在数组列表中查找项目索引的更好方法？

对于 Android 应用程序，我有以下功能

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;
}

这是写一个函数来获取元素位置的“最佳”方式吗？或者我应该利用 Java 中的一个高级本机函数？

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最佳答案

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.

小开

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.

小开

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.

小开

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).

小开 
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

小开

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

小开

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

小开

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)

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