Java获取集合的最后一个元素

Question

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?

Answer 1

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;
}

Answer 2

Iterables.getLast from Google Guava. It has some optimization for List s and SortedSet s too.

Answer 3

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;
}

Answer 4

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)

Answer 5

这应该可以在不转换为列表/数组的情况下工作：

collectionName.stream().reduce((prev, next) -> next).orElse(null)

Answer 6

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();
}

Answer 7

Or you can use a for-each loop:

Collection<X> items = ...;
X last = null;
for (X x : items) last = x;

Answer 8

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);

Answer 9

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

Answer 10

To avoid some of the problems mentioned above (not robust for nulls etc etc), to get first and last element in a list an approach could be

import java.util.List;

public static final <A> A getLastElement(List<A> list) {
    return list != null ? getElement(list, list.size() - 1) : null;
}

public static final <A> A getFirstElement(List<A> list) {
    return list != null ? getElement(list, 0) : null;
}   

private static final <A> A getElement(List<A> list, int pointer) {
    A res = null;
    if (list.size() > 0) {
        res = list.get(pointer);            
    }
    return res;
}

The convention adopted is that the first/last element of an empty list is null...