Sort ArrayList of Array in Java

Question

What is the best way to sort an ArrayList in Java?

Where String[] is...

String[] = new String[] { \"abc\", \"abc\", \"ab         


        

Answer 1

You write a Comparator that compares two String[] by the correct child, and then you pass it to Collections.sort(List<T> list, Comparator<? super T> c).

Answer 2

Use TreeSet or TreeMap http://download.oracle.com/javase/6/docs/api/java/util/TreeSet.html

Answer 3

Suppose we have a list of arrays [["x", "y","z"], ["a", "b", "c"], ["m", "n", "o"]]

We can use list.sort() instead of Collections.sort().

To sort, we can adopt a cleaner way using lambdas and method references.

Using lambda:

listOfStrings.sort((a, b)->a[1].compareTo(b[1]));

Using method reference:

listOfStrings.sort(Comparator.comparing(a->a[1]));

Outputs:

[a, b, c]
[m, n, o]
[x, y, z]

Answer 4

This is extremely easy to do with Java 8. Just write:

list.sort(Comparator.comparing(a -> a[1]));

For example, the following code:

List<String[]> list = Arrays.asList(
    new String[] { "abc", "abc", "abc", "abc", "abc", "abc", "abc" },
    new String[] { "xyz", "xyz", "xyz", "xyz", "xyz", "xyz", "xyz" },
    new String[] { "fgh", "fgh", "fgh", "fgh", "fgh", "fgh", "fgh" });

list.sort(Comparator.comparing(a -> a[1]));
list.stream().map(Arrays::toString).forEach(System.out::println);

Will yield the wanted result:

[abc, abc, abc, abc, abc, abc, abc]
[fgh, fgh, fgh, fgh, fgh, fgh, fgh]
[xyz, xyz, xyz, xyz, xyz, xyz, xyz]

Answer 5

Based on your edit: Your String[] should be a School object to contain your attributes. Make your School object implement Comparable and that will allow easy sorting with Collections.sort().

Answer 6

Start with Collections.sort, the one that takes a custom Comparator. You'll need to write a custom Comparator for this also.

For instance, assuming you want to rely on the natural ordering of Strings as defined in their compareTo method:

public static void main(String[] args) throws Exception {
        ArrayList<String[]> listOfStringArrays = new ArrayList<String[]>();
        listOfStringArrays.add(new String[] {"x","y","z"});
        listOfStringArrays.add(new String[] {"a","b","c"});
        listOfStringArrays.add(new String[] {"m","n","o"});
        Collections.sort(listOfStringArrays,new Comparator<String[]>() {
            public int compare(String[] strings, String[] otherStrings) {
                return strings[1].compareTo(otherStrings[1]);
            }
        });
        for (String[] sa : listOfStringArrays) {
            System.out.println(Arrays.toString(sa));
        }
        /* prints out 
          [a, b, c]
          [m, n, o]
          [x, y, z]
        */ 

    }