我怎样才能更快地对ArrayList排序？

Question

I'm doing a class project and I need to sort ArrayLists of custom objects based on the values of their int attributes.

I'm currenly using something like this:

public static void Sort(ArrayList <MyObject> objectList){

    for (int i = 0; i < list.size()-1; i++){

        for (int j = 0; j < list.size()-1; j++){

            if (objectList.get(j).getA() > objectList.get(j+1).getA()){

                Collections.swap(objectList, j, j+1);
            }
        }    
    }    
}

The program works well if the ArrayList has less than 10^4 elements. But if I try to sort 10^5 elements it takes several minutes, and I need to sort 10^6 elements. Any suggestions?

Answer 1

Use the List::sort method:

objectList.sort(Comparator.comparing(MyObject::getA));

As @lexicore has mentioned below, it seems like getA() returns a numeric type, in which case if it returns int then it's better to use comparingInt instead of comparing above, if it's long use comparingLong or if it's a float / double then use comparingDouble for better performance.

Answer 2

Currently, your implementation is O(n^2) which means that as the array grows, the time will expand quadratically.

Without going into a lot of details of using mergesort which is O(log(n) xn) , the fastest way is to use Java's built-in solution for sorting.

Collections.sort(list);

Link to API . There is also Collection.sort(list, comparator) which allows you to provide your own comparator .

Do you want it to be even faster? You can use the Java's new feature which allows you to sort with multiple cores in parallel. Here is the API . Notice that Arrays.sort() and Arrays.parallelSort() take an array as the first parameter. You will need to convert a list to an array using list.toArray() .

Finally, do note that List#sort() was introduced only in Java 8.