CODEWITHSUNDEEP
javavariablesloops
Lorenzo Vanmatterbone
Lorenzo Vanmatterbone

Reputation: 27

Get information outside a loop

I've a problem. I want to fill an array with objects containing different Informations. here is my loop

public FileRecord [] calcPos() throws IOException{
        for (int i = 0; i < getEFSFATmaxRecords(); i++){
        int blockNumber = i/5;
        int recordOffset = i%5;
        pos = (recordOffset*100+(getFsatPos() + 512 + 512*blockNumber)); 
        FileRecord rec = new FileRecord(pos,getHeader());
        array = new FileRecord[header.getMaxFileRecords()];

        array[i] = rec;
        System.out.println("FileName: " + array[i].getFileName());


            }
        return array;
    }

It should make different objects of FileRecord. The position depends on the running variable i. t Then the loop stores everything in the array and returns the array. Ive declared array as a global variable in this calss so I thought the changes inside the loop would directly affect the global array. But it doesnt work. what I'm doing wrong?

Upvotes: 0

Views: 84

Answers (3)

Nick Louloudakis
Nick Louloudakis

Reputation: 6005

The problem is that you do:

array = new FileRecord[header.getMaxFileRecords()];

INSIDE the method every time it is invoked (in fact, inside the loop!). This way, you are "setting" a new FileRecord[] object to the variable (and even worse, this happens many times in your method as the initialization is done in the loop). Each time this initialization happens, the variable "points to the new FileRecord[] object allocated in memory. The Object that was "pointed to" by array before is not used anymore, and will be destroyed, the when is responsibility of the garbage collector.

(http://javabook.compuware.com/content/memory/how-garbage-collection-works.aspx).

In simple words, you are "recreating" the array again and again inside your loop. Initialize the object only ONCE before using it in your method (for example in class constructor or in main, before using it in a sense).

Generally, I suggest that you don't use global variables. Search more on class encapsulation, a very important Object-Oriented Programming principle:

(http://www.tutorialspoint.com/java/java_encapsulation.htm).

Upvotes: 0

Sohaib
Sohaib

Reputation: 4694

You are re initializing your array in every iteration. Below is a correct version of the code you want:

public FileRecord [] calcPos() throws IOException{
     FileRecord[] array = new FileRecord[header.getMaxFileRecords()]; 
     for (int i = 0; i < getEFSFATmaxRecords(); i++){
       int blockNumber = i/5;
       int recordOffset = i%5;
       pos = (recordOffset*100+(getFsatPos() + 512 + 512*blockNumber)); 
       FileRecord rec = new FileRecord(pos,getHeader());
       array[i] = rec;
       System.out.println("FileName: " + array[i].getFileName());
    }
    return array;
}

As vogel says if the header.getMaxFileRecords() changes within the loop then your array may run out of bound.

Solution: An ArrayList should work.

Upvotes: 1

Andrew Wilkinson
Andrew Wilkinson

Reputation: 248

Within the array you are doing:

array = new FileRecord[header.getMaxFileRecords()];

This will re-create the array every interation and you'll lose the records stored in it.

You'll need to do this before the loop

Upvotes: 2

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