Update MoleculeDatabase.java

import java.io.Serializable;

import java.util.ArrayList;

package edu.bu.ec504.project;

/**

 * A class represents the molecule database

 */

public class MoleculeDatabase implements Serializable {

  public static void MoleculeDatabase() {

    private ArrayList<MoleculeDatabase> db;   // Molecule database

    /**

     * Private constructor to prevent instantiation from outside

     */

    private MoleculeDatabase() {

        this.db = new ArrayList<>();

    }

    /**

     * Add a new molecule into the database

     */

    public void addMolecule(Molecule molecule) {

      return null; // for now

    }

    /**

     * Find isomorphic molecule from the database

     */

    public Molecule find(Molecule molecule) {

      return null; // for now

    }

    /**

     * Save database to file system

     */

    public void save(String filename) {

      try {

        FileOutputStream fileOutStream = new FileOutputStream(filename);

        ObjectOutputStream objOutStream = new ObjectOutputStream(fileOutStream);

        objOutStream.writeObject(this.molecules);

        objOutStream.close();

        fileOutStream.close();

        System.out.println("Database saved successfully.");

      }

      catch (IOException e) {

        System.err.println("Error saving database: " + e.getMessage());

      }

    }

    /**

     * Load database from file system

     */

    public void load(String filename) {

      try {

        FileInputStream fileInStream = new FileInputStream(filename);

        ObjectInputStream objInStream = new ObjectInputStream(fileInStream);

        this.molecules = (ArrayList<Molecule>) objInStream.readObject();

        objInStream.close();

        fileInStream.close();

        System.out.println("Database loaded successfully.");

      }

      catch (IOException | ClassNotFoundException e) {

        System.err.println("Error loading database: " + e.getMessage());

      }

    }

}