Problems in ArrayList<Integer>

Question

import java.util.*;
import java.util.Random;

class ArraySorting {
public static void main(String[]args) {

    ArrayList<Integer> arr = new ArrayList<Integer>();

    Random generate = new Random();
    for (int nums = 0; nums < 20; nums++) {
      int randomnumbers = generate.nextInt(50);
      arr.add(randomnumbers);
    }

    System.out.println("First list of 20 generated numbers: ");
    System.out.println(arr);
    System.out.println("");

    int dupe = 0; 

    for (int n = 0; n < arr.size(); n++) {
        Integer check1 = arr.get(n); 

        for (int n2 = n+1; n2 < arr.size(); n2++) { 
            Integer check2 = arr.get(n2); 

            //remove second num if two numbers akike
            if (check1.equals(check2)) {
                arr.remove(check2);
                n2 = n2-1;
                dupe = 1; 

            }
        }
        n = n-dupe;
        dupe = 0;
    }

    System.out.println("Duplicates: " + (20 - arr.size()));

    for (int n3 = arr.size(); n3 < 20; ++n3) {
        int randomnumbers = generate.nextInt(50);
        arr.add(randomnumbers);

        //check for duplicates again
        for (int n = 0; n < arr.size(); n++) {
            Integer check1 = arr.get(n); 

            for (int n2 = n+1; n2 < arr.size(); n2++) { 
                Integer check2 = arr.get(n2); 

                if (check1.equals(check2)) {
                    arr.remove(check2);
                    n2 = n2-1;
                    dupe = 1; 
                }
            }
            n = n - dupe;
            dupe = 0;
        }
    }

    //before sort
    System.out.println(arr);
    System.out.println("");

    for(int a=0; a<20; a++){
        for (int b = 0; b < 19; b++) {
            if(arr[b] > arr[b+1]){
                int temporary = arr[b];
                arr[b] = arr[b+1];
                arr[b+1] = temporary;
            }
        }
    }

    System.out.println("\nSorted Array:\n");
    for (int a = 0; a < 20; a++) {
        System.out.println("Array [" + a + "]: " + arr[a]);
    }


}

}

Can anyone tell me what I did wrong for this one, I can't seem to generate the last part. Shouldn't the ArrayList arr = new ArrayList(); run same with the last part where arr[b] is work? I'm new to Java so would greatly appreciate if simple explaination/metaphor is ued provided with solution.

P.S: I'm not planning to use a library sort function like Collection, I'm required to use the sorting method at the last part.

Answer 1

The problem you are having is that you are trying to remove your duplicates before sorting. First, sort your integers, duplicates and all, and then remove the duplicates.

import java.util.ArrayList;
import java.util.Random;

public class ArraySorting {
    public static void main(String[]args) {

        ArrayList<Integer> arr = new ArrayList<Integer>();

        Random generate = new Random();
        for (int nums = 0; nums < 20; nums++) {
            int randomnumbers = generate.nextInt(10);
            arr.add(randomnumbers);
        }

        System.out.println("First list of 20 generated numbers: ");
        System.out.println(arr);
        System.out.println("");

        // SORT YOUR LIST FIRST
        bubbleSort(arr);
        System.out.println(arr);

        // NOW YOU CAN REMOVE YOUR DUPLICATES
        removeDuplicates(arr);
        System.out.println(arr);
    }

    public static void bubbleSort(ArrayList<Integer> list){
        for(int i = 0; i < list.size(); i++) {
            for(int j = 1; j < (list.size() -i); j++) {
                if(list.get(j - 1) > list.get(j)) {
                    int temp = list.get(j-1);
                    list.set(j-1, list.get(j));
                    list.set(j, temp);
                }                   
            }
        }       
    }

    public static void removeDuplicates(ArrayList<Integer> list){
        for(int i = 0; i < list.size(); i++) {
            if(i < list.size()-1) {
                int prev = list.get(i);
                int curr = list.get(i + 1);

                if(curr == prev) {
                    list.remove(list.get(i + 1));
                    i--;
                }
            }   
        }
    }   
}

Output

First list of 20 generated numbers: 
[9, 2, 2, 1, 3, 4, 0, 9, 5, 2, 5, 7, 4, 9, 0, 4, 0, 6, 6, 6]

[0, 0, 0, 1, 2, 2, 2, 3, 4, 4, 4, 5, 5, 6, 6, 6, 7, 9, 9, 9]
[0, 1, 2, 3, 4, 5, 6, 7, 9]

Answer 2

arr[a] is the syntax to access array elements. For ArrayLists you use arr.get(a). And to assign a value to an ArrayList, you use arr.set(b,value). You can't use the assignment operator.