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I have an ArrayList of objects where each object contains a string 'word' and a date. I need to check to see if the date has passed for a list of 500 words. The ArrayList could contain up to a million words and dates. The dates I store as integers, so the problem I have is attempting to find the word I am looking for in the ArrayList.

Is there a way to make this faster? In python I have a dict and mWords['foo'] is a simple lookup without looping through the whole 1 million items in the mWords array. Is there something like this in java?

    for (int i = 0;  i < mWords.size();  i++) {
        if ( word == mWords.get(i).word ) {
           return mWords.get(i);
        }
    }
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4 Answers 4

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If the words are unique then use HashMap. I mean, {"a", 1}, {"b", 2}

Map<String, Integer> wordsAndDates = new HashMap<String, Integer>();
wordsAndDates.put("a", 1);
wordsAndDates.put("b", 2);

and wordsAndDates.get("a") return 1

If not you shouldn't use HashMap because it overrides previous value. I mean 

wordsAndDates.put("a", 1);
wordsAndDates.put("b", 2);
wordsAndDates.put("a", 3);

and wordsAndDates.get("a") return 3

In such case you can use ArrayList and search in it

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2 Comments

Thanks guys! And thanks for the warning as it explains to me how the HashMap works.
you are welcome. it rejoices me that you learn something new today
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If you're not stuck with an ArrayList you should use some kind of hash based data structure. In this case it seems like a HashMap should fit nicely (it's pretty close to python's dict). This will give you an O(1) lookup time (compared to your current method of linear search).

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1

You want to use a Map in Java

Map<String,Integer> mWords = new HashMap<String, Integer>();
mWords.put ("foo", 112345);
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0

What about Collections.binarySearch() (NB: the list must be sorted) if ou are stuck with the ArrayList

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