选择ArrayList中没有collections.sort的排序

Question

So, why won't this code work it is returning the original list always (I haven't put the return statement, but can someone determine why the logic behind my selection sort algorithm is not working). Any help would be gladly appreciated!

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Scanner;

public class ArrayListDemo {
    public static void main(String [] args) {
        ArrayList <String> list = new ArrayList <String> (15);
        list.add("Wilson");
        list.add("Annans");
        list.add("Manning");
        list.add("Branday");
        list.add("Smittens");
        list.add(2, "Goddard Fey");
        list.set((list.size()) - 1, "Brand Brandon");
        list.add("Brand Boarders");
        selectionSort(list);
    }
    static void selectionSort(ArrayList<String> a) {
         int smallindex;
         for(int i = 0; i < a.size(); i++) {
              smallindex = i; // set first element as smallest
              for(int j = i + 1; j < a.size(); j++) { // find smallest
                   if (a.get(j).compareTo(a.get(smallindex)) > 0) {
                       swap(a, i, j);
                   }

              }
         }
    }

    static void swap(ArrayList<String> a, int index1, int index2) {
         String i_string = a.get(index1);
         String j_string = a.get(index2);
         String temp = i_string;
         i_string = j_string;
         j_string = temp;
    } 
}

Answer 1

Your swap(ArrayList<String> a, int index1, int index2) method creates local variables - i_string and j_string - and then swaps them. This has no impact on the contents of the input List a .

To change the value of the i 'th element of the List a you must call a.set(i,newValue) .

You can either implement the swap method yourself, or use the existing Collections.swap :

static void selectionSort(ArrayList<String> a) {
    int smallindex;
    for(int i = 0; i < a.size(); i++) {
        smallindex = i; // set first element as smallest
        for(int j = i + 1; j < a.size(); j++) { // find smallest
            if (a.get(j).compareTo(a.get(smallindex)) > 0) {
                Collections.swap(a, i, j);
            }
        }
    }
}

If you wish to learn how to implement swap correctly, you can take a look at the JDK implementation:

public static void swap(List<?> list, int i, int j) {
    final List l = list;
    l.set(i, l.set(j, l.get(i)));
}

As you can see, this method sets the value returned by l.set(j, l.get(i)) to be the new i 'th element of the List . l.set(j, l.get(i)) sets the value l.get(i) as the new j 'th element of the List and returns the previous j 'th element. Therefore the new i 'th element of the List becomes the previous j 'th element.