Integer sorting using Collections, using custom class in Java

Question

For instance:

class A{
    public int a;
    public String b;

    public A(int a, String b){
        this.a = a;
        this.b = b;
    }
}


main(){
    List<A> list = new Arraylist<>();
    list.add(new A(5, "abc"));
    list.add(new A(1, "aee"));
    list.add(new A(3, "adf"));
    list.add(new A(6, "aad"));
    list.add(new A(2, "xx"));
}

Is there any way to sort this list in ascending order using the integer in A class. I've already tried

Collection.sort(list) but getting some syntax error. What I'm doing wrong.

Konstantin Yovkov · Accepted Answer · 2015-10-08T08:17:54.663

1

You need a custom Comparator implementation:

Collections.sort(list, new Comparator<A>() {
    public int compare(A first, A second) {
        return Integer.compare(first.a, second.a);
    }
});

edited Oct 08 '15 at 08:17

answered Oct 08 '15 at 08:11

Konstantin Yovkov

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`return Integer.compare(first.a, second.a)` avoids unnecessary `Integer` creation. – Andy Turner Oct 08 '15 at 08:17

score 1 · Answer 2 · answered Oct 08 '15 at 08:14

1

Using Java8 List.sort and a lambda expression for the Comparator you can write:

list.sort((a1,a2) -> a1.a - a2.a);

answered Oct 08 '15 at 08:14

wero

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Rustam · Answer 3 · 2015-10-08T08:31:54.777

0

Use custom Comparator

Collections.sort(list, new Comparator<A>() {

                @Override
                public int compare(A o1, A o2) {

                if(o1.a==o2.a){
                    return 0;
                }else {
                   return o1.a>o2.a?1:-1;
                }
              }

            });

edited Oct 08 '15 at 08:31

answered Oct 08 '15 at 08:20

Rustam

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Your comparator doesn't isn't anti-symmetrical: it should return 0 for `o1.a == o2.a`. `Integer.compare(o1.a, o2.a)` is easier. – Andy Turner Oct 08 '15 at 08:27
@AndyTurner yes you are right. – Rustam Oct 08 '15 at 08:30

Aroniaina · Answer 4 · 2015-10-08T08:35:27.663

0

    List<A> listElements = new ArrayList<>();
    listElements.add(new A(5, "abc"));
    listElements.add(new A(1, "aee"));
    listElements.add(new A(3, "adf"));
    listElements.add(new A(6, "aad"));
    listElements.add(new A(2, "xx"));
    //Sorting
    Collections.sort(listElements, new Comparator<Pourtest.A>() {
        @Override
        public int compare(A o1, A o2) {
            return Integer.compare(o1.a, o2.a);
        }
    });
    //Verify
    for (A listElement : listElements) {
        System.out.println(listElement.a);
    }

edited Oct 08 '15 at 08:35

answered Oct 08 '15 at 08:20

Aroniaina

1,252
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Your comparator doesn't isn't anti-symmetrical: it should return 0 for o1.a == o2.a. Integer.compare(o1.a, o2.a) is easier. – Andy Turner Oct 08 '15 at 08:28

Integer sorting using Collections, using custom class in Java

4 Answers4