Java get last element of a collection
JavaCollectionsIteratorJava Problem Overview
I have a collection, I want to get the last element of the collection. What's the most straighforward and fast way to do so?
One solution is to first toArray(), and then return the last element of the array. Is there any other better ones?
Java Solutions
Solution 1 - Java
A Collection
is not a necessarily ordered set of elements so there may not be a concept of the "last" element. If you want something that's ordered, you can use a SortedSet
which has a last()
method. Or you can use a List
and call mylist.get(mylist.size()-1);
If you really need the last element you should use a List
or a SortedSet
. But if all you have is a Collection
and you really, really, really need the last element, you could use toArray()
or you could use an Iterator
and iterate to the end of the list.
For example:
public Object getLastElement(final Collection c) {
final Iterator itr = c.iterator();
Object lastElement = itr.next();
while(itr.hasNext()) {
lastElement = itr.next();
}
return lastElement;
}
Solution 2 - Java
Iterables.getLast
from Google Guava.
It has some optimization for List
s and SortedSet
s too.
Solution 3 - Java
It is not very efficient solution, but working one:
public static <T> T getFirstElement(final Iterable<T> elements) {
return elements.iterator().next();
}
public static <T> T getLastElement(final Iterable<T> elements) {
T lastElement = null;
for (T element : elements) {
lastElement = element;
}
return lastElement;
}
Solution 4 - Java
This should work without converting to List/Array:
collectionName.stream().reduce((prev, next) -> next).orElse(null)
Solution 5 - Java
Well one solution could be:
list.get(list.size()-1)
Edit: You have to convert the collection to a list before maybe like this: new ArrayList(coll)
Solution 6 - Java
A reasonable solution would be to use an iterator if you don't know anything about the underlying Collection, but do know that there is a "last" element. This isn't always the case, not all Collections are ordered.
Object lastElement = null;
for (Iterator collectionItr = c.iterator(); collectionItr.hasNext(); ) {
lastElement = collectionItr.next();
}
Solution 7 - Java
There isn't a last()
or first()
method in a Collection interface. For getting the last method, you can either do get(size() - 1)
on a List or reverse the List and do get(0)
. I don't see a need to have last()
method in any Collection API unless you are dealing with Stacks
or Queues
Solution 8 - Java
Or you can use a for-each loop:
Collection<X> items = ...;
X last = null;
for (X x : items) last = x;
Solution 9 - Java
If you have Iterable convert to stream and find last element
Iterator<String> sourceIterator = Arrays.asList("one", "two", "three").iterator();
Iterable<String> iterable = () -> sourceIterator;
String last = StreamSupport.stream(iterable.spliterator(), false).reduce((first, second) -> second).orElse(null);