Comments on: Fast Way to Get All Unique Values from a List in Java

By: Spiro

Spiro — Wed, 30 Mar 2011 12:48:45 +0000

The Oleg’a idea for a ArrayList:

public static ArrayList make_unique( ArrayList inArray){
String[] tmpLst= new String[inArray.size()];
for(int i=0; i< inArray.size(); i++){
tmpLst[i] =inArray.get(i);
}
Arrays.sort(tmpLst);
inArray.clear();
int lastArray =-1;
for(int i=1; i< tmpLst.length; i++){
if(lastArray==-1 || !tmpLst[i].equals(inArray.get(lastArray))) ){
inArray.add(tmpLst[i]);
lastArray++;
}
}
return inArray;
}

By: Help on my Java program

Help on my Java program — Mon, 10 May 2010 17:23:05 +0000

[...] Instead of looping through each letter of the given letter list (letters), Get a sub-list of all of the unique letters in the letter list (link: Fast way to get all unique value from a list in Java) [...]

By: Oleg

Oleg — Thu, 17 Dec 2009 08:30:47 +0000

one way to do it efficiently is to use Arrays.sort method on your array, using the default comparator for the types which implement Comparable or defying a new Comparator for your custom types. Than just iterate through the array to get unique values like in the code below

int[] a = new int[6];
a[0] = 10; a[1] = 100; a[2] = 100;
a[3] = 20; a[4] = 10; a[5] = 10;
Arrays.sort(a);
int value = a[0];
int i = 1;
while(i < a.length) {
if(a[i] == value) {
if(i == (a.length – 1)) {
System.out.println(a[i]);
}
} else {
System.out.println(a[i - 1]);
value = a[i];
if(i == (a.length – 1)) {
System.out.println(a[i]);
}
}
i++;
}

this will fine as long as a proper Comparator is provided to Arrays.sort() method

By: Titom

Titom — Thu, 17 Dec 2009 01:25:42 +0000

The example only works because the items you add to the List are added following a natural order. If you test it using:
x.add(“def”);
x.add(“abc”);
x.add(“abc”);
x.add(“ghi”);
x.add(“def”);
Lists.GetUniqueValues(x) will return a List that contains (“abc”, “def”, “ghi”) which is not what is expected… What is expected is (“def”, “abc”, “ghi”).

What I suggest is one of the following solutions:
1- You can include external free libraries in your application? Go with the Apache library Commons Collections, the SetUniqueList is what you are looking for(http://commons.apache.org/collections/api-release/org/apache/commons/collections/list/SetUniqueList.html),
2- You can’t include any external libraries? Create a new class UniqueArrayList that extends ArrayList and overrides the add methods so they check if the object to add is already in the list to prevent duplicates.

By: admin

admin — Sun, 13 Dec 2009 02:24:58 +0000

Stuart and Oleg suggest this is not the best way to approach the solution. What do you (or someone else) suggest as a way of doing it better? This forum is about sharing information.

By: Oleg

Oleg — Sat, 12 Dec 2009 23:22:24 +0000

not to mention it is not the most efficient way to do it

By: Stuart

Stuart — Fri, 14 Aug 2009 14:48:22 +0000

There’s only one problem I can see with both of those solutions: It relies on there not being any collision between hashCode() values. In general it’s the case, but the contract of the hashCode() method doesn’t guarantee it, so you could potentially lose unique values that hash to the same as something else.

By: shahzad

shahzad — Thu, 27 Nov 2008 10:11:22 +0000

return new ArrayList(new HashSet(values)); will serve the same purpose.