Replace while loop with break with Java streams and lambda expression

Question

I want to replace the following while with break condition with a Java stream and lambda expression. I am not able to find a way for this. I use Java 8.

Lets say if the input Arraylist(result) has sorted values: [1,2,3,4,5,10,12,30,40,50]

The output should contain values [1,2,3,4,5,10]

List<String> finalResult = new ArrayList();
    while (index < maxSize) {
        if (inputList.get(index) > 10)) {
            finalResult.add(inputList.get(index));
        } else {            
                break;              
        }
        index++;
    }

If I try using findFirst() to break the condition, then I'm not able to collect the list.

Optional<String> result =
inputList.stream().filter(obj -> some_condition_met).findFirst();

Answer 1

If you are using java 9 something like below is may be what you are looking for:

List<Integer> myList = List.of(1,2,3,4,5,10,12,30,40,50);
List<Integer> under5 = myList.stream()
                             .takeWhile(i -> i <= 5)
                             .collect(Collectors.toList());
System.out.println(under5);

Answer 2

1. For the provided input [1,2,3,4,5,10,12,30,40,50], the expected output [1,2,3,4,5,10] should be returned by simple filter:

static List<Integer> findBelow(List<Integer> inputList, Predicate<Integer> condition) {
    return inputList.stream()
                    .filter(condition::test)
                    .collect(Collectors.toList());
}

2. If a range of indexes needs to be specified, this should be implemented using IntStream::range:

static List<Integer> findBelow(List<Integer> inputList, int start, int end, Predicate<Integer> condition) {
    start = Math.max(0, start);
    end = Math.min(inputList.size(), end);
    return IntStream.range(start, end)
                    .filter(i -> condition.test(inputList.get(i)))
                    .mapToObj(inputList::get)
                    .collect(Collectors.toList());
}

---

Obviously, these implementations do not break out of the loop, and Java 8 Stream API does not have facilities for that.

The following workarounds can be offered:

3. Use findFirst to detect the index of the first non-match and then limit the range of index:

static List<Integer> findWithFirst(List<Integer> inputList, int start, int end, Predicate<Integer> condition) {
    start = Math.max(0, start);
    end = Math.min(inputList.size(), end);
    int firstNonMatch = IntStream.range(start, end)
                                 .filter(i -> !condition.test(inputList.get(i)))
                                 .findFirst()
                                 .orElse(maxSize);

    return IntStream.range(start, firstNonMatch) // no filter needed any longer
                    .mapToObj(inputList::get)
                    .collect(Collectors.toList());
}

4. Use boolean flag outside the stream to check if the inverted break condition is met while filtering the input list:

// using AtomicBoolean
static List<Integer> findAndBreak2(List<Integer> inputList, int start, int end, Predicate<Integer> condition) {
    AtomicBoolean breakFound = new AtomicBoolean(false);
    return IntStream.range(start, Math.min(inputList.size(), end))
                    .filter(i -> { 
                        breakFound.set(
                            breakFound.get() || !condition.test(inputList.get(i))
                        ); 
                        return !breakFound.get(); 
                    })
                    .mapToObj(inputList::get)
                    .collect(Collectors.toList());
}

// using static class field
static boolean found = false;
static List<Integer> findAndBreak(List<Integer> inputList, int start, int end, Predicate<Integer> condition) {
    found = false;
    return IntStream.range(start, Math.min(inputList.size(), end))
                    .filter(i -> { found |= !condition.test(inputList.get(i)); return !found; })
                    .mapToObj(inputList::get)
                    .collect(Collectors.toList());
}

Tests:

Condition i <= 10 is trivial, as it works for all cases. Invert condition i > 10 as in the OP code demonstrates break producing empty lists.

    Predicate<Integer> condition = (i) -> i > 10;
        
    List<Integer> inputList = Arrays.asList(1,2,3,4,5,10,12,30,40,50);
    List<Integer> finalResult = new ArrayList<>();
    int index = 0, maxSize = 10;
    while (index < maxSize) {
        if (condition.test(inputList.get(index))) {
            finalResult.add(inputList.get(index));
        } else {            
            break;              
        }
        index++;
    }
    
    System.out.println("loop: " + finalResult);

    System.out.println("plain filter: " + findBelow(inputList, condition));
    
    System.out.println("filter and range limit: " + findBelow(inputList, 0, maxSize, condition));
    
    System.out.println("filter and break static: " + findAndBreak(inputList, 0, maxSize, condition));

    System.out.println("filter and break atomic: " + findAndBreak2(inputList, 0, maxSize, condition));

    System.out.println("filter and findFirst: " + findWithFirst(inputList, 0, maxSize, condition));

Output:

loop: []
plain filter: [12, 30, 40, 50]
filter and range limit: [12, 30, 40, 50]
filter and break static: []
filter and break atomic: []
filter and findFirst: []

Answer 3

As far as i understand your question. Why are you trying to use a stream for this job. A simple loop would do the trick don't try complicating things. If the only thing is the code being too long extract it to a separate private method. The main point is stream is not what your looking for in this case. Hope this helped