用户输入一个整数，该整数将添加到数组中-Java

Question

psudeo code of what i want the add() method to do. Pretty much the user is prompted to type in a number. Once they type in a number, the number will be added to the array[0] and every other value will be pushed up one. However the array is only 20.length. So every array[20] is deleted. (ill add in the loop so they're still getting asked). Please help me make this add method, or show me a java method that does this. (not using the .list method). thanks in advance.

public static void main(String[] args) {
    int[] array = new int[1];
    Scanner sc = new Scanner(System.in);

    int num;
    num = sc.nextInt();
    add(num); 

    for(int i = 0; i < array.length; i++) {
        println(array[i]);
    }
}

public void add(int value) {
    array[0] = value;

    for(int i = 0; i < array.length; i++) {    
      if(array[i + 1] == already has value) {
         // then shift numbers one to the right;
      } else if(array.length > 20) {
         //then delete array[19];
      }
  }
}

Answer 1

You are going to want to shift the entire list left before adding the value. Consider this algorithm: 1. start at the second to last element of the list and shift it to the right overwriting what existed at the last element. 2. do this for all values down to the first one. 3. set array[0] to value

You are done.

Answer 2

Arrays are "contiguous blocks of memory", so in order to add a value to the start of these blocks, you'll have to first shift the rest values to the corresponding block.

public static int[] arr = {1,2,3,4,5,6,7,8,9,10};

public static void main(String[] args) {

    Scanner sc = new Scanner(System.in);
    int num;
    num = sc.nextInt();
    add(num);


    for(int i = 0; i < arr.length; i++) {
        System.out.println(arr[i]);
    }
}

public static void add(int value) {
    for(int i=arr.length-1;i>0;i--){
        arr[i] = arr[i-1];        //first shifting these values to make space for the new element at zero index
    }
    arr[0] = value;    
}   

Answer 3

You can do as MadProgrammer said in your comment or better use ArrayList :

From add(int index, E element)

Inserts the specified element at the specified position in this list. Shifts the element currently at that position (if any) and any subsequent elements to the right (adds one to their indices).

Example(Change it according to your need):

List<Integer> list = new ArrayList<Integer>(20);
public void add(int value) {

  list.add(0, value);
  for(int i = 0; i < array.length; i++) {
     list.add(index, value); // this will automatically shift element right
     if(list.size > 20) {
       list.remove(20);
      }
   }
}

Answer 4

Any reason why you don't wish to use ArrayList? In case no, any chance that this is not a programming assignment :)