How do you read the contents of a file into an ArrayList<String> in Java?
Here are the file contents:
cat
house
dog
.
.
.
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13 Answers
This Java code reads in each word and puts it into the ArrayList:
Scanner s = new Scanner(new File("filepath"));
ArrayList<String> list = new ArrayList<String>();
while (s.hasNext()){
list.add(s.next());
}
s.close();
Use s.hasNextLine() and s.nextLine() if you want to read in line by line instead of word by word.
3 Comments
WhiteFang34
Oh nice, shorter with
Scanner than BufferedReader and no exceptions to deal with. I guess I'm used to using BufferedReader prior to Java 5 when Scanner didn't exist, although I've been using Java 5 and 6 for many years. The Commons IO library of course provides the shortest answer if available as others have mentioned, so I generally use that now.
Hedley
Don't you need to specify the delimiter for the scanner, as the default delimiter matches whitespace? i.e. new Scanner(new File("filepath")).useDelimiter("\n");
Tschallacka
@Hedley from java7 on
new Scanner(new File("filepath")).useDelimiter(System.lineSeparator()) because it can differ per OSYou can use:
List<String> list = Files.readAllLines(new File("input.txt").toPath(), Charset.defaultCharset() );
Source: Java API 7.0
A one-liner with commons-io:
List<String> lines = FileUtils.readLines(new File("/path/to/file.txt"), "utf-8");
The same with guava:
List<String> lines =
Files.readLines(new File("/path/to/file.txt"), Charset.forName("utf-8"));
1 Comment
Fluffy
Why not use
Charsets.UTF_8 instead of forName?Simplest form I ever found is...
List<String> lines = Files.readAllLines(Paths.get("/path/to/file.txt"));
Comments
In Java 8 you could use streams and Files.lines:
List<String> list = null;
try (Stream<String> lines = Files.lines(myPathToTheFile))) {
list = lines.collect(Collectors.toList());
} catch (IOException e) {
LOGGER.error("Failed to load file.", e);
}
Or as a function including loading the file from the file system:
private List<String> loadFile() {
List<String> list = null;
URI uri = null;
try {
uri = ClassLoader.getSystemResource("example.txt").toURI();
} catch (URISyntaxException e) {
LOGGER.error("Failed to load file.", e);
}
try (Stream<String> lines = Files.lines(Paths.get(uri))) {
list = lines.collect(Collectors.toList());
} catch (IOException e) {
LOGGER.error("Failed to load file.", e);
}
return list;
}
List<String> words = new ArrayList<String>();
BufferedReader reader = new BufferedReader(new FileReader("words.txt"));
String line;
while ((line = reader.readLine()) != null) {
words.add(line);
}
reader.close();
1 Comment
user618712
how come i can't find the size of the arraylist with "line.size()"?
You can for example do this in this way (full code with exceptions handlig):
BufferedReader in = null;
List<String> myList = new ArrayList<String>();
try {
in = new BufferedReader(new FileReader("myfile.txt"));
String str;
while ((str = in.readLine()) != null) {
myList.add(str);
}
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
if (in != null) {
in.close();
}
}
1 Comment
Afshin Moazami
In Java 7+, you can use try-with-resources
//CS124 HW6 Wikipedia Relation Extraction
//Alan Joyce (ajoyce)
public List<String> addWives(String fileName) {
List<String> wives = new ArrayList<String>();
try {
BufferedReader input = new BufferedReader(new FileReader(fileName));
// for each line
for(String line = input.readLine(); line != null; line = input.readLine()) {
wives.add(line);
}
input.close();
} catch(IOException e) {
e.printStackTrace();
System.exit(1);
return null;
}
return wives;
}
Comments
Here's a solution that has worked pretty well for me:
List<String> lines = Arrays.asList(
new Scanner(new File(file)).useDelimiter("\\Z").next().split("\\r?\\n")
);
If you don't want empty lines, you can also do:
List<String> lines = Arrays.asList(
new Scanner(new File(file)).useDelimiter("\\Z").next().split("[\\r\\n]+")
);
Comments
To share some analysis info. With a simple test how long it takes to read ~1180 lines of values.
If you need to read the data very fast, use the good old BufferedReader FileReader example. It took me ~8ms
The Scanner is much slower. Took me ~138ms
The nice Java 8 Files.lines(...) is the slowest version. Took me ~388ms.
Comments
Here is an entire program example:
import java.io.File;
import java.io.FileNotFoundException;
import java.util.ArrayList;
import java.util.Scanner;
public class X {
public static void main(String[] args) {
File f = new File("D:/projects/eric/eclipseworkspace/testing2/usernames.txt");
try{
ArrayList<String> lines = get_arraylist_from_file(f);
for(int x = 0; x < lines.size(); x++){
System.out.println(lines.get(x));
}
}
catch(Exception e){
e.printStackTrace();
}
System.out.println("done");
}
public static ArrayList<String> get_arraylist_from_file(File f)
throws FileNotFoundException {
Scanner s;
ArrayList<String> list = new ArrayList<String>();
s = new Scanner(f);
while (s.hasNext()) {
list.add(s.next());
}
s.close();
return list;
}
}
Comments
Scanner scr = new Scanner(new File(filePathInString));
/*Above line for scanning data from file*/
enter code here
ArrayList<DataType> list = new ArrayList<DateType>();
/*this is a object of arraylist which in data will store after scan*/
while (scr.hasNext()){
list.add(scr.next()); } /*above code is responsible for adding data in arraylist with the help of add function */
Comments
Add this code to sort the data in text file.
Collections.sort(list);
1 Comment
Peter
Generally, answers are much more useful if they include an explanation of what the code is intended to do.