Two code files below : the actual logic & the test logic. Only the StringBuilder class is for review :)

Core Logic

package stringBuilder;

public class NoksStringBuilder {

    //initial capacity
    private static final int INITIAL_SIZE = 3;

    String[] stringList = new String[INITIAL_SIZE];

    //number of strings in the builder currently
    int size = 0;

    //total number of chars in the builder , sum total of length of each string
    int characterCount = 0;


    public void add(String s){
        if(size < stringList.length){
            stringList[size++] = s;
            characterCount += s.length();
        }
        else{
            String[] temp = new String[stringList.length*2];
        
            for(int i =0 ; i< stringList.length; i++){
                temp[i] = stringList[i];
            }
        
            stringList = temp;
            add(s);
        }
    }

    public String toString(){
        char[] output = new char[characterCount];
        int outputIndex = 0;
    
        for(int i = 0; i < size; i++){
            for(int j = 0; j < stringList[i].length(); j++){
                output[outputIndex++] = stringList[i].charAt(j);
            }
        }
    
        return new String(output);
    
    }

}

Test Class

package stringBuilder;

import java.util.Scanner;

public class Test {

    public static void main(String[] args){
        NoksStringBuilder stringBuilder = new NoksStringBuilder();

        Scanner inputScanner = new Scanner(System.in);
        int i = 0;
    
        while(i != 3){

            printMenu();
            i = Integer.parseInt(inputScanner.nextLine());
        
            if(i == 1){
                System.out.println("Enter string");
                stringBuilder.add(inputScanner.nextLine());
            }
            else if( i == 2){
                System.out.println(stringBuilder.toString());
            }
        
        }
    
    
    }

    private static void printMenu() {
        System.out.println("StringBuilder Options :");
        System.out.println("1.Add");
        System.out.println("2.ToString");
        System.out.println("3.Quit");
    }
    
    

}