Please Solve Whole program with completed code

Abstract Concept of a stack of distinct strings:

A stack of distinct strings, $p,$ is a collection of strings with a top where a string cannot exist more than

once in the collection. For example, the collection of strings $\{"$ab$","$cd$","$ae$","$bd$"\}$ is a stack with top

$="$bd$".$ The only operations supported are the addition of a string at the top, known as push, and the

deletion of a string from the top, known as pop.

Implementation of a stack of distinct strings:

The following class, StackOfDistinctstrings, represents a stack of distinct strings.

Click on Project $>$ New Project in the Netbeans program and save it as $"$Ex$1"$ on your lab$4$ directory.

Create a new class called "StackOfDistinctStrings" and copy the code in it$.$ For this class:

a$)$ Write the abstraction function in the Overview clause.

b$)$ Write the rep invariant in the Overview clause.

b$)$ Fill in the body of the method repOK$().$

b$)$ Fill in the body of the method tostring $()$

import java.util.ArrayList;

public class StackOfDistinctStrings $\{$

$//$ Overview: StacksOfDistinctStrings are mutable, bounded

$//$ collection of distinct strings that operate in

$//$ LIFO $($Last$-$In$-$First$-$Out$)$ order.

$//$

$//$ The abstraction function is:

$//$ a$)$ Write the abstraction function here

$//$

$//$

$//$

$//$

$//$ The rep invariant is:

$//$ b$)$ Write the rep invariant here

$//$

$//$

$//$

$//$

$//$the rep

private ArrayList items;

$//$ constructor

public StackOfDistinctStrings$()\{$

$//$ EFFECTS: Creates a new StackOfDistinctStrings object

items $=$ new ArrayList$()$;

$\}$

public void push$($String element$)$ throws Exception $\{$

$//$ MODIFIES: this

$//$ EFFECTS: Appends the element at the top of the stack

$//$ if the element is not in the stack, otherwise

$//$ does nothing.

if$($element $==$ null$)$ throw new Exception$()$;

if$($false $==$ items.contains$($element$))$

items.add$($element$)$;

$\}$

public String pop$()$ throws Exception $\{$

$//$ MODIFIES: this

$//$ EFFECTS: Removes an element from the top of the stack

if $($items$.$size$()==0)$ throw new Exception$()$;

return items.remove$($items$.$size$()-1)$;

$\}$

public boolean repOK$()\{$

$//$ EFFECTS: Returns true if the rep invariant holds for this

$//$ object; otherwise returns false

$//$ c$)$ Write the code for the repOK$()$ here

$\}$

public String toString$()\{$

$//$ EFFECTS: Returns a string that contains the strings in the

$//$ stack and the top element. Implements the

$//$ abstraction function.

$//$ d$)$ Write the code for the toString$()$ here

$\}$

$\}$