::println) //could become a parameter Consumer
);
Replacing the if with a filter and his Predicate then executing a method on it.
This would give a solution that can be dynamic providing different Predicate and Consumer to the filter and forEach method. That would be the only reason to work on this conversion.

Yeah you can:
newFolders.forEach((newFolder) -> {
oldFolders.forEach((oldFolder) -> {
if (newFolder.getName().equals(oldFolder.getName())
&& !newFolder.getItemCount().equals(oldFolder.getItemCount())) {
// do stuff...
}
})
})
EDIT: But as #Kayaman mentions in the comments below this is not necessarily better than just using nested for loops.
This is a good read for when you should and shouldn't consider using streams:
In Java, what are the advantages of streams over loops?

Try with this:
newFolders.stream().filter(newFolder ->
oldFolders
.stream()
.anyMatch(oldFolder->
newFolder.getName().equals(oldFolder.getName()) &&
!newFolder.getItemCount().equals(oldFolder.getItemCount())
)).forEach(folder -> {
//dostuff
});

Is there a way to check if an object contains certain data? [duplicate]

I want to filter a java.util.Collection based on a predicate.

Java 8 (2014) solves this problem using streams and lambdas in one line of code:
List<Person> beerDrinkers = persons.stream()
.filter(p -> p.getAge() > 16).collect(Collectors.toList());
Here's a tutorial.
Use Collection#removeIf to modify the collection in place. (Notice: In this case, the predicate will remove objects who satisfy the predicate):
persons.removeIf(p -> p.getAge() <= 16);
lambdaj allows filtering collections without writing loops or inner classes:
List<Person> beerDrinkers = select(persons, having(on(Person.class).getAge(),
greaterThan(16)));
Can you imagine something more readable?
Disclaimer: I am a contributor on lambdaj

Assuming that you are using Java 1.5, and that you cannot add Google Collections, I would do something very similar to what the Google guys did. This is a slight variation on Jon's comments.
First add this interface to your codebase.
public interface IPredicate<T> { boolean apply(T type); }
Its implementers can answer when a certain predicate is true of a certain type. E.g. If T were User and AuthorizedUserPredicate<User> implements IPredicate<T>, then AuthorizedUserPredicate#apply returns whether the passed in User is authorized.
Then in some utility class, you could say
public static <T> Collection<T> filter(Collection<T> target, IPredicate<T> predicate) {
Collection<T> result = new ArrayList<T>();
for (T element: target) {
if (predicate.apply(element)) {
result.add(element);
}
}
return result;
}
So, assuming that you have the use of the above might be
Predicate<User> isAuthorized = new Predicate<User>() {
public boolean apply(User user) {
// binds a boolean method in User to a reference
return user.isAuthorized();
}
};
// allUsers is a Collection<User>
Collection<User> authorizedUsers = filter(allUsers, isAuthorized);
If performance on the linear check is of concern, then I might want to have a domain object that has the target collection. The domain object that has the target collection would have filtering logic for the methods that initialize, add and set the target collection.
UPDATE:
In the utility class (let's say Predicate), I have added a select method with an option for default value when the predicate doesn't return the expected value, and also a static property for params to be used inside the new IPredicate.
public class Predicate {
public static Object predicateParams;
public static <T> Collection<T> filter(Collection<T> target, IPredicate<T> predicate) {
Collection<T> result = new ArrayList<T>();
for (T element : target) {
if (predicate.apply(element)) {
result.add(element);
}
}
return result;
}
public static <T> T select(Collection<T> target, IPredicate<T> predicate) {
T result = null;
for (T element : target) {
if (!predicate.apply(element))
continue;
result = element;
break;
}
return result;
}
public static <T> T select(Collection<T> target, IPredicate<T> predicate, T defaultValue) {
T result = defaultValue;
for (T element : target) {
if (!predicate.apply(element))
continue;
result = element;
break;
}
return result;
}
}
The following example looks for missing objects between collections:
List<MyTypeA> missingObjects = (List<MyTypeA>) Predicate.filter(myCollectionOfA,
new IPredicate<MyTypeA>() {
public boolean apply(MyTypeA objectOfA) {
Predicate.predicateParams = objectOfA.getName();
return Predicate.select(myCollectionB, new IPredicate<MyTypeB>() {
public boolean apply(MyTypeB objectOfB) {
return objectOfB.getName().equals(Predicate.predicateParams.toString());
}
}) == null;
}
});
The following example, looks for an instance in a collection, and returns the first element of the collection as default value when the instance is not found:
MyType myObject = Predicate.select(collectionOfMyType, new IPredicate<MyType>() {
public boolean apply(MyType objectOfMyType) {
return objectOfMyType.isDefault();
}}, collectionOfMyType.get(0));
UPDATE (after Java 8 release):
It's been several years since I (Alan) first posted this answer, and I still cannot believe I am collecting SO points for this answer. At any rate, now that Java 8 has introduced closures to the language, my answer would now be considerably different, and simpler. With Java 8, there is no need for a distinct static utility class. So if you want to find the 1st element that matches your predicate.
final UserService userService = ... // perhaps injected IoC
final Optional<UserModel> userOption = userCollection.stream().filter(u -> {
boolean isAuthorized = userService.isAuthorized(u);
return isAuthorized;
}).findFirst();
The JDK 8 API for optionals has the ability to get(), isPresent(), orElse(defaultUser), orElseGet(userSupplier) and orElseThrow(exceptionSupplier), as well as other 'monadic' functions such as map, flatMap and filter.
If you want to simply collect all the users which match the predicate, then use the Collectors to terminate the stream in the desired collection.
final UserService userService = ... // perhaps injected IoC
final List<UserModel> userOption = userCollection.stream().filter(u -> {
boolean isAuthorized = userService.isAuthorized(u);
return isAuthorized;
}).collect(Collectors.toList());
See here for more examples on how Java 8 streams work.

Use CollectionUtils.filter(Collection,Predicate), from Apache Commons.

"Best" way is too wide a request. Is it "shortest"? "Fastest"? "Readable"?
Filter in place or into another collection?
Simplest (but not most readable) way is to iterate it and use Iterator.remove() method:
Iterator<Foo> it = col.iterator();
while( it.hasNext() ) {
Foo foo = it.next();
if( !condition(foo) ) it.remove();
}
Now, to make it more readable, you can wrap it into a utility method. Then invent a IPredicate interface, create an anonymous implementation of that interface and do something like:
CollectionUtils.filterInPlace(col,
new IPredicate<Foo>(){
public boolean keepIt(Foo foo) {
return foo.isBar();
}
});
where filterInPlace() iterate the collection and calls Predicate.keepIt() to learn if the instance to be kept in the collection.
I don't really see a justification for bringing in a third-party library just for this task.

Consider Google Collections for an updated Collections framework that supports generics.
UPDATE: The google collections library is now deprecated. You should use the latest release of Guava instead. It still has all the same extensions to the collections framework including a mechanism for filtering based on a predicate.

Wait for Java 8:
List<Person> olderThan30 =
//Create a Stream from the personList
personList.stream().
//filter the element to select only those with age >= 30
filter(p -> p.age >= 30).
//put those filtered elements into a new List.
collect(Collectors.toList());

Since the early release of Java 8, you could try something like:
Collection<T> collection = ...;
Stream<T> stream = collection.stream().filter(...);
For example, if you had a list of integers and you wanted to filter the numbers that are > 10 and then print out those numbers to the console, you could do something like:
List<Integer> numbers = Arrays.asList(12, 74, 5, 8, 16);
numbers.stream().filter(n -> n > 10).forEach(System.out::println);

I'll throw RxJava in the ring, which is also available on Android. RxJava might not always be the best option, but it will give you more flexibility if you wish add more transformations on your collection or handle errors while filtering.
Observable.from(Arrays.asList(1, 2, 3, 4, 5))
.filter(new Func1<Integer, Boolean>() {
public Boolean call(Integer i) {
return i % 2 != 0;
}
})
.subscribe(new Action1<Integer>() {
public void call(Integer i) {
System.out.println(i);
}
});
Output:
1
3
5
More details on RxJava's filter can be found here.

The setup:
public interface Predicate<T> {
public boolean filter(T t);
}
void filterCollection(Collection<T> col, Predicate<T> predicate) {
for (Iterator i = col.iterator(); i.hasNext();) {
T obj = i.next();
if (predicate.filter(obj)) {
i.remove();
}
}
}
The usage:
List<MyObject> myList = ...;
filterCollection(myList, new Predicate<MyObject>() {
public boolean filter(MyObject obj) {
return obj.shouldFilter();
}
});

How about some plain and straighforward Java
List<Customer> list ...;
List<Customer> newList = new ArrayList<>();
for (Customer c : list){
if (c.getName().equals("dd")) newList.add(c);
}
Simple, readable and easy (and works in Android!)
But if you're using Java 8 you can do it in a sweet one line:
List<Customer> newList = list.stream().filter(c -> c.getName().equals("dd")).collect(toList());
Note that toList() is statically imported

Are you sure you want to filter the Collection itself, rather than an iterator?
see org.apache.commons.collections.iterators.FilterIterator
or using version 4 of apache commons org.apache.commons.collections4.iterators.FilterIterator

Since java 9 Collectors.filtering is enabled:
public static <T, A, R>
Collector<T, ?, R> filtering(Predicate<? super T> predicate,
Collector<? super T, A, R> downstream)
Thus filtering should be:
collection.stream().collect(Collectors.filtering(predicate, collector))
Example:
List<Integer> oddNumbers = List.of(1, 19, 15, 10, -10).stream()
.collect(Collectors.filtering(i -> i % 2 == 1, Collectors.toList()));

Let’s look at how to filter a built-in JDK List and a MutableList using Eclipse Collections.
List<Integer> jdkList = Arrays.asList(1, 2, 3, 4, 5);
MutableList<Integer> ecList = Lists.mutable.with(1, 2, 3, 4, 5);
If you wanted to filter the numbers less than 3, you would expect the following outputs.
List<Integer> selected = Lists.mutable.with(1, 2);
List<Integer> rejected = Lists.mutable.with(3, 4, 5);
Here’s how you can filter using a Java 8 lambda as the Predicate.
Assert.assertEquals(selected, Iterate.select(jdkList, each -> each < 3));
Assert.assertEquals(rejected, Iterate.reject(jdkList, each -> each < 3));
Assert.assertEquals(selected, ecList.select(each -> each < 3));
Assert.assertEquals(rejected, ecList.reject(each -> each < 3));
Here’s how you can filter using an anonymous inner class as the Predicate.
Predicate<Integer> lessThan3 = new Predicate<Integer>()
{
public boolean accept(Integer each)
{
return each < 3;
}
};
Assert.assertEquals(selected, Iterate.select(jdkList, lessThan3));
Assert.assertEquals(selected, ecList.select(lessThan3));
Here are some alternatives to filtering JDK lists and Eclipse Collections MutableLists using the Predicates factory.
Assert.assertEquals(selected, Iterate.select(jdkList, Predicates.lessThan(3)));
Assert.assertEquals(selected, ecList.select(Predicates.lessThan(3)));
Here is a version that doesn't allocate an object for the predicate, by using the Predicates2 factory instead with the selectWith method that takes a Predicate2.
Assert.assertEquals(
selected, ecList.selectWith(Predicates2.<Integer>lessThan(), 3));
Sometimes you want to filter on a negative condition. There is a special method in Eclipse Collections for that called reject.
Assert.assertEquals(rejected, Iterate.reject(jdkList, lessThan3));
Assert.assertEquals(rejected, ecList.reject(lessThan3));
The method partition will return two collections, containing the elements selected by and rejected by the Predicate.
PartitionIterable<Integer> jdkPartitioned = Iterate.partition(jdkList, lessThan3);
Assert.assertEquals(selected, jdkPartitioned.getSelected());
Assert.assertEquals(rejected, jdkPartitioned.getRejected());
PartitionList<Integer> ecPartitioned = gscList.partition(lessThan3);
Assert.assertEquals(selected, ecPartitioned.getSelected());
Assert.assertEquals(rejected, ecPartitioned.getRejected());
Note: I am a committer for Eclipse Collections.

With the ForEach DSL you may write
import static ch.akuhn.util.query.Query.select;
import static ch.akuhn.util.query.Query.$result;
import ch.akuhn.util.query.Select;
Collection<String> collection = ...
for (Select<String> each : select(collection)) {
each.yield = each.value.length() > 3;
}
Collection<String> result = $result();
Given a collection of [The, quick, brown, fox, jumps, over, the, lazy, dog] this results in [quick, brown, jumps, over, lazy], ie all strings longer than three characters.
All iteration styles supported by the ForEach DSL are
AllSatisfy
AnySatisfy
Collect
Counnt
CutPieces
Detect
GroupedBy
IndexOf
InjectInto
Reject
Select
For more details, please refer to https://www.iam.unibe.ch/scg/svn_repos/Sources/ForEach

The Collections2.filter(Collection,Predicate) method in Google's Guava library does just what you're looking for.

This, combined with the lack of real closures, is my biggest gripe for Java.
Honestly, most of the methods mentioned above are pretty easy to read and REALLY efficient; however, after spending time with .Net, Erlang, etc... list comprehension integrated at the language level makes everything so much cleaner. Without additions at the language level, Java just cant be as clean as many other languages in this area.
If performance is a huge concern, Google collections is the way to go (or write your own simple predicate utility). Lambdaj syntax is more readable for some people, but it is not quite as efficient.
And then there is a library I wrote. I will ignore any questions in regard to its efficiency (yea, its that bad)...... Yes, i know its clearly reflection based, and no I don't actually use it, but it does work:
LinkedList<Person> list = ......
LinkedList<Person> filtered =
Query.from(list).where(Condition.ensure("age", Op.GTE, 21));
OR
LinkedList<Person> list = ....
LinkedList<Person> filtered = Query.from(list).where("x => x.age >= 21");

In Java 8, You can directly use this filter method and then do that.
List<String> lines = Arrays.asList("java", "pramod", "example");
List<String> result = lines.stream()
.filter(line -> !"pramod".equals(line))
.collect(Collectors.toList());
result.forEach(System.out::println);

JFilter http://code.google.com/p/jfilter/ is best suited for your requirement.
JFilter is a simple and high performance open source library to query collection of Java beans.
Key features
Support of collection (java.util.Collection, java.util.Map and Array) properties.
Support of collection inside collection of any depth.
Support of inner queries.
Support of parameterized queries.
Can filter 1 million records in few 100 ms.
Filter ( query) is given in simple json format, it is like Mangodb queries. Following are some examples.
{ "id":{"$le":"10"}
where object id property is less than equals to 10.
{ "id": {"$in":["0", "100"]}}
where object id property is 0 or 100.
{"lineItems":{"lineAmount":"1"}}
where lineItems collection property of parameterized type has lineAmount equals to 1.
{ "$and":[{"id": "0"}, {"billingAddress":{"city":"DEL"}}]}
where id property is 0 and billingAddress.city property is DEL.
{"lineItems":{"taxes":{ "key":{"code":"GST"}, "value":{"$gt": "1.01"}}}}
where lineItems collection property of parameterized type which has taxes map type property of parameteriszed type has code equals to GST value greater than 1.01.
{'$or':[{'code':'10'},{'skus': {'$and':[{'price':{'$in':['20', '40']}}, {'code':'RedApple'}]}}]}
Select all products where product code is 10 or sku price in 20 and 40 and sku code is "RedApple".

I wrote an extended Iterable class that support applying functional algorithms without copying the collection content.
Usage:
List<Integer> myList = new ArrayList<Integer>(){ 1, 2, 3, 4, 5 }
Iterable<Integer> filtered = Iterable.wrap(myList).select(new Predicate1<Integer>()
{
public Boolean call(Integer n) throws FunctionalException
{
return n % 2 == 0;
}
})
for( int n : filtered )
{
System.out.println(n);
}
The code above will actually execute
for( int n : myList )
{
if( n % 2 == 0 )
{
System.out.println(n);
}
}

Use Collection Query Engine (CQEngine). It is by far the fastest way to do this.
See also: How do you query object collections in Java (Criteria/SQL-like)?

Using java 8, specifically lambda expression, you can do it simply like the below example:
myProducts.stream().filter(prod -> prod.price>10).collect(Collectors.toList())
where for each product inside myProducts collection, if prod.price>10, then add this product to the new filtered list.

Some really great great answers here. Me, I'd like to keep thins as simple and readable as possible:
public abstract class AbstractFilter<T> {
/**
* Method that returns whether an item is to be included or not.
* #param item an item from the given collection.
* #return true if this item is to be included in the collection, false in case it has to be removed.
*/
protected abstract boolean excludeItem(T item);
public void filter(Collection<T> collection) {
if (CollectionUtils.isNotEmpty(collection)) {
Iterator<T> iterator = collection.iterator();
while (iterator.hasNext()) {
if (excludeItem(iterator.next())) {
iterator.remove();
}
}
}
}
}

The simple pre-Java8 solution:
ArrayList<Item> filtered = new ArrayList<Item>();
for (Item item : items) if (condition(item)) filtered.add(item);
Unfortunately this solution isn't fully generic, outputting a list rather than the type of the given collection. Also, bringing in libraries or writing functions that wrap this code seems like overkill to me unless the condition is complex, but then you can write a function for the condition.

https://code.google.com/p/joquery/
Supports different possibilities,
Given collection,
Collection<Dto> testList = new ArrayList<>();
of type,
class Dto
{
private int id;
private String text;
public int getId()
{
return id;
}
public int getText()
{
return text;
}
}
Filter
Java 7
Filter<Dto> query = CQ.<Dto>filter(testList)
.where()
.property("id").eq().value(1);
Collection<Dto> filtered = query.list();
Java 8
Filter<Dto> query = CQ.<Dto>filter(testList)
.where()
.property(Dto::getId)
.eq().value(1);
Collection<Dto> filtered = query.list();
Also,
Filter<Dto> query = CQ.<Dto>filter()
.from(testList)
.where()
.property(Dto::getId).between().value(1).value(2)
.and()
.property(Dto::grtText).in().value(new string[]{"a","b"});
Sorting (also available for the Java 7)
Filter<Dto> query = CQ.<Dto>filter(testList)
.orderBy()
.property(Dto::getId)
.property(Dto::getName)
Collection<Dto> sorted = query.list();
Grouping (also available for the Java 7)
GroupQuery<Integer,Dto> query = CQ.<Dto,Dto>query(testList)
.group()
.groupBy(Dto::getId)
Collection<Grouping<Integer,Dto>> grouped = query.list();
Joins (also available for the Java 7)
Given,
class LeftDto
{
private int id;
private String text;
public int getId()
{
return id;
}
public int getText()
{
return text;
}
}
class RightDto
{
private int id;
private int leftId;
private String text;
public int getId()
{
return id;
}
public int getLeftId()
{
return leftId;
}
public int getText()
{
return text;
}
}
class JoinedDto
{
private int leftId;
private int rightId;
private String text;
public JoinedDto(int leftId,int rightId,String text)
{
this.leftId = leftId;
this.rightId = rightId;
this.text = text;
}
public int getLeftId()
{
return leftId;
}
public int getRightId()
{
return rightId;
}
public int getText()
{
return text;
}
}
Collection<LeftDto> leftList = new ArrayList<>();
Collection<RightDto> rightList = new ArrayList<>();
Can be Joined like,
Collection<JoinedDto> results = CQ.<LeftDto, LeftDto>query().from(leftList)
.<RightDto, JoinedDto>innerJoin(CQ.<RightDto, RightDto>query().from(rightList))
.on(LeftFyo::getId, RightDto::getLeftId)
.transformDirect(selection -> new JoinedDto(selection.getLeft().getText()
, selection.getLeft().getId()
, selection.getRight().getId())
)
.list();
Expressions
Filter<Dto> query = CQ.<Dto>filter()
.from(testList)
.where()
.exec(s -> s.getId() + 1).eq().value(2);

My answer builds on that from Kevin Wong, here as a one-liner using CollectionUtils from spring and a Java 8 lambda expression.
CollectionUtils.filter(list, p -> ((Person) p).getAge() > 16);
This is as concise and readable as any alternative I have seen (without using aspect-based libraries)
Spring CollectionUtils is available from spring version 4.0.2.RELEASE, and remember you need JDK 1.8 and language level 8+.

I needed to filter a list depending on the values already present in the list. For example, remove all values following that is less than the current value. {2 5 3 4 7 5} -> {2 5 7}. Or for example to remove all duplicates {3 5 4 2 3 5 6} -> {3 5 4 2 6}.
public class Filter {
public static <T> void List(List<T> list, Chooser<T> chooser) {
List<Integer> toBeRemoved = new ArrayList<>();
leftloop:
for (int right = 1; right < list.size(); ++right) {
for (int left = 0; left < right; ++left) {
if (toBeRemoved.contains(left)) {
continue;
}
Keep keep = chooser.choose(list.get(left), list.get(right));
switch (keep) {
case LEFT:
toBeRemoved.add(right);
continue leftloop;
case RIGHT:
toBeRemoved.add(left);
break;
case NONE:
toBeRemoved.add(left);
toBeRemoved.add(right);
continue leftloop;
}
}
}
Collections.sort(toBeRemoved, new Comparator<Integer>() {
#Override
public int compare(Integer o1, Integer o2) {
return o2 - o1;
}
});
for (int i : toBeRemoved) {
if (i >= 0 && i < list.size()) {
list.remove(i);
}
}
}
public static <T> void List(List<T> list, Keeper<T> keeper) {
Iterator<T> iterator = list.iterator();
while (iterator.hasNext()) {
if (!keeper.keep(iterator.next())) {
iterator.remove();
}
}
}
public interface Keeper<E> {
boolean keep(E obj);
}
public interface Chooser<E> {
Keep choose(E left, E right);
}
public enum Keep {
LEFT, RIGHT, BOTH, NONE;
}
}
This will bee used like this.
List<String> names = new ArrayList<>();
names.add("Anders");
names.add("Stefan");
names.add("Anders");
Filter.List(names, new Filter.Chooser<String>() {
#Override
public Filter.Keep choose(String left, String right) {
return left.equals(right) ? Filter.Keep.LEFT : Filter.Keep.BOTH;
}
});

In my case, I was looking for list with specific field null excluded.
This could be done with for loop and fill the temporary list of objects who have no null addresses.
but Thanks to Java 8 Streams
List<Person> personsList = persons.stream()
.filter(p -> p.getAdrress() != null).collect(Collectors.toList());
#java #collection #collections #java8 #streams

With Guava:
Collection<Integer> collection = Lists.newArrayList(1, 2, 3, 4, 5);
Iterators.removeIf(collection.iterator(), new Predicate<Integer>() {
#Override
public boolean apply(Integer i) {
return i % 2 == 0;
}
});
System.out.println(collection); // Prints 1, 3, 5

An alternative (more lightweight) alternative to Java collection streams is the Ocl.java library, which uses vanilla collections and lambdas: https://github.com/eclipse/agileuml/blob/master/Ocl.java
For example, a simple filter and sum on an ArrayList words
could be:
ArrayList<Word> sel = Ocl.selectSequence(words,
w -> w.pos.equals("NN"));
int total = Ocl.sumint(Ocl.collectSequence(sel,
w -> w.text.length()));
Where Word has String pos; String text; attributes. Efficiency seems similar to the streams option, eg, 10000 words are processed in about 50ms in both versions.
There are equivalent OCL libraries for Python, Swift, etc. Basically Java collection streams has re-invented the OCL operations ->select, ->collect, etc, which existed in OCL since 1998.

Limit a stream and find out if there are pending elements

I have the following code that I want to translate to Java 8 streams:
public ReleaseResult releaseReources() {
List<String> releasedNames = new ArrayList<>();
Stream<SomeResource> stream = this.someResources();
Iterator<SomeResource> it = stream.iterator();
while (it.hasNext() && releasedNames.size() < MAX_TO_RELEASE) {
SomeResource resource = it.next();
if (!resource.isTaken()) {
resource.release();
releasedNames.add(resource.getName());
}
}
return new ReleaseResult(releasedNames, it.hasNext(), MAX_TO_RELEASE);
}
Method someResources() returns a Stream<SomeResource> and ReleaseResult class is as follows:
public class ReleaseResult {
private int releasedCount;
private List<String> releasedNames;
private boolean hasMoreItems;
private int releaseLimit;
public ReleaseResult(List<String> releasedNames,
boolean hasMoreItems, int releaseLimit) {
this.releasedNames = releasedNames;
this.releasedCount = releasedNames.size();
this.hasMoreItems = hasMoreItems;
this.releaseLimit = releaseLimit;
}
// getters & setters
}
My attempt so far:
public ReleaseResult releaseReources() {
List<String> releasedNames = this.someResources()
.filter(resource -> !resource.isTaken())
.limit(MAX_TO_RELEASE)
.peek(SomeResource::release)
.map(SomeResource::getName)
.collect(Collectors.toList());
return new ReleasedResult(releasedNames, ???, MAX_TO_RELEASE);
}
The problem is that I can't find a way to know if there are pending resources to process. I've thought of using releasedNames.size() == MAX_TO_RELEASE, but this doesn't take into account the case where the stream of resources has exactly MAX_TO_RELEASE elements.
Is there a way to do the same with Java 8 streams?
Note: I'm not looking for answers like "you don't have to do everything with streams" or "using loops and iterators is fine". I'm OK if using an iterator and a loop is the only way or just the best way. It's just that I'd like to know if there's a non-murky way to do the same.

Since you don’t wanna hear that you don’t need streams for everything and loops and iterators are fine, let’s demonstrate it by showing a clean solution, not relying on peek:
public ReleaseResult releaseReources() {
return this.someResources()
.filter(resource -> !resource.isTaken())
.limit(MAX_TO_RELEASE+1)
.collect(
() -> new ReleaseResult(new ArrayList<>(), false, MAX_TO_RELEASE),
(result, resource) -> {
List<String> names = result.getReleasedNames();
if(names.size() == MAX_TO_RELEASE) result.setHasMoreItems(true);
else {
resource.release();
names.add(resource.getName());
}
},
(r1, r2) -> {
List<String> names = r1.getReleasedNames();
names.addAll(r2.getReleasedNames());
if(names.size() > MAX_TO_RELEASE) {
r1.setHasMoreItems(true);
names.remove(MAX_TO_RELEASE);
}
}
);
}
This assumes that // getters & setters includes getters and setters for all non-final fields of your ReleaseResult. And that getReleasedNames() returns the list by reference. Otherwise you would have to rewrite it to provide a specialized Collector having special non-public access to ReleaseResult (implementing another builder type or temporary storage would be an unnecessary complication, it looks like ReleaseResult is already designed exactly for that use case).
We could conclude that for any nontrivial loop code that doesn’t fit into the stream’s intrinsic operations, you can find a collector solution that basically does the same as the loop in its accumulator function, but suffers from the requirement of always having to provide a combiner function. Ok, in this case we can prepend a filter(…).limit(…) so it’s not that bad…
I just noticed, if you ever dare to use that with a parallel stream, you need a way to reverse the effect of releasing the last element in the combiner in case the combined size exceeds MAX_TO_RELEASE. Generally, limits and parallel processing never play well.

I don't think there's a nice way to do this. I've found a hack that does it lazily. What you can do is convert the Stream to an Iterator, convert the Iterator back to another Stream, do the Stream operations, then finally test the Iterator for a next element!
Iterator<SomeResource> it = this.someResource().iterator();
List<String> list = StreamSupport.stream(Spliterators.spliteratorUnknownSize(it, Spliterator.ORDERED), false)
.filter(resource -> !resource.isTaken())
.limit(MAX_TO_RELEASE)
.peek(SomeResource::release)
.map(SomeResource::getName)
.collect(Collectors.toList());
return new ReleaseResult(list, it.hasNext(), MAX_TO_RELEASE);

The only thing I can think of is
List<SomeResource> list = someResources(); // A List, rather than a Stream, is required
List<Integer> indices = IntStream.range(0, list.size())
.filter(i -> !list.get(i).isTaken())
.limit(MAX_TO_RELEASE)
.collect(Collectors.toList());
List<String> names = indices.stream()
.map(list::get)
.peek(SomeResource::release)
.map(SomeResource::getName)
.collect(Collectors.toList());
Then (I think) there are unprocessed elements if
names.size() == MAX_TO_RELEASE
&& (indices.isEmpty() || indices.get(indices.size() - 1) < list.size() - 1)