如何搜索列表中的元素？

Question

I have two lists for eg :

List 1 : has only one element

List<String> ids=new ArrayList<String>();

List 2 : has 1000 objects

List<ABC> abc=  new ArrayList<ABC>();

a.matIDS

Note: matIDS is String collection (for eg : abc,def,ghi)

for(ABC a : abc){
    for(String id : a.matIDs()){
        if(ids.contains(id)){
            LOG.info("ID found:::");
        }else{
            LOG.info("ID NOT found:::");
        }
    }
}

question:

In list 1 there is only 1 element where as in list 2 there are 1000. Do i need to check all those 1000 to find the 1 element?

Is there any better way?

Answer 1

If you really need to rapidly lookup a value (or values) in one list against another collection, then perhaps the fastest data structure for this would be to search against a set:

Set<String> set = new HashSet<>(abc);

Then, you may iterate the first list and lookup each entry in constant time in the other collection:

for (String id : ids) {
    if (set.contains(id)) {
        LOG.info("ID found:::");
    }
    else {
        LOG.info("ID NOT found:::");
    }
}

This is an improvement over your current brute force approach, which was O(n*m) , where n and m are the sizes of the ids and abc lists. Now, the running time is just O(m) , the size of the ids list.

Answer 2

If by 'better' you mean clearer then perhaps you might consider using streams:

abc.stream().flatMap(ABC::matIDs).anyMatch(ids::contains);

Whether you consider this 'better' depends on what you are after.

If you are regularly checking whether a particular ID is in the list then you could collect the IDs in a set:

Set<String> abcIDs = abc.stream().flatMap(ABC::matIDs).collect(toSet());

Then checking if a particular string is in the set is trivial without having to go back to the original list.

Answer 3

You can optimize your existing code to exit the loops when match found.

booean isFound=false;
for(ABC a : abc){
    for(String id : a.matIDs()){
        if(ids.contains(id)){
            isFound=true;
            break;
        }
    }
    if(isFound)
        break;
}
if(isFound)
    LOG.info("ID found:::");
else
    LOG.info("ID NOT found:::");

You may also use streams,

boolean isFound=abc.stream().flatMap(e-> e.matIDS.stream()).anyMatch(ids::contains);
if(isFound)
    LOG.info("ID found:::");
else
    LOG.info("ID NOT found:::");

To find which elements matched, you can use filter and collect in a Set

  Set<String>  matchedElements=abc.stream()
                .flatMap(e-> e.matIDS.stream())
                .filter(ids::contains)
                .collect(Collectors.toSet());

Hope it helps.