How do I make a loop that would find each distinct number inside an ArrayList in java?

Question

I have a java class called StudentList and I want to make a method which would go through each student and check their group numbers. Then add them to an arraylist, if the group number is already inside the arraylist then it would skip it. But how do I make a loop that would skip duplicates?

I've tried using the for loop but I can't think of a solution past this one point:

public ArrayList<Integer> getAllGroupNumbers() {
    ArrayList<Integer> a = new ArrayList<Integer>();
    for(int i = 0; i < list.size(); i++)
    {
        if(list.get(i).getGroupNumber() != ...)
    }
}

What's your point in using a list when you don't want duplicates? — Tom, Oct 19 '19 at 16:17
I'm following a class diagram. Would it be easier to just use an array? — Audrius Šaučiūnas, Oct 19 '19 at 16:19
With Java 8 you have streams operations for that. Is the `for` loop mandatory? — Matthieu, Oct 19 '19 at 16:19
We haven't touched streams and it specifically says to use a loop — Audrius Šaučiūnas, Oct 19 '19 at 16:20
Possible duplicate of [How do I remove repeated elements from ArrayList?](https://stackoverflow.com/questions/203984/how-do-i-remove-repeated-elements-from-arraylist) — Tom, Oct 19 '19 at 16:24
Also possible:[JAVA: Preventing Duplicate Entries to an ArrayList](//stackoverflow.com/q/20611751) — Tom, Oct 19 '19 at 16:27

score 1 · Accepted Answer · edited Oct 19 '19 at 16:25

1

You can use .contains method in ArrayList to check if an element is already present in the list. Sample code would be like the one follows.

for(int i = 0; i < list.size(); i++)
{
  Integer groupNumber = list.get(i).getGroupNumber();
  if( !a.contains(groupNumber) ) {
     a.add(groupNumber);
  }
}

edited Oct 19 '19 at 16:25

Tom

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answered Oct 19 '19 at 16:22

Subramanian Mariappan

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How do I make a loop that would find each distinct number inside an ArrayList in java?

1 Answers1