Thread: How to find indexes of ALL occurrences of object in ArrayList

Hi,

I am interested to find all the indexes where an element that I specify occurs in an ArrayList. Is there already a method for this? I see in the documentation that there are two similar methods that return the index of the first appearance of the element (indexOf) and the last index of (lastIndexOf). Is there a method to return all indexes? Am I missing something? Or do I need to write my own code to achieve this?

On a probably-separate note: what is an Iterator? Does that have anything to do with this?

Basically what I want is to have returned to me a list of all the indexes where an object appears in a list, e.g. <pre> List<Elements> foundElements = variableArrayList.getAllIndexesOf(varElement);</pre>

Thanks much,
-Daniel

Okay so the collections class doesnt have a method for this, you can however iterate through the whole list and check each element and store each index of the occurrence in a separate arraylist. Even if the collections class had this method it would also have to iterate through it, so a pseudocode for this could be:

PHP Code:

for(int i = 0; i < (length of the list to be checked); i++)
{
      if(object at index i is = to the object to be checked)
     {
         then add its index(or i in this case) to a temporary arraylist.
     }
}

    After everything in the loop is done, then return the arraylist containing the indexes. 


I hope that made sense.

Sorry I am new to posting code on a forum, I will fix it.

Originally Posted by nafty

Hi,

I am interested to find all the indexes where an element that I specify occurs in an ArrayList. Is there already a method for this? I see in the documentation that there are two similar methods that return the index of the first appearance of the element (indexOf) and the last index of (lastIndexOf). Is there a method to return all indexes? Am I missing something? Or do I need to write my own code to achieve this?

On a probably-separate note: what is an Iterator? Does that have anything to do with this?

Basically what I want is to have returned to me a list of all the indexes where an object appears in a list, e.g. <pre> List<Elements> foundElements = variableArrayList.getAllIndexesOf(varElement);</pre>

Thanks much,
-Daniel

A Iterator is just an manner to iterate over a collection. When you look to the API it has no methods to say when a element is in the collection.