如何排序哈希映射列表

Question

I have a List of HashMap such as below

ArrayList l = new ArrayList ();
HashMap m = new HashMap ();
m.add("site_code","AL");
m.add("site_name","Apple");
l.add(m);
m = new HashMap();
m.add("site_code","JL");
m.add("site_name","Cat");
l.add(m);
m = new HashMap();
m.add("site_code","PL");
m.add("site_name","Banana");
l.add(m)

I'd like to sort the list based on site_name . So in the end it would be sorted as.

Apple, Banana, Cat

I was trying something like this:

Collections.sort(l, new Comparator(){
           public int compare(HashMap one, HashMap two) {
              //what goes here?
           }
});

Answer 1

If you make your collections generic, it will end up looking about like this:

Collections.sort(l, new Comparator<HashMap<String, String>>(){ 
        public int compare(HashMap<String, String> one, HashMap<String, String> two) { 
            return one.get("site_name").compareTo(two.get("site_name"));
        } 
});

If you can't use generics because you're stuck on a 1.4 or earlier platform, then you'll have to cast the get 's to String .

(Also, as a matter of style, I'd prefer declaring the variables as List and Map rather than ArrayList and HashMap . But that's not relevant to the question.)

Answer 2

I think this is a great time to think about a redesign. From your example, it looks like all of your objects have the same two fields - site_name and site_code . In that case, why not define your own class rather than using a HashMap ?

public class Site implements Comparable<Site> {
    private String site_name;
    private String site_code;

    // getters and setters, equals, and hashCode

    public int compareTo(Site other) {
        return this.site_name.compareTo(other.getSiteName);
    }
}

And then you can just use Collections.sort() .

Answer 3

Something like:

String codeOne = (String)one.get("site_code");
String codeTwo = (String)two.get("site_code");

return codeOne.compareTo(codeTwo);

I haven't compiled or tested this, but it should be along these lines.