21

I have an ArrayList in Java, and I need to find all occurrences of a specific object in it. The method ArrayList.indexOf(Object) just finds one occurrence, so it seems that I need something else.

Cœur
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missrg
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  • By "specific object" do you mean the references should be `equals` or `==` – Peter Lawrey Dec 16 '12 at 11:05
  • In my case it's an ArrayList of integers, so both of them can be used. But actually are there cases that only one of them can be used? – missrg Dec 16 '12 at 11:10
  • @missrg.. Always use `equals` to compare object contents. Whether `Integer`, or any other objects. – Rohit Jain Dec 16 '12 at 11:11
  • @Rohit Thanks for the advice :) Can you please explain why it is better? – missrg Dec 16 '12 at 11:14
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    @missrg.. `==` operator only compares the value of `reference`, and not the actual content of the objects. Thus you compare references to two different object with same value, you would get false result with `==` and true result with `equals` method. You can get immense resource on this topic on internet. just google - "equals v/s ==" – Rohit Jain Dec 16 '12 at 11:18
  • Not really an answer since I don't know if you can use [Guava](http://code.google.com/p/guava-libraries/wiki/GuavaExplained), but it's quite straightforward with it: Iterables.filter(list, Predicates.equalTo(object)) – Francisco Paulo Dec 16 '12 at 11:22
  • @Rohit Thanks soo much, I hadn't figure out this difference :) – missrg Dec 16 '12 at 11:24
  • " I need to find all occurances of a specific object in it. " it clearly says what he want! –  Dec 16 '12 at 11:32
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    'equals' may look at the contents of two objects and compare the values in those objects. If you `==` then they are the same specific object, not just two objects which happen to contain the same values. This means that while `new Integer(1234).equals(new Integer(1234))` it is the case that `new Integer(1234) != new Integer(1234)` as they are not the same object. – Peter Lawrey Dec 16 '12 at 11:40

6 Answers6

23

I don't think you need to be too fancy about it. The following should work fine:

static <T> List<Integer> indexOfAll(T obj, List<T> list) {
    final List<Integer> indexList = new ArrayList<>();
    for (int i = 0; i < list.size(); i++) {
        if (obj.equals(list.get(i))) {
            indexList.add(i);
        }
    }
    return indexList;
}
Michael
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André C. Andersen
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6

I suppose you need to get all indices of the ArrayList where the object on that slot is the same as the given object.

The following method might do what you want it to do:

public static <T> int[] indexOfMultiple(List<T> list, T object) {
    List<Integer> indices = new ArrayList<>();
    for (int i = 0; i < list.size(); i++) {
        if (list.get(i).equals(object)) {
            indices.add(i);
        }
    }
    // ArrayList<Integer> to int[] conversion
    int[] result = new int[indices.size()];
    for (int i = 0; i < indices.size(); i++) {
        result[i] = indices.get(i);
    }
    return result;
}

It searches for the object using the equals method, and saves the current array index to the list with indices. You're referring to indexOf in your question, which uses the equals method to test for equality, as said in the Java documentation:

Searches for the first occurence of the given argument, testing for equality using the equals method.

Update

Using Java 8 streams it'll become much easier:

public static <T> int[] indexOfMultiple(List<T> list, T object) {
    return IntStream.range(0, list.size())
        .filter(i -> Objects.equals(object, list.get(i)))
        .toArray();
}
MC Emperor
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  • Well, I mentioned indexOf method because I was hoping there may be a similar method appropriate for my case, like it is indexOf(char,int) for Strings. Thanks for the answer and the explanation :) – missrg Dec 16 '12 at 11:30
4

This is similar to this answer, just uses stream API instead.

List<String> words = Arrays.asList("lorem","ipsum","lorem","amet","lorem");
String str = "lorem";
List<Integer> allIndexes =
        IntStream.range(0, words.size()).boxed()
                .filter(i -> words.get(i).equals(str))
                .collect(Collectors.toList());
System.out.println(allIndexes); // [0,2,4]
Community
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Anton Balaniuc
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2

iterate over all elements, don't break the loop

each element of the ArrayList compare with your object ( arrayList.get(i).equals(yourObject) )

if match than the index ( i ) should be stored into a separate ArrayList ( arraListMatchingIndexes).

Sometimes in this way I do a "remove all", when I need the positions too.

I hope it helps!

2

Do

for (int i=0; i<arrList.size(); i++){
    if (arrList.get(i).equals(obj)){
        // It's an occurance, add to another list
    }
}

Hope this helps.

Sri Harsha Chilakapati
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    @user529543 depends. Sometimes, you need to check if the certain instance of an object does not occur multiple times in an array, then you will use `==`, if equal object means "contain the same data", then it"s the `.equals()` version – Jiří Nov 10 '18 at 19:14
0

Java 8+

If you want to precompute the indexes of every value in the List, Collectors.groupingBy can be used on an IntStream of indexes.

import java.util.stream.Collectors;
import java.util.stream.IntStream;
//...
List<Integer> list = Arrays.asList(1, 2, 2, 1, 4, 5, 4, 3, 4, 5, 0);
final Map<Integer, List<Integer>> indexMap = IntStream.range(0, list.size()).boxed()
        .collect(Collectors.groupingBy(list::get));
//Map of item value to List of indexes at which it occurs in the original List

Then, to find all the indexes of a specific value, use get on the Map in constant time.

List<Integer> indexes = indexMap.get(value);

Demo

Unmitigated
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