Checking if an ArrayList contains another ArrayList as element

Question

I have a list of lists and I would like to add a list to it, without duplicate. In other to do that, I would like to check if that list is already contained in the main list. I have written something like this

import java.util.ArrayList; public class Test{ public static void main(String [] args) { ArrayList<ArrayList<String>> Main = new ArrayList<>(); ArrayList<String> temp = new ArrayList<>(); temp.add("One"); temp.add("Two"); temp.add("Three"); Main.add(temp);// add this arraylist to the main array list ArrayList<String> temp1 = new ArrayList<>(); temp1.add("One"); temp1.add("Two"); temp1.add("Three"); if(!Main.containsAll(temp1)) // check if temp1 is already in Main { Main.add(temp1); } } }

When I print the contents of Main, I obtain both temp and temp1. How can I fix this?

You should be doing Main.add(new ArrayList<String>(temp));. Directly adding temp would set a reference to the temp variable (object) and that's not something you'd want. — progyammer
– progyammer, Commented Sep 28, 2016 at 18:13

Nicolas Filotto · Accepted Answer · 2016-09-28 18:21:21Z

You can use the method List#contains() since contains will check for instances of ArrayList that are equal to the provided ArrayList which will be the case here as temp.equals(temp1) returns true since the method equals of an AbstractList compares their content and here the content of those ArrayList is equal.

if(!Main.contains(temp1)) // check if temp1 is already in Main { Main.add(temp1); }

Tunaki · Accepted Answer · 2016-09-28 18:21:34Z

The issue here is that you're getting confused with the use of containsAll and the list of lists.

containsAll is a method that checks whether this collection contains all of the elements of the given collection. In this case:

This collection has 1 element, which is a List<String>;
The given collection has 3 elements, which are "One, "Two" and "Three".

And clearly, this collection, which only contains a List<String> (which is ["First, "Two", "Three"]), does not contain the 3 elements; it only contains a list of those three elements.

So what you really want here isn't containsAll, but contains, i.e. you want to check whether your list contains another list (and not its elements).

The following works:

if (!Main.contains(temp1)) { Main.add(temp1); }

and will result in Main being [[One, Two, Three]], added just once.

The side question is: why does it work? Well now, the question is: does my List<List<String>>, which is [[One, Two, Three]], contains this List<String>, which is [One, Two, Three]? Since two lists are equal when they have the same size and all of their elements are equal, it does contain it.

Roland Illig · Accepted Answer · 2016-09-28 18:17:06Z

Since you want to avoid duplicate lists (and not check for the elements of the inner lists), just use Main.contains instead of Main.containsAll.

This will check whether the Main list already contains a list with the elements you are about to add.

Wojtek · Accepted Answer · 2016-09-28 18:25:37Z

What would you do if it was about ArrayList<Integer>? There is such a method named contains(). To check if your main list contains some object (another list) just invoke this function passing it as the parameter.

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