while loop - where to initiate object in java

Question

This question relates to my answer of another of my question. The original question is here

Can anyone explain why the bad code fails in the way explained in the comments (by the wy it is in pseudo code)

 // bad code
        ResultSet rs = getDataFromDBMS("Select * from [tableName];");

            //temp uses a collection member within it to hold a list of column names to data value pairs (hashmap<String,String>)
        Object temp = new objectToHoldInfoFromResultSet();

        // loop over the result set
        while (rs.next)// for each row in the result set
        {
            for (int i = 1; i <= rs.getNumberColums; i++) {
                temp.add(infoAboutColumn);
            }
            temp.printInfo();// always prints correct (ie current) info
                    //the print just takes the hashmap member and asks for a       
                    //toString() on it
            anotherObject(makeUseOf(temp));// always uses info from first
                   //iteration. Essentially grabs the hashMap member of temp, and puts
                   //this into another collection of type 
                   //HashMap< HashMap<String,String>, temp> see the linked question
                   //for more detail.

        }

        // Seemingly each loop into the while the temp.doSomethingToData(); uses
        // the temp object created in the first iteration

        // good code
        ResultSet rs = getDataFromDBMS("Select * from [tableName];");

        // loop over the result set
        while (rs.next)// for each row in the result set
        {
            Object temp = new objectToHoldInfoFromResultSet();// moving
                                                                // declaration
                                                                // of temp into
                                                                // the while
                                                                // loop solves
                                                                // the problem.
            for (int i = 1; i <= rs.getNumberColums; i++) {
                temp.add(infoAboutColumn);
            }
            temp.printInfo();// always prints correct (ie current) info

            anotherObject(makeUseOf(temp));// also uses the correct (ie current)
                                            // info.

        }

Answer 1

We can't reliably answer this without knowing what temp.printInfo() and makeUseOf() are doing. It is easy to implement them to behave the way you describe though.

When you instantiate temp outside the loop, you will be using the same object throughout all iterations of the loop. Thus it is possible for it to gather data from each iteration (e.g. into a collection). And then it is possible for methods to get data accumulated in the current iteration, as well as from any previous iteration, which may cause problems if it was not intended.

So let's assume temp contains a collection and in each iteration a column from the resultset is added to it. Now if temp.printInfo() is implemented to print info about the last element in this collection, while makeUseOf() is implemented to do something with the first element in the collection, you get the behaviour you observed.

OTOH when you instantiate temp inside the loop, you will get a new, distinct object in each iteration, which won't "remember" any data from earlier iterations. Thus even with the implementations of temp.printInfo() and makeUseOf() outlined above, you will get correct results.