comparing 2 ArrayList in Java

Question

I got 2 ArrayList

List fileArray = new ArrayList(); List fileTxt = new ArrayList();

List<String> newDoc = new ArrayList<String>();        

while((fileName = fileRead.readLine()) !=null){
    fileArray.add(fileName);
}
fileRead.close();

while((txtName = txtRead.readLine()) !=null){
    fileTxt.add(txtName);
}
txtRead.close();

I want to compare fileArray data with fileTxt data, but don't know how to do that, because to I'm thinking to compare I need to get the data first. And to do that I need to looping the array and using the .get like

for(int a=0;a<fileTxt.size();a++){
    System.out.println(fileTxt.get(a));
}

I know this is wrong, but anyone can help me.

note: I want to use something like contains method but can't because the data is 5 million rows. (not equals method)

Updated: I want to find a word in fileTxt that appeared in the fileArray. (so basically its like contains)

Define "compare", precisely. If you want to test that they are equal, then just use list1.equals(list2). If you want something else, you need to define it first. — JB Nizet, Oct 28 '18 at 17:56
Also, you probably don't want to be storing 5M records using an ArrayList — Nicholas K, Oct 28 '18 at 17:58
Are you trying to find the differences between the lines, possibly ignoring whitespace? Compare could mean many things. — Peter Lawrey, Oct 28 '18 at 17:58
Possible duplicate of [Java Compare Two Lists](https://stackoverflow.com/questions/2762093/java-compare-two-lists) — Emmanuel H, Oct 28 '18 at 18:05

score 0 · Answer 1 · answered Oct 28 '18 at 18:01

0

To compare 2 values in your 2 arraylists, you do this:

if(fileArray.get(index).equals(fileTxt.get(index)))

answered Oct 28 '18 at 18:01

Rino Leonardino

17
8

With this method, you need a for loop. And if a list has a value twice, and the second only once, it will succeed with your code, whereas both are different. – Emmanuel H Oct 28 '18 at 18:03

score 0 · Answer 2 · answered Oct 28 '18 at 18:43

There is a wondefull function.

System.out.println(array1.equals(array2));

Then, you might want to check your answer by hands:

    ArrayList like1 = new ArrayList();
    ArrayList like2 = new ArrayList(); 
    ...

    boolean flag = true;
    if (!Math.min(like1.size(), like2.size()) == Math.max(like1.size(), like2.size())) 
    {
        flag = false;
    }
    for (int i = 0; i < Math.min(like1.size(), like2.size()); i++) {
        if (!like1.get(i).equals(like2.get(i))) {
            flag = false;
            break;
        }
    }

score -2 · Answer 3 · answered Oct 28 '18 at 18:02

You can do this using the removeAll() method.

fileArray.removeAll(fileTxt);
fileTxt.removeAll(fileArray);

If one of them still have data, this means that the two List where different.

But storing 5 millions data in a Java list is not something I will advise you. Perhaps you should think about something else.

comparing 2 ArrayList in Java

3 Answers3