Question

I’ve a program where the loop in question looks something like this

int numOfWords = 1000;
int avgSizeOfWord = 20;
while(all documents are not read) {
    char[][] wordsInDoc = new char[numOfWords][avgSizeOfWord];
    for(int i=0; i<numWordsInDoc; i++) {
        wordsInDoc[i] = getNextWord();
    }
    processWords(wordsInDoc);
}

I was wondering what happens behind the scene when this loop gets executed. When does the garbage collector collect the memory that has been assigned for each document? Is their a better way (wrt memory usage) to do the same?

Any insight is appreciated.

Answer 1

Well you’re definitely wasting memory – you’re allocating all of the “sub-arrays” and then overwriting them. You’d be better off with:

while(all documents are not read) {
    char[][] wordsInDoc = new char[numOfWords][];
    for(int i=0; i < numWordsInDoc; i++) {
        wordsInDoc[i] = getNextWord();
    }
    processWords(wordsInDoc);
}

Now what does processWords actually do? If it doesn’t stash the array anywhere, you could reuse it:

char[][] wordsInDoc = new char[numOfWords][];
while(all documents are not read) {
    for(int i=0; i < numWordsInDoc; i++) {
        wordsInDoc[i] = getNextWord();
    }
    processWords(wordsInDoc);
}

I would definitely perform the first change, but probably not the second.

As for when exactly garbage collection occurs – that’s implementation-specific.