How to check if an object exists in a LinkedList in java?

Question

A LinkedList contains a set of Integer[]. Each Integer[] in the list has 2 numbers. Ex of the linked list:

Integer[]{1,2}, Integer[]{2,3}.....

Before adding another Integer[] to this LinkedList, I wanto check if another Integer[] with the same data already exists.

Ex: Object to add = Integer[] {2,3}. But this already exists in the LinkedList.

So I want to avoid adding this.

How to verify that object already exists? Is there an inbuild function that can be used? contains() didnt do the trick.

If your list contained a class that holds two integers, and if that class overrides equals and hashCode, reflecting what those integers are, then simply call contains(...) on the list. If you went this route, then best to make the class immutable. — Hovercraft Full Of Eels
– Hovercraft Full Of Eels, Commented Nov 23, 2017 at 16:05
Consider using a Set and creating a class for your integer-pairs with correct equals and hashcode methods. — Henrik Aasted Sørensen
– Henrik Aasted Sørensen, Commented Nov 23, 2017 at 16:06
Possible duplicate of Using contains on an ArrayList with integer arrays — Arnaud
– Arnaud, Commented Nov 23, 2017 at 16:08
Use a Set<Point>> instead: no duplicates as .equals okay and add then returns false. — Joop Eggen
– Joop Eggen, Commented Nov 23, 2017 at 16:11

cнŝdk · Accepted Answer · 2017-11-23 16:42:54Z

5

I think you better use a specific class if you are treating coordinates, as an Integer[] is useless for only two numbers, and will cause some problems with contains() and other List methods like .sort() as well.

You better create a Coordinate class, which will hold the two values:

public class Coordinate{
    private int x;
    private int y;

    //getters and setters, constructor
    @Override
    public boolean equals(Object o) {
        if (o == this) return true;
        if (!(o instanceof Coord)) {
            return false;
        }
        Coordinate coord = (Coordinate) o;
        return coord.x == x &&
                coord.y == y;
    }

    @Override
    public int hashCode() {
        int result = 17;
        result = 31 * result + x;
        result = 31 * result + y;
        return result;
    }
}

Then you can use:

LinkedList<Coordinate>

Note:

Note that using a Set implementation will be better here, it will prevent having duplicates in the set of coordinates, so we don't need to check for it manually.

edited Nov 23, 2017 at 16:42

answered Nov 23, 2017 at 16:13

cнŝdk

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5 Comments

Sync Over a year ago

Abbreviations are bad practice and discouraged unless they are universally accepted such as Utils. Coordinate is therefore preferred.

Hovercraft Full Of Eels Over a year ago

Caveat: If you're going to use the x and y fields within equals and hashCode, then they should be immutable (as per my comment to the OP's question).

Shubhendu Pramanik Over a year ago

@chsdk Rather than a class we can use Stream with Set to make it short.

cнŝdk Over a year ago

@HovercraftFullOfEels I just tried to produce a quick class for the example purpose, yes it would be better defined it as immutable. Thank you for the tip anyway :)

cнŝdk Over a year ago

@ShubhenduPramanik Yes it would be another good alternative to use Stream and yes for the Set point, I was about pointing it out.

Adrien Brunelat · Accepted Answer · 2017-11-23 16:26:04Z

2

Well, you can do it the dumb way:

boolean exists = false;
for (Integer[] integers : list) {  // list being the LinkedList
    if (Arrays.equals(integers, value)) {
        exists = true;
        break;
    }
}
if (!exists) {
    list.add(value);
}

edited Nov 23, 2017 at 16:26

answered Nov 23, 2017 at 16:11

Adrien Brunelat

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1 Comment

Adrien Brunelat Over a year ago

I'd suggest doing what chsdk posted though.

Shubhendu Pramanik · Accepted Answer · 2017-11-23 16:41:35Z

2

You can use Stream with Set to solve your problem like below:

 List<Set<Integer>> list = new LinkedList<>();
    list.add(Stream.of(1, 2).collect(Collectors.toSet()));

    Set<Integer> s1 = new HashSet<>();
    s1.add(1);
    s1.add(2);

    System.out.println(list.contains(s1));

    Set<Integer> s2 = new HashSet<>();
    s2.add(1);
    s2.add(4);
    System.out.println(list.contains(s2));

O/P:

true

false

N.B: You can use ArrayList because yo preserve the sequence as well.

edited Nov 23, 2017 at 16:41

answered Nov 23, 2017 at 16:32

Shubhendu Pramanik

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Comments

yegodm · Accepted Answer · 2017-11-23 17:08:38Z

0

If you really really want to do that with contains() (or have no other choice by whatever reason), you can implement it like that:

    final Integer[] newPair = {2, 3};
    final boolean exists = values.contains(new Object()
    {
        // note that List.contains() javadoc explicitly specifies that 
        // newPair is used as the receiver not the argument for equals()
        @Override
        public final boolean equals(final Object listElement)
        {
            final Integer[] otherPair = (Integer[]) listElement;
            return Arrays.equals(newPair, otherPair);
        }
    });

answered Nov 23, 2017 at 17:08

yegodm

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