我不知道为什么我的数组没有在此冒泡排序中保存其随机值

Question

I wanted to do a simple bubble sort exercise but I don't know what should I change in order to make this work but also not to screw with the drawing method that I have. Here is my code:

package sorting;

import java.awt.Graphics;
import javax.swing.JFrame;

public class Sorting extends JFrame{

    public int[] values = new int[800];

    public static void main(String[] args) {
        Sorting sort = new Sorting();
    }

    public Sorting(){
        setSize(800, 500);
        setResizable(false);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setVisible(true);
        setTitle("Sorting");
    }

    @Override
    public void paint(Graphics g){        
        for (int i = 0; i < 800; i++){
            values[i] = (int)(Math.random()*500);
            for (int k = 0; k < 800; k++){
                g.drawLine(k, 500, k, 500-values[k]);
            }
        }
        for (int j = 0; j < 800; j++){
            for (int a = 0; a < 800 - j - 1; a++){
                int r = values[j];
                int b = values[j + 1];
                if (a > b){
                    swap(values, j, j+1);
                }
            }
        }
    }

    private void swap(int[] arr, int a, int b) {
        int temp = arr[a];
        arr[a] = arr[b];
        arr[b] = temp;
    }
}

My guess is that my values array doesn't have the values created in paint method saved and that's why the swapping method isn't working but I'm not sure.

This is my output

Answer 1

if (a > b){
    swap(values, j, j+1);
}

Should be

if (r > b){
    swap(values, j, j+1);
}

Answer 2

Following is the fixed for loop. You are using incorrect index, as well as comparing incorrect values.

for (int j = 0; j < 800; j++){
    for (int a = 0; a < 800 - j - 1; a++){
        int r = values[a];
        int b = values[a + 1];
        if (r > b){
            swap(values, a, a+1);
        }
    }
}

Answer 3

In bubble sort you would be looping through whole array and if there is a pair of two numbers at index i and i+1 that does not seem to be sorted, you swap values at those indexes and you proceed like that till the end of your current loop. Then you have to loop through again because there was a unsorted part detected. The algorithm ends when you loop through all entries and you detect no change to the array.

I would propose to use simple buuble sort implementation with while and for loop :

boolean isSorted = false; // assume array is not sorted

while (!isSorted) {  //repeat until array is sorted

     isSorted = true;  // assume array is sorted

     for (int i = 0; i < values.length - 1; i++) { //loop through array
         int a = values[i];  //get first value to compare
         int b = values[i + 1];  // get second value to compare

         if (a < b) {  // if first value is less than second, swap them
             swap(values, i, i + 1);
             isSorted = false;  // there was a change so array is not actually sorted! So we will have to loop through whole array again
         }
     }
}