Question

I’m making an arraylist of numbers, appropriately

private ArrayList<Integer> numbers = new ArrayList();

and I have to check if they’re all unique. So I have this code:

public boolean isUnique()
{
    ArrayList<Integer> checkNumbers = new ArrayList();

    for(int i = 1; i<=numbers.size(); i++)
    {
        if(numbers.contains(i) && !checkNumbers.contains(i))
        {
            checkNumbers.add(i);
            return true;
        }           
    }

    return false;
}

The idea is, I have to take in a square number (n) of integer inputs, unique from 1 to n.

But no matter what I add to numbers (13 2 13 2), it always returns true.

What’s wrong with my logic here?

Answer 1

if the list can contain more numbers than n and all you want is to verify that 1 .. n all exist and without duplication, then your code should be modified to this:

public boolean isUnique()
{
    ArrayList<Integer> checkNumbers = new ArrayList();

    for(int i = 1; i<=numbers.size(); i++)
    {
        if(numbers.contains(i))
        {
           if (!checkNumbers.contains(i)) 
             checkNumbers.add(i);
           else 
            return false; 
        }
        else{
            return false;
        }           
    }

    return true;
}

if on the other hand the list can’t contain more than n elements, you don’t need the other list at all:

public boolean isUnique()
    {

     if (numbers.size()<n)
        return false;  

        for(int i = 1; i<=numbers.size(); i++)
        {
            if(!numbers.contains(i))
              return false; 

        }

        return true;
    }