如何在List中获取唯一值

Question

I have 2 text files with data. I am reading these files with BufferReader and putting the data of one column per file in a List<String> .

I have duplicated data in each one, but I need to have unique data in the first List to confront with the duplicated data in the second List .

How can I get unique values from a List ?

Answer 1

It can be done one one line by using an intermediate Set :

List<String> list = new ArrayList<>(new HashSet<>(list));

In java 8, use distinct() on a stream:

List<String> list = list.stream().distinct().collect(Collectors.toList());

Alternatively, don't use a List at all; just use a Set (like HashSet) from the start for the collection you only want to hold unique values.

Answer 2

Convert the ArrayList to a HashSet .

List<String> listWithDuplicates; // Your list containing duplicates
Set<String> setWithUniqueValues = new HashSet<>(listWithDuplicates);

If for some reason, you want to convert the set back to a list afterwards, you can, but most likely there will be no need.

List<String> listWithUniqueValues = new ArrayList<>(setWithUniqueValues);

Answer 3

In Java 8 :

     // List with duplicates
     List<String> listAll = Arrays.asList("A", "A", "B", "C", "D", "D");

     // filter the distinct 
     List<String> distinctList = listAll.stream()
                     .distinct()
                     .collect(Collectors.toList());

    System.out.println(distinctList);// prints out: [A, B, C, D]

this will also work with objects, but you will probably have to adapt your equals method.

Answer 4

i just realize a solution may be it can be helpful for other persons. first will be populated with duplicated values from BufferReader.

ArrayList<String> first = new ArrayList<String>();  

To extract Unique values i just create a new ArrayList like down:

ArrayList<String> otherList = new ArrayList<>();

    for(String s : first) {
        if(!otherList.contains(s))
            otherList.add(s);
    }

A lot of post in internet are all speaking to assign my Arraylist to a List , Set , HashTable or TreeSet. Can anyone explain the difference in theory and whitch one is the best tu use in practice ? thnks for your time guys.