Java program to create a custom Collection(ArrayList)

MyList.java(Interface)

package com.nt.util;

public interface MyList {

 public void add(Object obj);
}

MyArrayList.java(Class)

package com.nt.util;

public class MyArrayList implements MyList{

 private Object[] objArray;
 private static int elementCount=0;
 
 //default constructor
 public MyArrayList() {
  
  //initial size
  this.objArray=new Object[4];
 }
 
 //Logic to add elements
 public void add(Object obj) {
  
  //checks if capacity is == size
  if(this.capacity()==this.size())
  {
   //then it will create a new array
   Object[] tempArray=new Object[this.capacity()*2];
   int i=0;
   
   //it will copy all the element to new array
   for(;i<(this.size());i++)
   {
    tempArray[i]=this.objArray[i];
    
   }
   
   //it will insert the last element
   tempArray[elementCount++]=obj;
   
   //it will copy tempArray to objArray
   this.objArray=tempArray;
  }
  else
  {
   
   this.objArray[elementCount++]=obj;
   
  }
 }
 
 //returns the capacity
 private int capacity() {
  return this.objArray.length;
 }
 
 //returns the size
 private int size()
 {
  return elementCount;
 }
 
 @Override
 public String toString()
 {
  String str=java.util.Arrays.toString(this.objArray);

  return str.replaceAll("null","");
 }

}

ArrayListTest.java

package com.nt.test;

import com.nt.util.MyArrayList;
import com.nt.util.MyList;

public class ArraylistTest {

 public static void main(String[] args) {
  
  MyList list=new MyArrayList();
  list.add(23);
  list.add(45);
  list.add(43);
  list.add(3);
  list.add(55);
  System.out.println(list);
  
 }
}