Java 如何在列表中获取唯一值
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How to get unique values in List
提问by Junior Fulcrum
I have 2 text files with data. I am reading these files with BufferReaderand putting the data of one column per file in a List<String>.
我有 2 个带有数据的文本文件。我正在阅读这些文件BufferReader并将每个文件的一列数据放入List<String>.
I have duplicated data in each one, but I need to have unique data in the first Listto confront with the duplicated data in the second List.
我在每个文件中都有重复的数据，但我需要在第一个中有唯一的数据List来面对第二个中的重复数据List。
How can I get unique values from a List?
如何从 a 中获取唯一值List？
回答by Robby Cornelissen
Convert the ArrayListto a HashSet.
将 转换ArrayList为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.
如果出于某种原因，您想在之后将集合转换回列表，则可以，但很可能不需要。
List<String> listWithUniqueValues = new ArrayList<>(setWithUniqueValues);
回答by Junior Fulcrum
i just realize a solution may be it can be helpful for other persons. first will be populated with duplicated values from BufferReader.
我只是意识到一个解决方案可能对其他人有帮助。首先将填充来自 BufferReader 的重复值。
ArrayList<String> first = new ArrayList<String>();
To extract Unique values i just create a new ArrayList like down:
要提取唯一值，我只需创建一个新的 ArrayList，如下所示：
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.
互联网上的很多帖子都在谈论将我的 Arraylist 分配给 List 、 Set 、 HashTable 或 TreeSet 。任何人都可以解释理论上的差异，哪个是最好的实践？谢谢你们的时间。
回答by Bohemian
It can be done one one line by using an intermediate Set:
可以通过使用中间件一行一行地完成Set：
List<String> list = new ArrayList<>(new HashSet<>(list));
In java 8, use distinct()on a stream:
在 java 8 中，distinct()在流上使用：
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.
或者，根本不要使用 List；只需从一开始就为您只想保存唯一值的集合使用 Set（如 HashSet）。
回答by Jorciney
In Java 8:
在 Java 8 中：
// 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.
这也适用于对象，但您可能需要调整您的 equals 方法。
