[JAVA] How to add elements without specifying the length of the array

This is the first article. I decided to join an IT company as a member of society next year, and I am studying Java by myself. .. I'm a super beginner, so I don't think the code is helpful (it's too dirty and not good).

I looked for something like the ArrayList's ʻadd ()method using an array (such as ʻint []).

There is no choice but to change the length of the array one by one

it seems like. ..

To change the length of the array you need to make a copy of the array using System.arraycopy () For example, when changing from ʻint [3] to ʻint [5]

class Hairetsu {
	public static void main(String args[])
    	int[] a = new int[3]; //Original
    	a[0] = 1;
    	a[1] = 2;
    	a[2] = 3;
    	int[] b = new int[5]; //Copy to
    	System.arraycopy( a, 0, b, 0, a.length ); //(Array a,from 0 of a,To array b,from 0 in b,Minutes of all elements of a)copy!
    	for ( int i = 0 ; i < b.length ; i++ ){
    		System.out.println( b[i] );

Execution result


It becomes the code such as. So, if you apply this and try to make something like the ʻadd ()` method in the list, ...

class HairetsuList {
	static int[] list;
	public static void add(int a){
		int i = list.length;
		int[] copy = new int[i+1]; //Create a large array with 1 element from the beginning
		System.arraycopy( list , 0 , copy , 0 , i );
		copy[i] = a ;
		list = copy; //The length of the array is different, but there is this. ..

It looks like this, for the time being a test ...

class Test {
	public static void main(String args[])
		HairetsuList test = new HairetsuList();
		test.list = new int[0];
    	System.out.print("{ ");
    	for (int i=0 ; i < test.list.length; i++){
    		System.out.print( test.list[i]+" ");

Execution result

{ 4 5 3 }

Aside from the childishness of the code, it was pretty good. You can use a list etc. normally. .. This time, I had to use arrays for corporate training tasks, so I tried my best to think about it, but programming is difficult. ..

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