在Java中对ArrayList进行排序

Question

I use this function in java:

Collections.sort(an Arraylist, new CustomComparator);

The compare method in CustomComparator class will return an int. What this means to the sort procedure? what's the number of this value and how will affect the sorting procedure?

More specifically i want in compare method to compare two values. Here is my code:

    import java.util.Comparator;

    public abstract class CustomComparator implements Comparator<HLine> {
        @Override
        public int compare(HLine hl1, HLine hl2) {
            return hl1.y < hl2.y;
        }
    } 

and i call for sorting:

    Collections.sort(hlines, new comparator());

hlines is an Arraylist of a object's with a Point and two doubles. I want to compare the second double in two object's.

Answer 1

Basically, as stated in the Javadoc of Comparator.compare and Comparable.compareTo these methods return

a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.

Meaning that if you call

new Integer(1).compareTo(0)

it will return a negative integer, which indicates, that 0 has to be ordered before 1.

new Integer(1).compareTo(1)

it will return 0, which indicates, that both values has to be ordered on the same level.

new Integer(1).compareTo(2)

it will return a positive integer, which indicates, that 2 has to be ordered after 1.

To fix your codesample, you need to rewrite compare() so it will return an Integer , as it is now it returns a boolean and will not compile.

Because you are trying to compare double s you can simply change to

    @Override
    public int compare(HLine hl1, HLine hl2) {
        return hl1.y - hl2.y;
    }

Answer 2

sort() method sort the elements but first they are compared.For the comparison purpose, sort() method can use compare() or compareTo() methods.

Now, if you want to sort the elements on the basis of only one attribute the use compareTo() method of comparable interface.

And if you want to sort the elements on the basis of more than one elements then use cmopare() method of comparator interface.