基于Java中属性数据的对象的订单清单

Question

I need to order my List with 'Foo' objects based on a Enum value that Foo has as an attribute. I found 'Comparators', and I have used them before. But I always had some integer / double attribute, which is more clearer for me.

Because this question might be vague, i'll make some spoonfed code so you guys can see what I mean / want. Thanks a bunch!

/* Enum class */
public enum SomeEnum {
   CONSTANT_ALPHA, CONSTANT_BETA, CONSTANT_DELTA;
}

/* Class 'Foo', this is my Foo object */
public class Foo {
    private SomeEnum enum; /* Every 'Foo' has a enum type */

    public Foo(SomeEnum enum){ 
        this.enum = enum; 
    }

    public SomeEnum getEnum(){ 
        return enun;
    }
}

/* Imagine this below is some controller class*/

 Foo alpha1 = new Foo(SomeEnum.CONSTANT_ALPHA);
 Foo beta1 = new Foo(SomeEnum.CONSTANT_BETA);
 Foo alpha2 = new Foo(SomeEnum.CONSTANT_ALPHA);
 Foo delta1 = new Foo(SomeEnum.CONSTANT_DELTA);
 Foo beta2 = new Foo(SomeEnum.CONSTANT_BETA);

 List<Foo> list = new ArrayList<>();
 list.add(alpha1);
 //list.add(beta1) etc etc (all those 5 Foos)

 Collections.sort(list, someComparatorIDontKnowHowToMake);

 System.out.println(list.toString()); /* Not sure how to correctly print a List<> object, but imagine I would print the contents of the list*/

 /* The result of the list should be: */
 /* alpha1, alpha2, beta1, beta2, delta1*/

In conclusion or TL;DR:

I need to order the objects in the list based with the same Enum types. I do not need to have 'alpha' before 'delta' specifically. I just want that the Foo objects that have the same enum value are next to eachother in the List.

If this didn't make sense, please do ask!

Thanks.

Answer 1

You can simply compare enum elements of Foo

foos.sort(new Comparator<Foo>() {

    @Override
    public int compare(Foo o1, Foo o2) {
        //Add null check
        return  o1.getEnum().compareTo(o2.getEnum());
    }
});

Answer 2

Can't test the code right know, so there might be some syntax issues, but to the point: enum instances are distinct integers, so you can compare Foo instances just by them.

Collections.sort(list, new Comparator<Foo>(){
        public compare(Foo first, Foo second){
            return first.getEnum().comapareTo(second.getEnum());
        }
    });

for(Foo item : list){
    System.out.println(item);
}

Note that this code assumes that getEnum() won't return null for any instance in the list and that you will have toString() method implemented for Foo.

edit , yep, always too late.