How to sort hashmap String, Collection<String []>> by String[index]

Question

i want sort this Map:

Map<String, Collection<String[]>> map = new HashMap<String, Collection<String[]>>();

In map is for example:

String key = "MyKey";
Collection<String[]> values = new ArrayList<String[]>();
map.put(key, values);

String[] firstValues = { "John", "21", "M"};
values.add(firstValues); //Name, Age, Gender

String[] secondValues = { "Jane", "31", "W"};
values.add(secondValues);

etc.

so finnaly, i have: map with:

Mykey : [["John", "21", "M"], ["Eve", "31", "W"]];

And i want do:

Collection<String[]> values = map.get(key);

and then sort by specific index, for example by first index(name) alphabetically, or by second index(age). So after first sort will be:

Mykey : [["Eve", "31", "W"], ["John", "21", "M"]];

after second sort:

Mykey : [["John", "21", "M"], ["Eve", "31", "W"]];

A Collection is not necessarily ordered. You might want to use List instead. — Vatev
– Vatev, Commented Jul 28, 2012 at 20:47

Marko Topolnik · Accepted Answer · 2012-07-28 21:01:51Z

5

Your code looks to me like something a developer experienced in dynamic languages such as Ruby would write. You'll probably find out that this is not your best avenue with Java. You should model your String[] with an appropriate object containing those strings as properties.

Also I think that the map part of your question is not very relevant—you are in effect dealing with a collection of objects that you need to sort by one of the properties. That you happen to have several such collections in a map doesn't influence the solution.

The sorting itself is done as @Havard Geithus has already advised you.

answered Jul 28, 2012 at 21:01

Marko Topolnik

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Håvard Malm Geithus · Accepted Answer · 2015-05-18 19:53:46Z

3

Use the API java.util.Collections.sort(List list, Comparator c) which specifies a comparator, and implement it as you wish (one for name, and another one for age)

Also, instead of using String[]{name, age, gender} as elements, create a class Person having these variables as members. I would probably place the two comparators you want inside this class as well:

public static final Comparator<Person> AGE_COMPARATOR = new Comparator<Person>() {
    @Override
    public int compare(Person p1, Person p2) {
        // implement
    }
};

Now you could simply call

Collections.sort(map.get(key), Person.AGE_COMPARATOR);

Assuming you used the List interface instead of the Collection interface as @Vatev suggested:

Map<String, List<Person>> map = new HashMap<String, List<Person>>();

edited May 18, 2015 at 19:53

answered Jul 28, 2012 at 20:47

Håvard Malm Geithus

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