how would I complete this code without calling any built-in java collection framework classes like ArrayList, LinkedList, etc?

import java.util.Iterator;

class CallStack implements Iterable {

// You'll want some instance variables here

public CallStack() {

//setup what you need

}

public void push(T item) {

//push an item onto the stack

//you may assume the item is not null

//O(1)

}

public T pop() {

//pop an item off the stack

//if there are no items on the stack, return null

//O(1)

//replace this dummy return statement

return null;

}

public T peek() {

//return the top of the stack (but don't remove it)

//if there are no items on the stack, return null

//O(1)

//replace this dummy return statement

return null;

}

public String toString() {

//Create a string of the stack where each item

//is separated by a space. The top of the stack

//should be shown to the right and the bottom of

//the stack on the left.

//Hint: Reuse the provided code from another class

//instead of writing this yourself!

//O(n)

//replace this dummy return statement

return null;

}

public void clear() {

//remove everything from the stack

//O(1)

}

public int size() {

//return the number of items on the stack

//O(1)

//replace this dummy return statement

return -1;

}

public boolean isEmpty() {

//return whether or not the stack is empty

//O(1)

//replace this dummy return statement

return false;

}

@SuppressWarnings("unchecked")

public Object[] toArray() {

//Return an array representation of the stack.

//The top of the stack should be element 0

//in the array.

//O(n)

//replace this dummy return statement

return null;

}

public Iterator iterator() {

//Return an iterator that traverses from

//the top of the stack to the bottom of

//the stack.

//The iterator's hasNext() and next() methods

//must both be O(1)

//next() should throw a NullPointerException

//if you try to use next when there are no

//more items

//replace this dummy return statement

return null;

}

public static void main(String[] args) {

CallStack s1 = new CallStack<>();

s1.push("a");

s1.push("b");

CallStack s2 = new CallStack<>();

s2.push(1);

s2.push(2);

s2.push(3);

if(s1.toString().equals("a b") && s1.toArray()[0].equals("b") && s1.toArray()[1].equals("a") && s1.toArray().length == 2) {

System.out.println("Yay 1");

}

if(s1.peek().equals("b") && s2.peek().equals(3) && s1.size() == 2 && s2.size() == 3) {

System.out.println("Yay 2");

}

if(s1.pop().equals("b") && s2.pop().equals(3) && s1.size() == 1 && s2.size() == 2) {

System.out.println("Yay 3");

}

if(s1.toString().equals("a") && s1.peek().equals("a") && s2.peek().equals(2) && s1.pop().equals("a") && s2.pop().equals(2) && s1.size() == 0 && s2.size() == 1) {

System.out.println("Yay 4");

}

if(s1.toString().equals("") && s1.peek() == null && s2.peek().equals(1) && s1.pop() == null && s2.pop().equals(1) && s1.size() == 0 && s2.size() == 0) {

System.out.println("Yay 5");

}

s2.push(10);

s2.push(20);

s2.push(30);

if(s1.isEmpty() && s1.toArray().length == 0 && !s2.isEmpty()) {

s2.clear();

if(s2.isEmpty()) {

System.out.println("Yay 6");

}

}

CallStack s3 = new CallStack<>();

s3.push(3);

s3.push(2);

s3.push(1);

int i = 1;

for(Integer item : s3) {

if(i == item) System.out.println("Yay " + (6+i));

else

System.out.println(item);

i++;

}

}

}