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?

ArrayList has a 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.

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

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).

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.

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.

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)

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

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