Unexpected values in HashSet before method invocation in Java

Question

package main;

import java.io.*;
import java.util.*;

public class IsBSTTree {

     Set<Integer> set = new HashSet<>();

        public static void main(String[] args) throws IOException {
            BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
            PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
            String[] line = in.readLine().split(" ");
            IsBSTTree solution = new IsBSTTree();
            Node root = solution.buildTree(line, 0, line.length - 1);
            if (solution.isBstTree(root, Integer.MIN_VALUE, Integer.MAX_VALUE)) {
                out.println("Yes");
            } else {
                out.println("No");
            }
            out.flush();
        }

        Boolean isBstTree(Node node, int min, int max) {
            
            if (node == null) 
                return true;

            if (node.data <= min || node.data >= max) 
                return false;
            if (set.contains(node.data)) 
                return false;
            
            set.add(node.data);

            if (isBstTree(node.left, min, node.data) || 
                    isBstTree(node.right, node.data, max)) {
                return false;
            }

            return false;
        }

        public Node buildTree(String[] arr, int from, int to) {
            if (from > to) return null;

            int middle = from + (to - from) / 2;
            Node node = new Node();
            node.data = Integer.valueOf(arr[middle]);
            node.left = buildTree(arr, from, middle - 1);
            node.right = buildTree(arr, middle + 1, to);
            return node;
        }

        class Node {
            int data;
            Node left;
            Node right;
        }
    }

---

Problem Description

• The set variable is supposed to track the node values to detect duplicates and validate the BST.
• However, while debugging, I noticed that set already contains values before the isBstTree method is invoked.
• I can't understand how values are being added to the set before the method runs. Expected Behavior The set should be empty when the isBstTree method starts, and only get populated during the method's execution.

Actual Behavior

The set contains values before the isBstTree method is called.

Questions

1. How is it possible for set to contain values before the isBstTree method is invoked?
2. What changes are necessary to ensure that the set behaves as expected?

Any help or insights would be greatly appreciated!