[Java] Object operation of ArrayList class (AOJ ④ Inversion of sequence)

Object operation of ArrayList class (variable length array)

• The list is ** a collection in which the elements under it are ordered **
• ArrayList has a data structure that uses arrays internally, and ** elements can be added and deleted even afterwards **
• cf: Array is not possible
• Reading and writing (random access) of the value by the index value is ** accessible in a certain time regardless of the position **
• ** Frequent insertion and deletion of values is slow **
• If the memory area reserved at the time of insertion is exceeded, memory reallocation will occur automatically (→ Declare the size if the number of elements is expected in advance).

Definition

• ArrayList <type> List name = new ArrayList <type> ();
• Specify the class name in
• Allows you to store ** objects of the specified class as elements **
• If you want to create a list that handles basic types, define it in the wrapper class (ex: int type wrapper class is Integer class)

Sorting elements

• ** sort method ** of Collections class **
• Sort list elements in ascending natural order
• Collections.sort (ArrayList object);
• Sort elements in descending order
• Collections.sort (ArrayList object, Collections.reverseOrder ());

Elements in reverse order

• ** reverse method ** of Collections class **
• Collections.reverse (list);

import java.util.ArrayList;
import java.util.List;
import java.util.Collections;
import java.util.Arrays;

public class Main {
	public static void main(String[] args) {
	    
    ArrayList<Integer> nums = new ArrayList<Integer>(Arrays.asList(50,80,46));
    nums.add(1);
    System.out.println(nums); //[50, 80, 46, 1]

    //Sort in ascending order with the sort function
    Collections.sort(nums); 
    //Display and confirm
    System.out.println(nums);  //[1, 46, 50, 80]
    //Sort in descending order with the sort function
    Collections.sort(nums, Collections.reverseOrder());
    //Display and confirm
    System.out.println(nums);  //[80, 50, 46, 1]

    //Get the second value from the list and display it
    System.out.println(nums.get(2)); //46

    System.out.println("The number of data:" + nums.size()); //The number of data:4

    for (int i = 0 ; i < nums.size() ; i++){
      int num = nums.get(i);
      System.out.print(num+" "); //80 50 46 1 
    }
  }
}   

Inversion of sequence

Create a program that outputs the given sequence in reverse order. *Input The input is given in the following format: n a1 a2 . . . an n represents the length of the sequence and ai represents the i-th number. *Output Output the reverse sequence of numbers in one row. Insert a space between the elements of the sequence (note that there is no space after the last number). *Constraints

• n ≤ 100
• 0 ≤ ai < 1000

import java.util.ArrayList;
import java.util.Collections;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		int n =scanner.nextInt();
	
        ArrayList<Integer> nums = new ArrayList<Integer>();

        for(int i=1;i<=n;i++){
		    int num = scanner.nextInt();
		    nums.add(num);
        }
        
       /*
Sort in ascending order
         Collections.sort(nums)
Sort in descending order
         Collections.sort(nums, Collections.reverseOrder());
Check size
         System.out.println(nums.size()); 
       */
       Collections.reverse(nums);

        for (int i = 0 ; i < n ; i++){

            if (i==n-1){
                System.out.print(nums.get(i));
            }
            else{
            System.out.print(nums.get(i)+" ");
            }
        }
        System.out.println();
	}
}