Is it possible to modify private member of a class directly from main function?

Question

This is the code which modifies the private String stk[] indirectly from main method when sorting option is called from main method:
import java.io.*;
class Stack
    private String stk[];
    private int tos;
    private int size;
    Stack()
        size=5;
        stk=new String[size];
        tos=-1;
    Stack(int sz)
        size=sz;
        stk=new String[size];
        tos=-1;
    boolean push(String s)
        if(tos==size-1) return false;
        stk[++tos]=s;
        return true;
    String pop()
        if(tos<0) return "Stack Underflow";
        return stk[tos--];
    String[] display()//array type function to return an array called "stk"
        return stk;
    int returnSize()
        return tos;
class myStack
    public static void main(String args[]) throws IOException
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        Stack S=new Stack();
        int opt=-1;
        while(opt!=6)
            System.out.println("\n\n\n");
            System.out.println("[1] - Create Stack");
            System.out.println("[2] - Push");
            System.out.println("[3] - Pop");
            System.out.println("[4] - Display");
            System.out.println("[5] - Display List In Ascending Order");
            System.out.println("[6] - Exit");
            System.out.print("Option: ");
            opt=Integer.parseInt(br.readLine());
            if(opt==1)
                System.out.print("\n\nEnter the size of stack: ");
                int size=Integer.parseInt(br.readLine());
                S=new Stack(size);
                System.out.print("\nStack Created...");
            else if(opt==2)
                System.out.print("\n\nEnter String: ");
                String s=br.readLine();
                if(S.push(s))
                    System.out.print("\nSuccessfull...");
                else
                    System.out.print("\nStack Overflow...");
            else if(opt==3)
                System.out.print("\nItem Deleted: "+S.pop());
            else if(opt==4)
                int sz=S.returnSize();
                System.out.print("\n\n\nStack Contains: "+(sz+1)+" Item(s)\n");
                String st[]=S.display();
                while(sz>=0)
                    System.out.println(st[sz]);
                    sz--;
            else if(opt==5)
                int s=S.returnSize();
                String stc[]=S.display();
                for(int i=0;i<=s;i++)
                    for(int j=i+1;j<=s;j++)
                        if(stc[j].compareTo(stc[i])<0)
                            String t=stc[i];
                            stc[i]=stc[j];
                            stc[j]=t;
                System.out.println(stc[i]);
            else if(opt>6)
                System.out.print("\nPress 6 To Exit....");

Answer 1

Short answer is: no.
Long answer is: you should not try to. Information hiding is the fundamental principle of OOP. This means that the code dealing with an object has no knowledge about the objects inner structure. It only knows the methods provided by its interface.
You should declare all object properties private and prevent any other code outside the owning class to manipulate it. This includes leaking properties via "getter" methods. In your example the display() method is bad in two ways:
its name does not convey its purpose.
it a "getter" that returns the private property stk to the caller who may change it. Eg. the caller could add or delete an entry without changing tos accordingly.
bye
TPD