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Is their a prebuilt method that works differently than removeAll to remove all of a certain value in an array list?

I have a method to delete duplicates in arrayLists that looks like this:

The first 2 for for loops work and set it correctly but I don't know what to do for the second for loop to delete all values of -1 from the ArrayList or is their a better way to do this?

public static ArrayList<Integer> deleteDuplicates(ArrayList<Integer> a) {    
    for (int i = 0; i < a.size(); i++) {
      for (int j = (i + 1); j < a.size(); j++) { 
        if (a.get(i) == a.get(j) && i != j) {
          a.set(j, -1); 
        }
      }
    }
    for (int i = 0; i < a.size(); i++) {
      if (a.get(i) == -1) {
        a.removeAll(int -1);
      }
    }

The removeAll(int -1) thing at the bottom doesn't compile, I just left it there so you can see what I'm trying to do.

EDIT:

replaced the for loop at the bottom with this: 

  for (int i = 0; i < a.size(); i++) {
      if (a.get(i) == -1) {
        a.remove(i);
        i--;
      }
    }

pretty sure that works.

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5
  • Please explain this line if (a.get(i) == -1) Commented Mar 17, 2016 at 0:55
  • If you want to avoid duplicates have you considered using a set/hashset? Commented Mar 17, 2016 at 0:57
  • a.remove(i); Since you are using for loop to check every element one by one there is no need to use removeall, just use a.remove(i); There is a better way to remove the duplicates by putting them into a set, set cannot contain duplicated value so it removes the duplicate automatically. Hope it helps. Commented Mar 17, 2016 at 0:58
  • 2
    Be very careful with List<Integer> - there is remove(int) which removes the item at a specified index, and remove(Integer) which removes the object specified. Due to autoboxing conventions, it can be very easy to call the wrong method, often resulting in a lot of frustrating debugging. Commented Mar 17, 2016 at 0:58
  • 1
    removeAll(Collections.singleton(-1)) might be what you're looking for. Commented Mar 17, 2016 at 0:59

2 Answers 2

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A possibly less efficient, but far more readable way of accomplishing this is to use a Set to filter the duplicates:

public static ArrayList<Integer> deleteDuplicates(ArrayList<Integer> a) {
   return new ArrayList<Integer>(new TreeSet<Integer>(a));
}

For most cases, like very large Lists, this will be a better option.

Given the option, avoid the need to convert an ArrayList all together by declaring your initial ArrayList<Integer> as a Set<Integer>. If so, this method would be come unnecessary.

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6 Comments

I tried the TreeSet thing and I get the error message: .java:22: error: illegal start of expression ArrayList<Integer> b = ArrayList<>(new TreeSet<Integer>(a)); ^ 1 error
I updated the code in the answer to help correct your issue.
this is declaring a set so I have the line of code: ArrayList<Integer> b = ArrayList<Integer>(new TreeSet<Integer>(a)); and then I have a really basic for loop that deletes the duplicates after its sorted but the first line gives me the error message: Generate.java:90: error: cannot find symbol ArrayList<Integer> b = ArrayList<Integer>(new TreeSet<Integer>(a)); ^ symbol: variable ArrayList location: class Generate
and these: Generate.java:90: error: cannot find symbol ArrayList<Integer> b = ArrayList<Integer>(new TreeSet<Integer>(a)); ^ symbol: variable Integer location: class Generate Generate.java:90: error: cannot find symbol ArrayList<Integer> b = ArrayList<Integer>(new TreeSet<Integer>(a)); ^ symbol: class TreeSet location: class Generate 3 errors
you need to have an import statement for both ArrayList and TreeSet, both can be imported with the statement at the top of your class: import java.util.*;
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As others have pointed out, the most elegant way would be to use a Set; which takes care of duplicate values automatically.

I would even go one step further: if you have control over the input data ... then do not use an ArrayList at all. It is simply a waste of memory and CPU cycles to first fill a list with values; to then transform it into a Set to eliminate duplicates; to then turn that into a list again. You are constantly creating new objects without the need to.

Instead: use a set right from the beginning. So, if possible change the code that "receives" the input data to directly store within a set.

( side note: the different collection classes like HashSet, ArrayList, ... have different "cost" for different kinds of operations. In that sense you might want to check this question to determine which collection type best fits your needs )

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