使用迭代器遍历列表并比较连续元素

Question

I want to compare each element in a single list and the code directly below provides me my solution:

for (int i = 0; i < list.size(); i++) {
    for (int j = i + 1; j < list.size(); j++) {
        // compare 
    }
}

It compares elements index wise, for instance comparing four elements:

0 to 1
0 to 2
0 to 3
1 to 2
1 to 3
2 to 3

I want to achieve the same as above using an iterator. My implementation using an iterator:

int i = 0;
for (Iterator<Tuple> it = list.iterator(); it.hasNext();) {
    Tuple tB = null;
    Tuple tA = it.next();

    for(int j = 0; j == i; j++) {
        try {
            tB = it.next();

            if(tA.Username.equals(tB.Username) && .....) {
                System.out.println("Match");
            } else {
                System.out.println("Not Matched");
            }
            i++;
        } catch(NoSuchElementException exc) {
            throw exc;
        }   
    }
}

Tuple is a class with some string properties.

This code only compares the first element with the others and throws NoSuchElementException , but I want to move from inner for-loop to the outer for-loop to keep traversing. How can I achieve this?

Answer 1

Hi you get the Exception because of your it.next() within the inner for loop. You need a second Iterator.

 for (Iterator<Tuple> it = list.iterator(); it.hasNext();) 
{
       Tuple tA = it.next();
       List<Tuple> tmpList = new ArrayList<Tuple>(list);
       tmpList.remove(tA);
  for( Iterator<Tuple> it2 = tmpList.iterator(); it2.hasNext();)     {
        try{
            Tuple tB=it2.next();

            if(tA.Username.equals(tB.Username) && .....)
            {
                System.out.println("Match");
            }
            else
            {
                System.out.println("Not Matched");
            }
            i++;
        }
        catch(NoSuchElementException exc)
        {
            throw exc;
        }   
    }
}

Answer 2

To compliment shsvd98 answer , you can use a ListIterator instead of an Iterator . ListIterator extends Iterator and provides more functionality.

For instance the following code does exactly what your first code block does. It also doesn't compare an element with itself and there's no need to create another List after each outer for-loop iteration.

Tuple tA = null;
Tuple tB = null;

for (ListIterator<Tuple> one = list.listIterator(); one.hasNext();) {
    // set 'tA' here, not in the nested for loop
    tA = one.next();

    // initialize 'two' with the next index of 'one'
    for(ListIterator<Tuple> two = list.listIterator(one.nextIndex()); two.hasNext();) {
        // set 'tB' here
        tB = two.next();

        // debug line used in output below this code block
        // System.out.print("tA:" + tA + " to tB:" + tB + " ");

        try {
            if(tA.username.equals(tB.username)) {
                System.out.println("Match");
            } else {
                System.out.println("Not Matched");
            }
        } catch(NoSuchElementException exc) {
            throw exc;
        }  

    }
}

I ran the above code, switching out Tuple with String .

Populated ArrayList with ["a", "b", "c", "b"]

Output:
tA:a to tB:b Not Matched
tA:a to tB:c Not Matched
tA:a to tB:b Not Matched
tA:b to tB:c Not Matched
tA:b to tB:b Match
tA:c to tB:b Not Matched

If your curious about the benefits of using ListIterator over Iterator , then the ListIterator documentation provides more insight.