class CategoryItem
{
    int id;
    String name;

    // add other fields if you want...

    CategoryItem(int id, String name)
    {
        this.id = id;
        this.name = name;
    }
 public String toString()
    {
        return name;
    }
}

public void LoadCategory()
{
        try {
            Class.forName("com.mysql.jdbc.Driver");
            con1 = DriverManager.getConnection("jdbc:mysql://localhost/javapos","root","");
            insert = con1.prepareStatement("SELECT * FROM category");
             ResultSet rs = insert.executeQuery();
             jComboBox1.removeAllItems();

             while(rs.next())
             {
                 jComboBox1.addItem(CategoryItem(rs.getInt(1),rs.getString(2)));
             }
             }
        catch (Exception e) {
            }
}

String name =txtproduct.getText();
            String dec =txtdec.getText();

           CategoryItem item = (CategoryItem)jComboBox1.getSelectedItem();

            Class.forName("com.mysql.jdbc.Driver");
            con1 = DriverManager.getConnection("jdbc:mysql://localhost/javapos","root","");
            insert = con1.prepareStatement("insert into product (productname,description,category)values(?,?,?)");
            insert.setString(1,name);
            insert.setString(2,dec);
             insert.setInt(3,item.id);
            insert.executeUpdate();
            JOptionPane.showMessageDialog(this, "Sucsessfully Saved");