How to determine if a List is sorted in Java?

Question

I would like a method that takes a List<T> where T implements Comparable and returns true or false depending on whether the list is sorted or not.

What is the best way to implement this in Java? It's obvious that generics and wildcards are meant to be able to handle such things easily, but I'm getting all tangled up.

It would also be nice to have an analogous method to check if the list is in reverse order.

Answer 1

To check whether a list or any data structure for that matter is a task that only takes O(n) time. Just iterate over the list using the Iterator Interface and run through the data (in your case you already have it ready as a type of Comparable) from start to end and you can find whether its sorted or not easily

Answer 2

This is an operation that will take O(n) time (worst case). You will need to handle two cases: where the list is sorted in descending order, and where the list is sorted in ascending order.

You will need to compare each element with the next element while making sure that the order is preserved.

Answer 3

Easy:

List tmp = new ArrayList(myList.size());
Collections.sort(tmp);
boolean sorted = tmp.equals(myList);

Or (if elements are comparable):

Object prev;
for( Object elem : myList ) {
    if( prev != null && prev.compareTo(elem) > 0 ) {
        return false;
    }
    prev = elem;
}
return true;

Or (if elements are not comparable):

Object prev;
for( Object elem : myList ) {
    if( prev != null && myComparator.compare(prev,elem) > 0 ) {
        return false;
    }
    prev = elem;
}
return true;

The implementations fail for lists containing null values. You have to add appropriate checks in this case.

Answer 4

Simply use the iterator to look through the contents of the List<T>.

public static <T extends Comparable> boolean isSorted(List<T> listOfT) {
    T previous = null;
    for (T t: listOfT) {
        if (previous != null && t.compareTo(previous) < 0) return false;
        previous = t;
    }
    return true;
}

Answer 5

Guava provides this functionality through its awesome Ordering class. An Ordering is a Comparator++. In this case, if you have a list of some type that implements Comparable, you could write:

boolean sorted = Ordering.natural().isOrdered(list);

This works for any Iterable, not just List, and you can handle nulls easily by specifying whether they should come before or after any other non-null elements:

Ordering.natural().nullsLast().isOrdered(list);

Also, since you mentioned that you'd like to be able to check for reverse order as well as normal, that would be done as:

Ordering.natural().reverse().isOrdered(list);

Answer 6

This is what I would do:

public static <T extends Comparable<? super T>> boolean isSorted(List<T> list) {
    if (list.size() != 0) {
        ListIterator<T> it = list.listIterator();
        for (T item = it.next(); it.hasNext(); item = it.next()) {
            if (it.hasPrevious() && it.previous().compareTo(it.next()) > 0) {
                return false;
            }
        }

    }
    return true;
}

Answer 7

Here's a generic method that will do the trick:

public static <T extends Comparable<? super T>>
        boolean isSorted(Iterable<T> iterable) {
    Iterator<T> iter = iterable.iterator();
    if (!iter.hasNext()) {
        return true;
    }
    T t = iter.next();
    while (iter.hasNext()) {
        T t2 = iter.next();
        if (t.compareTo(t2) > 0) {
            return false;
        }
        t = t2;
    }
    return true;
}

Answer 8

One simple implementation on arrays:

public static <T extends Comparable<? super T>> boolean isSorted(T[] a, int start, int end) {
    while (start<end) {
        if (a[start].compareTo(a[start+1])>0) return false;
        start++;
    }
    return true;
}

Converting for lists:

public static <T extends Comparable<? super T>> boolean isSorted(List<T> a) {
    int length=a.size();
    if (length<=1) return true;

    int i=1;
    T previous=a.get(0);
    while (i<length) {
        T current=a.get(i++);
        if (previous.compareTo(current)>0) return false;
        previous=current;
    }
    return true;
}