class StackInfo {
    int start, size, capacity;
}

private int numberOfElements(StackInfo[] info) {
    int size = 0;
    for (StackInfo si : info) {
        size += si.size;
    }
    return size;
}

private int numberOfElements(StackInfo[] info) {
    return (int) Arrays.stream(info).map(s -> s.size).count();
    //       ^^
    //     to convert long
}

private int numberOfElementsStream(StackInfo[] info) {
    return Arrays.stream(info)    // Stream<StackInfo>
            .mapToInt(sd -> sd.size)   //IntStream of their size
            .sum();   // sum of all
}

Arrays.stream(info).mapToInt(StackInfo::getSize).sum();

Arrays.stream(info).reduce((s1, s2) -> s1.size + s2.size);