CODEWITHSUNDEEP
javaarraylistcontains
Rakim
Rakim

Reputation: 1107

Quick way to search through arrayList

I have an arraylist Arraylist<String[]> and I am trying to find a quick way of looking for a specific value on a specific index of the String[]. My ArrayList is always going to contain String[] of length 2. And what I want to do is look through the ArrayList for any String[] that has a specific value i.e. str[1]="value". I know that I can iterate through the ArrayList taking every single element (String[]) and then looking for the value using str[1].equals("value") but I was wondering if there is a quicker way of doing it by maybe using contains() of the ArrayList or something. thanks

PS: I don't know the value of the first element of my array (str[0]) so I cannot construct a new String[] and check if the ArrayList contains that

Upvotes: 1

Views: 2444

Answers (3)

Erick G. Hagstrom
Erick G. Hagstrom

Reputation: 4945

LinkedHashMap

How about using a LinkedHashMap<String, String[]> instead of the ArrayList? The key would be the String[1] value. Linked because that gives you predictable iteration order.

An auxiliary Map

Or you could create a Map<String, Integer> where the key is the String[1] value and the value is the index of your String[] in the ArrayList.

ArrayList<String[]> al = new ArrayList<>();
Map<String, Integer> alIndexMap = new HashMap<>();
// ...
Integer nextIndex = al.size();
al.add(someStringArray);
alIndexMap.put(someStringArray[1], nextIndex);

If you keep al and alIndexMap in sync all the time, you'll always know where in al is the array that you're looking for.

Guava BiMap

Google's Guava has some classes that implement their BiMap interface. It's like a Map, but it works both ways, i.e. you can use the value as a key if you want.

Upvotes: 1

burna
burna

Reputation: 2962

If you look into ArrrayList#contains - you will see, that this method also iterates through all elements.

You could use a parallelStream - so that the ArrayList can be searched by more threads.

    List<String[]> listToSearchIn = new ArrayList<String[]>();

    List<String[]> matches = listToSearchIn.parallelStream()
            .filter((element) -> element[1].equals("value"))
            .collect(Collectors.toList());

Upvotes: 1

Eugen Hotaj
Eugen Hotaj

Reputation: 383

Unless your ArrayList is sorted then O(n) efficiency is the best you can do. Unless by "quick way" you mean a method which does the iteration for you. Even in this scenario the answer is no since you're wanting to check data inside the String[] objects themselves.

Upvotes: 0

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