Remove elements from arraylist based on a condition

Question

I have an ArrayList whose each element is of type DataType, where DataType is a class:

class DataType{
 String dId;
 String dType;
 String rId;
}

I need to remove all such elements from the list whose rId is equal to any other element's dID. i.e. if DataType D1 has value of dID as "abc" and DataType D2 has value of rID as "abc", than remove both D1 and D2 from the list.

Could someone please suggest the most appropriate approach for doing this.

No, you try something and tell us if and why your attempt doesn't work. — Sotirios Delimanolis, Dec 16 '13 at 02:10
Questions asking for code must demonstrate a minimal understanding of the problem being solved. Include attempted solutions, why they didn't work, and the expected results. See also: Stack Overflow question checklist — Brian Roach, Dec 16 '13 at 02:11
@Makoto This is probably just personal issues (namely mine), but I've had issues deleting from `List`s while iterating them... — MadProgrammer, Dec 16 '13 at 02:14
Just use a [Set](http://docs.oracle.com/javase/6/docs/api/java/util/Set.html)? — Elliott Frisch, Dec 16 '13 at 02:14
@MadProgrammer which issue are you referring to? ConcurrentModificationException? If so, you'll need a CopyOnWriteArrayList or something that handles concurrency. — user2910265, Dec 16 '13 at 02:23
@user2910265 That does assume you have something other than `List` ;) - And as I say, it's probably just me ;) — MadProgrammer, Dec 16 '13 at 02:26
@ElliottFrisch: a Set would only make sense if equality is based solely on the dId property and on nothing else. — Hovercraft Full Of Eels, Dec 16 '13 at 02:54

score 4 · Answer 1 · answered Dec 16 '13 at 02:14

The easiest would be to traverse the list once and create a HashMap<String, List<DataType>>. You will map every object to their dID which forms the primary key.

After that you can iterate over your ArrayList, check the rId of the current object and see if it's in the HashMap. HashMap has O(1) lookup time so this should be a non issue. If the value is present, remove the current value (you're using an Iterator to prevent a ConcurrentModificationException) and remove the objects inside the value-part of the key-value pair as well.

Make sure you have correctly implemented .equals(Object o) and .hashcode().

Remove elements from arraylist based on a condition

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