Retrieve specific data from database and display

You're printing the entire list contents every time you add a record from the result set. In the first iteration, list.size() == 3, so it prints:

Item0:P0001 Item1:1 Item2:37.0

In the second iteration, list.size() == 6, so it prints:

Item0:P0001 Item1:1 Item2:37.0 Item3:P0002 Item4:2 Item5:666.0

Thereby making your eventual output:

Item0:P0001 Item1:1 Item2:37.0 Item0:P0001 Item1:1 Item2:37.0 Item3:P0002 Item4:2 Item5:666.0

Moving your for loop outside the while statement should fix it.

while(rs.next()){
  list.add(rs.getString("Product_ID"));
  list.add(rs.getString("Order_Quantity"));
  list.add(rs.getString("Sub_Total"));
}

for(int i = 0;i<list.size();i++){
  String item = list.get(i);
  System.out.println("Item" + i + ":" + item);
}

First of all this is not the correct way of using PreparedStatement . You should use it in following way:

String sql = "Select Product_ID,Order_Quantity,Sub_Total from Order_Menu Where Order_ID=?";
pst = conn.prepareStatement(sql);
pst.setString(order_id_1);
rs = pst.executeQuery();

Now, the reason for your output.

You are adding records in list assuming that at each index of list only one product is placing. But it is not the case. For example , if your program gets product P0001 then you are adding p001 at index 0 , Order_Quantity 1 at index 1 and subtotal 37 at index 2. So , you are getting such output. I would suggest you to create a separate class for Product as follows:

class Product
{
  String product_id;
  String order_quantity;
  String sub_total;
  public Product(String product_id, String order_quantity, String sub_total)
  {
    this.product_id = product_id;
    this.order_quantity = order_quantity;
    this.sub_total = sub_total;
  }
  public void setProduct_id(String product_id)
  {
    this.product_id = product_id;
  }
  public void setOrder_quantity(String order_quantity)
  {
    this.order_quantity = order_quantity;
  }
  public void setSub_total(String sub_total)
  {
    this.sub_total = sub_total;
  }
  public String getProduct_id()
  {
    return product_id;
  }
  public String getOrder_quantiy()
  {
    return order_quantity;
  }
  public String getSub_total() 
  {
    return sub_total;
  }
  @Override
  public boolean equals(Object obj)
  {
    if ( obj instanceof Product)
    {
        temp = (Product)obj;
        if (product_id.equals(temp.product_id))
        {
          return true;
        }
    }
    return false;
  }
  @Override
  public int hashCode()//override it in such a way so that you get different hashCode for each product ID.
  {
    return product_id.hashCode();
  }
  @Override
  public String toString()
  {
    return "product_id:"+product_id+", order_quantity:"+order_quantity+", sub_total:"+sub_total;
  }
}

And finally, You can use the object of Product class as follows:

List<Product> list = new ArrayList<Product>();
while(rs.next()){
 Product product = new Product(rs.getString("Product_ID"),rs.getString("Order_Quantity"),rs.getString("Sub_Total"));
  list.add(product);
 }
 for(int i = 0;i<list.size();i++){
 Product item = list.get(i);
 System.out.println("Item" + i + ":" + item);
}

You are not clearing your list in between iterations. This means that when you process the first row, your list will contain three items, when you display the second row, your list will contain the items from the first row and the second row...

Why don't you forget the list and just

for (int i = 0; rs.next(); ++i){

    System.out.println("Item " + i);
    System.out.println("Product ID    : " + rs.getString("Product_ID"));
    System.out.println("Order Quantity: " + rs.getString("Order_Quantity"));
    System.out.println("Sub_Total     : " + rs.getString("Sub_Total"));

}

and of course don't forget to wrap the whole thing in try finally and:

rs.close();
pst.close();

Make sure you are iterating list out side the aray list

while(){
    //implementation
   }
   for(){
    //Iterate your list
    }

Say your database table is as follows:

Product_ID  Order_Quantity  Sub_Total 
P0001       1               37.0
P0002       2               666.0

Your while loop adds elements to your list as:

First iteration:

list=["P0001","1","37.0"]

Second iteration:

list=["P0001","1","37.0","P0002","2","666.0"]

So when the while loop executes the first time, You are getting the output as:

Item0:P0001
Item1:1
Item2:37.0

In the second iteration, since list is ["P0001","1","37.0","P0002","2","666.0"],hence you get further output.

Item0:P0001
Item1:1
Item2:37.0
Item3:P0002
Item4:2
Item5:666.0

combining the above two outputs,you are getting:

Item0:P0001
Item1:1
Item2:37.0
Item0:P0001
Item1:1
Item2:37.0
Item3:P0002
Item4:2
Item5:666.0

So you need two dimensional data structure.I would suggest add a one dimensional array of strings to the list as:

String sql = "Select Product_ID,Order_Quantity,Sub_Total from Order_Menu Where    Order_ID='"+order_id_1+"'";
    pst = conn.prepareStatement(sql);
    rs = pst.executeQuery();
    String[] row=new String[3]
    while(rs.next()){

        row[0]=rs.getString("Product_ID");
        row[1]=rs.getString("Order_Quantity");
        row[2]=rs.getString("Sub_Total");

        list.add(row);
}