Java: How to read a text file

Question

I want to read a text file containing space separated values. Values are integers. How can I read it and put it in an array list?

Here is an example of contents of the text file:

1 62 4 55 5 6 77 

I want to have it in an arraylist as [1, 62, 4, 55, 5, 6, 77]. How can I do it in Java?

Answer 1

You can use Files#readAllLines() to get all lines of a text file into a List<String>.

for (String line : Files.readAllLines(Paths.get("/path/to/file.txt"))) {     // ... } 

Tutorial: Basic I/O > File I/O > Reading, Writing and Creating text files

---

You can use String#split() to split a String in parts based on a regular expression.

for (String part : line.split("\\s+")) {     // ... } 

Tutorial: Numbers and Strings > Strings > Manipulating Characters in a String

---

You can use Integer#valueOf() to convert a String into an Integer.

Integer i = Integer.valueOf(part); 

Tutorial: Numbers and Strings > Strings > Converting between Numbers and Strings

---

You can use List#add() to add an element to a List.

numbers.add(i); 

Tutorial: Interfaces > The List Interface

---

So, in a nutshell (assuming that the file doesn't have empty lines nor trailing/leading whitespace).

List<Integer> numbers = new ArrayList<>(); for (String line : Files.readAllLines(Paths.get("/path/to/file.txt"))) {     for (String part : line.split("\\s+")) {         Integer i = Integer.valueOf(part);         numbers.add(i);     } } 

---

If you happen to be at Java 8 already, then you can even use Stream API for this, starting with Files#lines().

List<Integer> numbers = Files.lines(Paths.get("/path/to/test.txt"))     .map(line -> line.split("\\s+")).flatMap(Arrays::stream)     .map(Integer::valueOf)     .collect(Collectors.toList()); 

Tutorial: Processing data with Java 8 streams