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Best way to replace nested loop concatenate string using java stream

I have a list of names and a list of versions. I want to get all permutations which are constructed by concatenating the string from two lists. I am using two for loop to do this but I want to switch to a more functional style approach. Here is my solution:
List<String> names = new ArrayList<>();
List<String> versions = new ArrayList<>();
List<String> result = new ArrayList<>();
names.forEach(name -> versions.stream().map(version -> result.add(name.concat(version))));
Is there a better way to do it?

You are looking for the "Cartesian Product" of names and versions — basically the return set/list from the aforementioned sets/lists.
final Stream<List<String>> result = names.stream()
.flatMap(s1 -> versions.stream().flatMap(s2 -> Stream.of(Arrays.asList(s1, s2))));
result.forEach(System.out::println);
Keep in mind that operation is super expensive. Google's Guava have this implemented also under com.google.common.collect.Sets.cartesianProduct(s1, s2).

You should look forward to use flatMap while streaming over names and then performing map operation further correctly as:
List<String> result = names.stream() // for each name
.flatMap(name -> versions.stream() // for each version
.map(version -> name.concat(version))) // concat version to the name
.collect(Collectors.toList()); // collect all such names

Or a bit tidier:
final List<String> result = names.stream() // Stream the Names...
.flatMap(name -> versions.stream() // ...together with Versions.
.map (version -> name.concat(version))) // Combine Name+Version
.collect(Collectors.toList()); // & collect in List.

Stream: Filter on children, return the parent

Assume a class MyClass:
public class MyClass {
private final Integer myId;
private final String myCSVListOfThings;
public MyClass(Integer myId, String myCSVListOfThings) {
this.myId = myId;
this.myCSVListOfThings = myCSVListOfThings;
}
// Getters, Setters, etc
}
And this Stream:
final Stream<MyClass> streamOfObjects = Stream.of(
new MyClass(1, "thing1;thing2;thing3"),
new MyClass(2, "thing2;thing3;thing4"),
new MyClass(3, "thingX;thingY;thingZ"));
I want to return every instance of MyClass that contains an entry "thing2" in myCSVListOfThings.
If I wanted a List<String> containing myCSVListOfThings this could be done easily:
List<String> filteredThings = streamOfObjects
.flatMap(o -> Arrays.stream(o.getMyCSVListOfThings().split(";")))
.filter("thing2"::equals)
.collect(Collectors.toList());
But what I really need is a List<MyClass>.
This is what I have right now:
List<MyClass> filteredClasses = streamOfObjects.filter(o -> {
Stream<String> things = Arrays.stream(o.getMyCSVListOfThings().split(";"));
return things.anyMatch(s -> s.equals("thing2"));
}).collect(Collectors.toList());
But somehow it does not feel right. Any cleaner solution than opening a new Stream inside of a Predicate?

Firstly, I recommend you to add extra method to MyClass public boolean containsThing(String str), so you can transform you code like this:
List<MyClass> filteredClasses = streamOfObjects
.filter(o -> o.containsThing("thing2"))
.collect(Collectors.toList());
Now you can implement this method as you want depends on input data: splitting into Stream, splitting into Set, even searching of substring (if it's possible and has sense), caching result if you need.
You know much more about usage of this class so you can make right choice.

One solution is to use a pattern matching that avoids the split-and-stream operation:
Pattern p=Pattern.compile("(^|;)thing2($|;)");
List<MyClass> filteredClasses = streamOfObjects
.filter(o -> p.matcher(o.getMyCSVListOfThings()).find())
.collect(Collectors.toList());
Since the argument to String.split is defined as regex pattern, the pattern above has the same semantic as looking for a match within the result of split; you are looking for the word thing2 between two boundaries, the first is either, the beginning of the line or a semicolon, the second is either, the end of the line or a semicolon.
Besides that, there is nothing wrong with using another Stream operation within a predicate. But there are some ways to improve it. The lambda expression gets more concise if you omit the obsolete local variable holding the Stream. Generally, you should avoid holding Stream instances in local variables as chaining the operations directly will reduce the risk of trying to use a Stream more than one time. Second, you can use the Pattern class to stream over the resulting elements of a split operation without collecting them all into an array first:
Pattern p=Pattern.compile(";");
List<MyClass> filteredClasses = streamOfObjects
.filter(o -> p.splitAsStream(o.getMyCSVListOfThings()).anyMatch("thing2"::equals))
.collect(Collectors.toList());
or
Pattern p=Pattern.compile(";");
List<MyClass> filteredClasses = streamOfObjects
.filter(o -> p.splitAsStream(o.getMyCSVListOfThings()).anyMatch(s->s.equals("thing2")))
.collect(Collectors.toList());
Note that you could also rewrite your original code to
List<MyClass> filteredClasses = listOfObjects.stream()
.filter(o -> Arrays.asList(o.getMyCSVListOfThings().split(";")).contains("thing2"))
.collect(Collectors.toList());
Now, the operation within the predicate is not a Stream but a Collection operation, but this doesn’t change the semantic nor the correctness of the code…

As I see it you have three options.
1) look for particular entry in the String without spliting it - still looks messy
List<MyClass> filteredClasses = streamOfObjects
.filter(o -> o.getMyCSVListOfThings().contains(";thing2;"))
.collect(Collectors.toList());
2) map twice - still messy
List<MyClass> filteredClasses = streamOfObjects
.map(o -> Pair<MyClass, List<String>>.of(o, toList(o.getMyCSVListOfThings()))
.filter(pair -> pair.getRight().contains("thing2"))
.map(pair -> pair.getLeft())
.collect(Collectors.toList());
where toList is a method that will convert String to List
3) create additional field - method I'd suggest
Extend class MyClass - add field to the class
List<String> values;
And initialize it in the constructor:
public MyClass(Integer myId, String myCSVListOfThings) {
this.myId = myId;
this.myCSVListOfThings = myCSVListOfThings;
this.values = toList(myCSVListOfThings);
}
And then in the stream simply:
List<MyClass> filteredClasses = streamOfObjects
.filter(o -> o.getValues().contains("thing2"))
.collect(Collectors.toList());
Of course field values can be initialized in LAZY mode during first getValues method call if you want.

This is similar to the issue, Getting only required objects from a list using Java 8 Streams, posted a year earlier. I think the solution I left there is applicable here.
There's a library called com.coopstools.cachemonads. It extends the java stream (and Optional) classes to allow caching of entities for later use.
The solution can be found with:
List<Parent> goodParents = CacheStream.of(parents)
.cache()
.map(Parent::getChildren)
.flatMap(Collection::stream)
.map(Child::getAttrib1)
.filter(att -> att > 10)
.load()
.distinct()
.collect(Collectors.toList());
where, parents is an array or stream.
For clarity, the cache method is what stores the parents; and the load method is what pulls the parents back out. And If a parent does not have children, a filter will be needed after the first map to remove the null lists.
More specifically, for your issue:
List<Parent> goodParents = CacheStream.of(streamOfObjects)
.cache()
.map(o -> o.getMyCSVListOfThings().split(";"))
.flatMap(Collection::stream)
.filter("thing2"::equals)
.load()
.collect(Collectors.toList())
This library can be used in any situation where operations need to be performed on children, including map/sort/filter/etc, but where an older entity is still needed. There may be more lines than some of the other answers, but each line is very clean and straight forward.
Please let me know if this answer is helpful.
The code can be found at https://github.com/coopstools/cachemonads or can be downloaded from maven:
<dependency>
<groupId>com.coopstools</groupId>
<artifactId>cachemonads</artifactId>
<version>0.2.0</version>
</dependency>
(or, gradle, com.coopstools:cachemonads:0.2.0)

Java lambda expression -- mapping and then modifying a list?

Using a Java 8 lambda expression, I'm trying to do something like this.
List<NewObject> objs = ...;
for (OldObject oldObj : oldObjects) {
NewObject obj = oldObj.toNewObject();
obj.setOrange(true);
objs.add(obj);
}
I wrote this code.
oldObjects.stream()
.map(old -> old.toNewObject())
.forEach({new.setOrange("true")})
.collect(Collectors.toList());
This is invalid code because I'm then trying to do .collect() on what's returned by .forEach(), but forEach is void and does not return a list.
How should this be structured?

You can use Stream's peek method, which returns the Stream because it's an intermediate operation. It normally isn't supposed to have a side effect (it's supposed to be "non-interfering"), but in this case, I think the side effect (setOrange(true)) is intended and is fine.
List<NewObject> newObjects =
oldObjects.stream()
.map(OldObject::toNewObject)
.peek( n -> n.setOrange(true))
.collect(Collectors.toList());
It's about as verbose as your non-streams code, so you can choose which technique to use.

You can use peek.
List<NewObject> list = oldObjects.stream()
.map(OldObject::toNewObject)
.peek(o -> o.setOrange(true))
.collect(Collectors.toList());
Alternatively, you can mutate the elements after forming the list.
List<NewObject> list = oldObjects.stream()
.map(OldObject::toNewObject)
.collect(Collectors.toList());
list.forEach(o -> o.setOrange(true));

Java 8 list to map with stream

I have a List<Item> collection.
I need to convert it into Map<Integer, Item>
The key of the map must be the index of the item in the collection.
I can not figure it out how to do this with streams.
Something like:
items.stream().collect(Collectors.toMap(...));
Any help?
As this question is identified as possible duplicate I need to add that my concrete problem was - how to get the position of the item in the list and put it as a key value

You can create a Stream of the indices using an IntStream and then convert them to a Map :
Map<Integer,Item> map =
IntStream.range(0,items.size())
.boxed()
.collect(Collectors.toMap (i -> i, i -> items.get(i)));

One more solution just for completeness is to use custom collector:
public static <T> Collector<T, ?, Map<Integer, T>> toMap() {
return Collector.of(HashMap::new, (map, t) -> map.put(map.size(), t),
(m1, m2) -> {
int s = m1.size();
m2.forEach((k, v) -> m1.put(k+s, v));
return m1;
});
}
Usage:
Map<Integer, Item> map = items.stream().collect(toMap());
This solution is parallel-friendly and does not depend on the source (you can use list without random access or Files.lines() or whatever).

Don't feel like you have to do everything in/with the stream. I would just do:
AtomicInteger index = new AtomicInteger();
items.stream().collect(Collectors.toMap(i -> index.getAndIncrement(), i -> i));
As long as you don't parallelise the stream this will work and it avoids potentially expensive and/or problematic (in the case of duplicates) get() and indexOf() operations.
(You cannot use a regular int variable in place of the AtomicInteger because variables used from outside a lambda expression must be effectively final. Note that when uncontested (as in this case), AtomicInteger is very fast and won't pose a performance problem. But if it worries you you can use a non-thread-safe counter.)

This is updated answer and has none of the problems mentioned in comments.
Map<Integer,Item> outputMap = IntStream.range(0,inputList.size()).boxed().collect(Collectors.toMap(Function.identity(), i->inputList.get(i)));

Using a third party library (protonpack for example, but there are others) you can zip the value with its index and voila:
StreamUtils.zipWithIndex(items.stream())
.collect(Collectors.toMap(Indexed::getIndex, Indexed::getValue));
although getIndex returns a long, so you may need to cast it using something similar to:
i -> Integer.valueOf((int) i.getIndex())

Eran's answer is usually the best approach for random-access lists.
If your List isn't random access, or if you have a Stream instead of a List, you can use forEachOrdered:
Stream<Item> stream = ... ;
Map<Integer, Item> map = new HashMap<>();
AtomicInteger index = new AtomicInteger();
stream.forEachOrdered(item -> map.put(index.getAndIncrement(), item));
This is safe, if the stream is parallel, even though the destination map is thread-unsafe and is operated upon as a side effect. The forEachOrdered guarantees that items are processed one-at-a-time, in order. For this reason it's unlikely that any speedup will result from running in parallel. (There might be some speedup if there are expensive operations in the pipeline before the forEachOrdered.)

What is the Aggregate/Reduce version of this loop using Java streams?

I'm trying to come up to speed on the Streams API, but I'm really used to the simplicity of the C# Linq Extension functions and the ability to use the yield keyword to create iterators. Normally I would use:
list.Aggregate(set, (acc, a) => { acc.add(a.Id); return acc});
Or something like that, but I'm not immediately seeing how this maps to the Streams API.
List<SomeObject> objs = ...
Set<String> ids = new HashSet<>();
for (SomeObject a : objs) {
ids.add(a.getId());
}
assertThat(ids.size(), objs.size());
EDIT:
Changed SomeObject.getId() to a.getId() in the for loop.

The following statement should be equivalent to the for-loop in your example.
Set<String> ids = objs.stream()
.map(a -> a.getId())
.collect(Collectors.toSet());
You can also use a method reference instead of a lambda expression:
Set<String> ids = objs.stream()
.map(SomeObject::getId)
.collect(Collectors.toSet());