How can I get a first element from a sorted list?

Question

I used Collections.sort(playersList); to sort a List. So, I think playersList is sorted now. But how can I get the first element of the list? playersList[0] does not work.

Answer 1

playersList.get(0)

Java has limited operator polymorphism. So you use the get() method on List objects, not the array index operator ([])

Answer 2

You have to access lists a little differently than arrays in Java. See the javadocs for the List interface for more information.

playersList.get(0)

However if you want to find the smallest element in playersList, you shouldn't sort it and then get the first element. This runs very slowly compared to just searching once through the list to find the smallest element.

For example:

int smallestIndex = 0;
for (int i = 1; i < playersList.size(); i++) {
    if (playersList.get(i) < playersList.get(smallestIndex))
        smallestIndex = i;
}

playersList.get(smallestIndex);

The above code will find the smallest element in O(n) instead of O(n log n) time.

Answer 3

That depends on what type your list is, for ArrayList use:

list.get(0);

for LinkedList use:

list.getFirst();

if you like the array approach:

list.toArray()[0];

Answer 4

Using Java 8 streams, you can turn your list into a stream and get the first item in a list using the .findFirst() method.

List<String> stringsList = Arrays.asList("zordon", "alpha", "tommy");
Optional<String> optional = stringsList.stream().findFirst();
optional.get(); // "zordon"

The .findFirst() method will return an Optional that may or may not contain a string value (it may not contain a value if the stringsList is empty).

Then to unwrap the item from the Optional use the .get() method.

Answer 5

Since Java 21, simply using List.getFirst:

System.out.println(playersList.getFirst());

Note: Throws NoSuchElementException if the list is empty.

Additional details in JEP 431: Sequenced Collections.

Answer 6

Matthew's answer is correct:

list.get(0);

To do what you tried:

list[0];

you'll have to wait until Java 7 is released:

devoxx conference http://img718.imageshack.us/img718/11/capturadepantalla201003cg.png

Here's an interesting presentation by Mark Reinhold about Java 7

_{It looks like parleys site is currently down, try later :(}

Answer 7

If your collection is not a List (and thus you can't use get(int index)), then you can use the iterator:

Iterator iter = collection.iterator();
if (iter.hasNext()) {
    Object first = iter.next();
}

Answer 8

If you just want to get the minimum of a list, instead of sorting it and then getting the first element (O(N log N)), you can use do it in linear time using min:

<T extends Object & Comparable<? super T>> T min(Collection<? extends T> coll)

That looks gnarly at first, but looking at your previous questions, you have a List<String>. In short: min works on it.

For the long answer: all that super and extends stuff in the generic type constraints is what Josh Bloch calls the PECS principle (usually presented next to a picture of Arnold -- I'M NOT KIDDING!)

Producer Extends, Consumer Super

It essentially makes generics more powerful, since the constraints are more flexible while still preserving type safety (see: what is the difference between ‘super’ and ‘extends’ in Java Generics)

Answer 9

    public class Main {

    public static List<String> list = new ArrayList();

    public static void main(String[] args) {

        List<Integer> l = new ArrayList<>();

        l.add(222);
        l.add(100);
        l.add(45);
        l.add(415);
        l.add(311);

        l.sort(null);
        System.out.println(l.get(0));
    }
}

without l.sort(null) returned 222

with l.sort(null) returned 45