How do I sort an ArrayList based on its object's contents

Question

Possible Duplicate:
Sorting an ArrayList of Contacts based on name?
Sorting of ArrayList

I have an ArrayList of type Person and I want to sort it based on the person's ages: person.getAge().

How can I do that, so that the list of people is ordered by their age?

Look into `Comparator` interface and `Collections.sort(...)` method — Victor Sorokin, Dec 02 '12 at 19:22
look this question http://stackoverflow.com/questions/1814095/sorting-an-arraylist-of-contacts-based-on-name?lq=1 — bentham, Dec 02 '12 at 19:24

Ted Hopp · Accepted Answer · 2012-12-02T19:41:33.973

2

ArrayList<Person> people = . . .;
Collections.sort(people, new Comparator<Person>() {
        @Override
        public int compare(Person o1, Person o2) {
            final int age1 = o1.getAge();
            final int age2 = o2.getAge();
            return age1 < age2 ? -1 : age1 > age2 ? 1 : 0;
        }
    }
);

In Java 7, you can use return Integer.compare(age1, age2); instead.

Alternatively, you can have Person implement Comparable<Person>. However, then you could only sort on the one attribute.

edited Dec 02 '12 at 19:41

answered Dec 02 '12 at 19:24

Ted Hopp

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Did you mean the static `Integer.compare(int, int)`? – Paul Bellora Dec 02 '12 at 19:29
@PaulBellora - Yes, thanks. I'll fix the typo. – Ted Hopp Dec 02 '12 at 19:41
The last approach still works in Java 8. Most of the number classes have a compare method, I assume Boolean has the same as well (not checked) – Zoe Sep 21 '17 at 15:55
@LunarWatcher - Yes, `Boolean` implements `Comparable`. It treats `true` as greater than `false`. – Ted Hopp Sep 24 '17 at 00:42

score 1 · Answer 2 · answered Dec 02 '12 at 19:24

Expanding on the comment, something along these lines:

Collections.sort(personList, new Comparator<Person>() {
    @Override
    public int compare(Person p1, Person p2) {
        return new Integer(p1.getAge()).compareTo(p2.getAge());
    }
});

Relevant documentation

Yogendra Singh · Answer 3 · 2012-12-02T19:39:08.250

0

Define your comparator, which compares using the age attribute Only. Use the same in collection sort method [sort(java.util.List, java.util.Comparator)](http://docs.oracle.com/javase/7/docs/api/java/util/Collections.html#sort(java.util.List, java.util.Comparator)) which accepts the comparator.

        Collections.sort(peopleList, new Comparator<Person>(){
             @Override
             public int compare(Person p1, Person p2) {
                return Integer.valueOf(p1.getAge()).compareTo(p2.getAge());
             }
         });

Or better do below for optimization as Integer.compareTo internally uses Integer.compare method.

       Collections.sort(peopleList, new Comparator<Person>(){
             @Override
             public int compare(Person p1, Person p2) {
                return Integer.compare(p1.getAge(), p2.getAge());
             }
         });

edited Dec 02 '12 at 19:39

answered Dec 02 '12 at 19:27

Yogendra Singh

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This generates a lot of garbage because it creates two `Integer` objects for each comparison. Much better to stick to primitive `int` comparisons. – Ted Hopp Dec 02 '12 at 19:29
@TedHopp Updated the answer with optimized version as well. – Yogendra Singh Dec 02 '12 at 19:34
@user521180 You can better use `Integer.compare` method which does the comparison of tow int values for you. I would advice not to implement a custom comparison. – Yogendra Singh Dec 02 '12 at 19:42

score 0 · Answer 4 · answered Dec 02 '12 at 19:31

0

//We can do using TreeMap
//i.e
TreeMap treemap=new TreeMap<int,Person>
//trace over person
for(Person p:YourPersonArrayList)
{
treemap.put(p.getAge(),p);
}
//Finally you get TreeMap which is 
//sorted on Persons age...

answered Dec 02 '12 at 19:31

Nagappa L M

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Code may not work because i am not sure about int key's in TreeMap,you can use TreeMap Collection to solve your problem – Nagappa L M Dec 02 '12 at 19:35

How do I sort an ArrayList based on its object's contents

4 Answers4