System.arraycopy(array2,0, arr,arr.length+1, array2.length);

package com.company;

import java.util.Arrays;
import java.util.Scanner;

public class Statfunc<T> {
    private  Object [] arr = {};
    public  void menMest(){
        for ( int i = 0; i < arr.length; i += 2 ) {
            T tmp = (T) arr[i];
            arr[i] = arr[i + 1];
            arr[i + 1] = tmp;
        }
    }
    public void add (double t){
        Object [] temp = new Object[arr.length+1];
        System.arraycopy(arr, 0,temp,0,arr.length);
        arr=temp;
        arr[arr.length-1]=t;    }

    public void addMas(int y) {
        Scanner scanner = new Scanner(System.in);
        Object array[] = new Object[y];
        Object array2[] = new Object[arr.length];
        System.out.println("ELEMENTS:");
        for (int i = 0; i < y; i++) {            array[i]= scanner.nextDouble();        }
        System.out.print("[");
        for (int i = 0; i < y; i++) {            System.out.print(array[i] + " ");        }
        System.out.println("]");
        System.out.println("WHERE");
        int x = scanner.nextInt();
        if (x == 0)
        {
          System.arraycopy(arr, 0, array2,0,arr.length);
          System.arraycopy(array,0, arr,0,array.length);
          System.arraycopy(array2,0, arr,arr.length+1, array2.length);
        }
        else if(x==arr.length+1)
        {
            for (int i = 0; i < y; i++) add((double)array[i]);    }

            else { }

        }

    @Override
    public String toString(){
        return Arrays.toString(arr);
    }
}


package com.company;

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scanner=new Scanner(System.in);
        int y;
        Statfunc <Double> myList = new Statfunc <> ();
        myList.add(5.0);        myList.add(2.0);        myList.add(0.0);        myList.add(1.0);
        System.out.println(myList);
        //topic#2
        myList.menMest();
        System.out.println(myList);
        System.out.println("HOW MANY ELEMENT IN MASSIV");
        y=scanner.nextInt();
        myList.addMas(y);
        System.out.println(myList);

    }

}