Java 8 Stream: How to compare current element with next element?

Question

How to get the next element from a List using Java 8 Streams?

If I am iterating over a List, I want to compare current with next element of the list.

Is it doable using Java 8 Stream?

Answer 1

My free StreamEx library allows you to process the pairs of the stream elements using additional pairMap intermediate operation. Like this:

StreamEx.of(input).pairMap((current, next) -> doSomethingWith(current, next)); 

Where input is a Collection, array or Stream. For example, this way you can easily check whether input is sorted:

boolean isSorted = StreamEx.of(input)                            .pairMap((current, next) -> next.compareTo(current))                            .allMatch(cmp -> cmp >= 0); 

There's also forPairs terminal operation which is a forEach analog to all pairs of input elements:

StreamEx.of(input).forPairs((current, next) -> doSomethingWith(current, next)); 

These features work nicely with any stream source (either random access or not) and fully support the parallel streams.

Answer 2

One way is to generate an IntStream of the indices, and fetch the List elements by their index. This is only efficient if the List supports random access (i.e. if your List is a LinkedList, it would be a bad idea, since list.get(i) doesn't take constant time).

For example :

IntStream.range(0,list.size()-1).forEach(i -> {     doSomething(list.get(i),list.get(i+1)); }); 

Another way is to store the last element in an array :

List<Element> list = ... Element[] arr = new Element[1]; list.stream().forEach(e -> {     if (arr[0] != null)         doSomething(arr[0],e);      arr[0]=e; }); 

This will only work for sequential streams.