CODEWITHSUNDEEP
javaarraylist
Chris Headleand
Chris Headleand

Reputation: 6193

Is it possible to find out if a value exists twice in an arraylist?

I have an integer arraylist.. 

ArrayList <Integer> portList = new ArrayList();

I need to check if a specific integer has already been entered twice. Is this possible in Java?

Upvotes: 10

Views: 37314

Answers (8)

Yanick Rochon
Yanick Rochon

Reputation: 53516

This will tell you if you have at least two same values in your ArrayList:

int first = portList.indexOf(someIntValue);
int last  = portList.lastIndexOf(someIntValue);
if (first != -1 && first != last) {
    // someIntValue exists more than once in the list (not sure how many times though)
}

If you really want to know how many duplicates of a given value you have, you need to iterate through the entire array. Something like this:

/**
 * Will return a list of all indexes where the given value
 * exists in the given array. The list will be empty if the
 * given value does not exist at all.
 *
 * @param List<E> list
 * @param E value
 * @return List<Integer>    a list of indexes in the list
 */
public <E> List<Integer> collectFrequency(List<E> list, E value) {
   ArrayList<Integer> freqIndex = new ArrayList<Integer>();
   E item;
   for (int i=0, len=list.size(); i<len; i++) {
       item = list.get(i);
       if ((item == value) || (null != item && item.equals(value))) {
           freqIndex.add(i);
       }
   }
   return freqIndex;
}

if (!collectFrequency(portList, someIntValue).size() > 1) {
    // Duplicate value
}

Or using the already availble method:

if (Collections.frequency(portList, someIntValue) > 1) {
    // Duplicate value
}

Upvotes: 4

Alexei
Alexei

Reputation: 15646

Here is my solution (in Kotlin):

    // getItemsMoreThan(list, 2) -> [4.45, 333.45, 1.1, 4.45, 333.45, 2.05, 4.45, 333.45, 2.05, 4.45] -> {4.45=4, 333.45=3}
    // getItemsMoreThan(list, 1)->  [4.45, 333.45, 1.1, 4.45, 333.45, 2.05, 4.45, 333.45, 2.05, 4.45] -> {4.45=4, 333.45=3, 2.05=2}
    fun getItemsMoreThan(list: List<Any>, moreThan: Int): Map<Any, Int> {
        val mapNumbersByElement: Map<Any, Int> = getHowOftenItemsInList(list)
        val findItem = mapNumbersByElement.filter { it.value > moreThan }
        return findItem
    }

    // Return(map) how often an items is list.
    // E.g.: [16.44, 200.00, 200.00, 33.33, 200.00, 0.00] -> {16.44=1, 200.00=3, 33.33=1, 0.00=1}
    fun getHowOftenItemsInList(list: List<Any>): Map<Any, Int> {
        val mapNumbersByItem = list.groupingBy { it }.eachCount()
        return mapNumbersByItem
    }

Upvotes: 0

Insektosaurus
Insektosaurus

Reputation: 31

I used the following solution to find out whether an ArrayList contains a number more than once. This solution comes very close to the one listed by user3690146, but it does not use a helper variable at all. After running it, you get "The number is listed more than once" as a return message.

public class Application {

    public static void main(String[] args) {

        ArrayList<Integer> list = new ArrayList<Integer>();

        list.add(4);
        list.add(8);
        list.add(1);
        list.add(8);

        int number = 8;

        if (NumberMoreThenOnceInArray(list, number)) {
            System.out.println("The number is listed more than once");
        } else {
            System.out.println("The number is not listed more than once");
        }
    }

    public static boolean NumberMoreThenOnceInArray(ArrayList<Integer> list, int whichNumber) {

        int numberCounter = 0;

        for (int number : list) {
            if (number == whichNumber) {
                numberCounter++;
            }
        }

        if (numberCounter > 1) {
            return true;
        }

        return false;
    }

}

Upvotes: 1

user3690146
user3690146

Reputation: 106

My solution

public static boolean moreThanOnce(ArrayList<Integer> list, int searched) 
{
    int numCount = 0;

    for (int thisNum : list) {
        if (thisNum == searched) 
            numCount++;
    }
    
    return numCount > 1;
}

Upvotes: 5

Survivor
Survivor

Reputation: 672

You could use something like this to see how many times a specific value is there:

System.out.println(Collections.frequency(portList, 1));
// There can be whatever Integer, and I use 1, so you can understand

And to check if a specific value is there more than once you could use something like this:

if ( (Collections.frequency(portList, x)) > 1 ){
    System.out.println(x + " is in portList more than once ");
}

Upvotes: 22

Nicholas
Nicholas

Reputation: 7501

If you are looking to do this in one method, then no. However, you could do it in two steps if you need to simply find out if it exists at least more than once in the List. You could do

int first = list.indexOf(object)
int second = list.lastIndexOf(object)
// Don't forget to also check to see if either are -1, the value does not exist at all.
if (first == second) {
    // No Duplicates of object appear in the list
} else {
    // Duplicate exists
}

Upvotes: 4

Tharika Madurapperuma
Tharika Madurapperuma

Reputation: 470

By looking at the question, we need to find out whether a value exists twice in an ArrayList. So I believe that we can reduce the overhead of "going through the entire list just to check whether the value only exists twice" by doing the simple check below.

public boolean moreThanOneMatch(int number, ArrayList<Integer> list) {

    int count = 0;
    for (int num : list) {
        if (num == number) {
            count ++ ;
            if (count == 2) {
                return true;
            }
        }
    }
    return false;
}

Upvotes: -2

Mike
Mike

Reputation: 2434

Set portSet = new HashSet<Integer>();
portSet.addAll(portList);
boolean listContainsDuplicates = portSet.size() != portList.size();

Upvotes: 1

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