CODEWITHSUNDEEP
javafor-looparraylist
Jacksonkr
Jacksonkr

Reputation: 32207

Better way to find index of item in ArrayList?

For an Android app, I have the following functionality

private ArrayList<String> _categories; // eg ["horses","camels"[,etc]]

private int getCategoryPos(String category) {
    for(int i = 0; i < this._categories.size(); ++i) {
        if(this._categories.get(i) == category) return i;
    }

    return -1;
}

Is that the "best" way to write a function for getting an element's position? Or is there a fancy shmancy native function in java the I should leverage?

Upvotes: 96

Views: 299066

Answers (8)

Jon Egeland
Jon Egeland

Reputation: 12613

ArrayList has an indexOf() method. Check the API for more, but here's how it works:

private ArrayList<String> _categories; // Initialize all this stuff

private int getCategoryPos(String category) {
  return _categories.indexOf(category);
}

indexOf() will return exactly what your method returns, fast.

Upvotes: 212

Fatih &#199;akıroğlu
Fatih &#199;akıroğlu

Reputation: 503

the best solution here

class Category(var Id: Int,var Name: String)
arrayList is Category list
val selectedPositon=arrayList.map { x->x.Id }.indexOf(Category_Id)
spinner_update_categories.setSelection(selectedPositon)

Upvotes: 2

Vasu Dev Garg
Vasu Dev Garg

Reputation: 121

Use indexOf() method to find first occurrence of the element in the collection.

Upvotes: 1

Pradeesh MP
Pradeesh MP

Reputation: 45

The best way to find the position of item in the list is by using Collections interface,

Eg, 

List<Integer> sampleList = Arrays.asList(10,45,56,35,6,7);
Collections.binarySearch(sampleList, 56);

Output : 2

Upvotes: -1

Hiren Patel
Hiren Patel

Reputation: 52800

ArrayList<String> alphabetList = new ArrayList<String>();
alphabetList.add("A"); // 0 index
alphabetList.add("B"); // 1 index
alphabetList.add("C"); // 2 index
alphabetList.add("D"); // 3 index
alphabetList.add("E"); // 4 index
alphabetList.add("F"); // 5 index
alphabetList.add("G"); // 6 index
alphabetList.add("H"); // 7 index
alphabetList.add("I"); // 8 index

int position = -1;
position = alphabetList.indexOf("H");
if (position == -1) {
    Log.e(TAG, "Object not found in List");
} else {
    Log.i(TAG, "" + position);
}

Output: List Index : 7

If you pass H it will return 7, if you pass J it will return -1 as we defined default value to -1.

Done

Upvotes: 17

yshavit
yshavit

Reputation: 43391

If your List is sorted and has good random access (as ArrayList does), you should look into Collections.binarySearch. Otherwise, you should use List.indexOf, as others have pointed out.

But your algorithm is sound, fwiw (other than the == others have pointed out).

Upvotes: 7

danca
danca

Reputation: 509

Java API specifies two methods you could use: indexOf(Object obj) and lastIndexOf(Object obj). The first one returns the index of the element if found, -1 otherwise. The second one returns the last index, that would be like searching the list backwards.

Upvotes: 4

rustybeanstalk
rustybeanstalk

Reputation: 2762

There is indeed a fancy shmancy native function in java you should leverage.

ArrayList has an instance method called

indexOf(Object o)

(http://docs.oracle.com/javase/6/docs/api/java/util/ArrayList.html)

You would be able to call it on _categories as follows:

_categories.indexOf("camels")

I have no experience with programming for Android - but this would work for a standard Java application.

Good luck.

Upvotes: 3

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