Android read line by line

How to read a file line by line in Java

августа 24, 2019 • Atta

Sometimes we want to read a file line by line to a string to process the content. A good example is reading a CSV file line by line and then splitting the line by comma ( , ) into multiple columns.

In Java, there are various options available to choose from when you need to read a file line by line.

1. Scanner

The Scanner class presents the simplest way to read a file line by line in Java. We can use Scanner class to open a file and then read its content line by line.

A Scanner breaks its input into tokens using a delimiter pattern, which is a new line in our case:

The hasNextLine() method returns true if there is another line in the input of this scanner without advancing the file read position.

To read data and move on to the next line, we should use the nextLine() method. This method moves the scanner past the current line and returns the rest of the current line, excluding any line separator at the end. The read position is then set to the beginning of the next line.

Since the nextLine() method continues to search through the input looking for a line separator, it may buffer all of the input searching for the line to skip if no line separators are present.

2. BufferedReader

The BufferedReader class provides an efficient way to read characters, arrays, and lines from a character-input stream.

As the name suggests, it buffers the characters up to 8MB (or 8192KB) which is large enough for most use cases. If the file you are reading is larger than the default buffer size, you can customize the default size:

The BufferedReader constructor accepts a Reader instance (like FileReader , InputStreamReader ) as character-input stream source. Here is a simple example that shows how to use it for reading a file line by line:

The readLine() method reads a line of text from the file and returns a string containing the contents of the line, excluding any line-termination characters or null.

Note: A null value does not mean that the string is empty. Rather it shows that the end of the file is reached.

Alternatively, you can use lines() method from BufferedReader class that returns a Stream of lines. You can easily convert this stream into a list or read the lines like the following:

3. Java 8 Stream

Java 8 Stream is another way (albeit cleaner) of reading a file line by line. We can use Files.lines() static method to initialize a lines stream like below:

In addition to simple API, streams are very useful for filtering, sorting and processing the data. Let us extend the above example and filter out the lines that end with a colon ( : ), then sort them alphabetically, and convert to uppercase:

4. New I/O API

Java New I/O API or NIO (classes in java.nio.* package) provides the Files.readAllLines() method to read a text file line by line into a List , as shown below:

5. RandomAccessFile

The RandomAccessFile class provides a non-blocking mode of reading and writing files. A random-access file behaves like a large array of bytes stored in the file system.

We can use RandomAccessFile to open a file in reading mode and then use its readLine() method to read line by line:

6. Apache Commons IO

The Apache Commons IO library contains utility classes, stream implementations, file filters, file comparators, and much more. Add the following to your build.gradle file to import the library in your project:

If you are using Maven, add the following to your pom.xml file:

We can now use FileUtils.readLines() the static method from Apache Commons IO that reads all lines from a file into a List :

Since Apache Commons IO reads all lines from the file at once, it may not be a good solution for reading large files. It will continue blocking the for loop execution in the above case until all lines are added to the lines object.

7. Okie

Okie is another open-source I/O library developed by Square for Android, Kotlin, and Java. It complements native java.io and java.nio packages to make it much easier to access, save, and process the data.

To import Okie in your project, add the following to the build.gradle file:

If you are using Maven, add the following to your pom.xml file:

Now we can use Okio.source() method to open a source stream to read a file. The returned Source interface is very small and has limited uses. Okie provides BufferedSource class to wrap the source with a buffer that makes your program run faster.

Let us have an example:

The readUtf8Line() method reads the data until the next line delimiter – either \n , \r\n , or the end of the file. It returns that data as a string, omitting the delimiter at the end. When it encounters empty lines, the method will return an empty string. If there isn’t no more data to read, it will return null .

Further Reading

You may be interested in other Java I/O articles:

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Java Read File Line by Line Example

Posted by: Yatin in Core Java February 5th, 2018 2 Comments Views

Hello readers, in this tutorial, we will see an example of how to Read a File Line by Line in Java 8. We will learn the Java 8 Stream’s API for reading a file’s content line by line and we will explore its different characteristics.

1. Introduction

These days in the programming universe reading the file content is one of the most habitual file manipulation tasks in Java. In the ancient Java world, the code to read the text file line by line was very tedious. In Java8, JDK developers have added new methods to the java.nio.file.Files class which has opened new gateways for the developers and these new methods offer an efficient reading of the files using the Streams.

Java8 has added the Files.lines() method to read the file data using the Stream . The attractiveness of this method is that it reads all lines from a file as a stream of strings. This method,

• Reads the file data only after a Terminal operation (such as forEach() , count() etc.) is executed on the Stream
• Reads the file content line-by-line by using the Streams
• Works by reading the Byte from a file and then decodes it into a Character using the UTF-8 character encoding

Do remember, Files.lines() is different from the Files.readAllLines() as the latter one reads all the lines of a file into a list of String elements. This is not an efficient way as the complete file is stored as a List resulting in extra memory consumption.

Java8 also provides another method i.e. Files.newBufferedReader() which returns a BufferedReader to read the content from the file. Since both Files.lines() and Files.newBufferedReader() methods return Stream, developers can use this output to carry out some extra processing.

Now, open up the Eclipse Ide and we will go over these methods to read a file line by line using the Java8 Lambda Stream.

2. Java 8 Read File Line by Line Example

2.1 Tools Used

We are using Eclipse Oxygen, JDK 1.8, and Maven.

2.2 Project Structure

Firstly, let’s review the final project structure if you’re confused about where you should create the corresponding files or folder later!

2.3 Project Creation

This section will show how to create a Java-based Maven project with Eclipse. In Eclipse IDE, go to File -> New -> Maven Project .

In the New Maven Project window, it will ask you to select the project location. By default, ‘Use default workspace location’ will be selected. Select the ‘Create a simple project (skip archetype selection)’ checkbox and just click on the next button to proceed.

It will ask you to ‘Enter the group and the artifact id for the project’. We will input the details as shown in the below image. The version number will be by default: 0.0.1-SNAPSHOT .

Click on Finish and the creation of a maven project is completed. If you see, it has downloaded the maven dependencies and a pom.xml file will be created. It will have the following code:

Developers can start adding the dependencies that they want. Let us start building the application!

3. Application Building

Below are the steps involved in explaining this tutorial.

3.1 Java Class Implementation

Let’s create the required Java files. Right-click on the src/main/java folder, New -> Package .

A new pop window will open where we will enter the package name as: com.jcg.java .

Once the package is created in the application, we will need to create the implementation class to show the Files.lines() and Files.newBufferedReader() methods usage. Right-click on the newly created package: New -> Class .

A new pop window will open and enter the file name as: ReadFileLineByLineDemo . The implementation class will be created inside the package: com.jcg.java .

3.1.1 Example of Reading File in Java8

Here is the complete Java program to read an input file line by line using the Lambda Stream in Java8 programming. In this tutorial we will talk about the 3 different approaches for reading a file:

• Approach 1: This approach (i.e. fileStreamUsingFiles(……) talks about reading a file into Java8 Stream and printing it line by line
• Approach 2: This approach (i.e. filterFileData(……) ) talks about reading a file into Java8 Stream (like the one we’ll be using in Approach 1) but also apply a filter to it (i.e. Read only those elements from the file that start with an alphabet ( s ) and print it onto the console). This approach gives an edge to apply criteria while reading the file
• Approach 3: This approach (i.e. fileStreamUsingBufferedReader(……) talks about a new Java8 method known as BufferedReader.lines(……) that lets the BufferedReader returns the content as Stream

Important points:

• In the above approaches, we have omitted the try-with-resources concept for simplicity and mainly focused on the new ways of reading the file. However, just for developers curiosity try-with-resources is a concept that ensures that each resource is closed at the end of the statement execution
• With enhancements in Java and introduction to Stream in Java8, developers have stopped using the BufferedReader and Scanner classes to read a file line by line as it does not offer the facilities like the ones offered by Java8 Streams API

Let us move ahead and understand the 3 different approaches with a practical example.

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Java read file line by line

Core Java Tutorial

Today we will look into different java read file line by line methods. Sometimes we have to read file line by line to a String, for example calling a method by passing each line as String to process it.

Java Read File line by line

We can read file line by line using different ways. Let’s look at some of them.

Java Read File line by line using BufferedReader

We can use java.io.BufferedReader readLine() method to read file line by line to String. This method returns null when end of file is reached. Below is a simple program showing example for java read file line by line using BufferedReader.

Java Read File line by line using Scanner

We can use Scanner class to open a file and then read its content line by line. Below is the scanner example to read file line by line and print it.

Java Read File line by line using Files

java.nio.file.Files is a utility class that contains various useful methods. Files readAllLines method can be used to read all the file lines into a list of string.

Java Read File line by line using RandomAccessFile

We can use RandomAccessFile to open a file in read mode and then use its readLine method to read file line by line.

That’s all for java read file line by line using different methods.

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Reading a File Line by Line in Java

In Computer Science, a file is a resource used to record data discretely in a computer’s storage device. In Java, a resource is usually an object implementing the AutoCloseable interface.

Reading files and resources have many usages:

• Statistics, Analytics, and Reports
• Machine Learning
• Dealing with large text files or logs

Sometimes, these files can be absurdly large, with gigabytes or terabytes being stored, and reading through them in entirety is inefficient.

Being able to read a file line by line gives us the ability to seek only the relevant information and stop the search once we have found what we’re looking for. It also allows us to break up the data into logical pieces, like if the file was CSV-formatted.

There are a few different options to choose from when you need to read a file line by line.

Scanner

One of the easiest ways of reading a file line by line in Java could be implemented by using the Scanner class. A Scanner breaks its input into tokens using a delimiter pattern, which in our case is the newline character:

The hasNextLine() method returns true if there is another line in the input of this scanner, but the scanner itself does not advance past any input or read any data at this point.

To read the line and move on, we should use the nextLine() method. This method advances the scanner past the current line and returns the input that wasn’t reached initially. This method returns the rest of the current line, excluding any line separator at the end of the line. The read position is then set to the beginning of the next line, which will be read and returned upon calling the method again.

Since this method continues to search through the input looking for a line separator, it may buffer all of the input while searching for the end of the line if no line separators are present.

Buffered Reader

The BufferedReader class represents an efficient way of reading the characters, arrays, and lines from a character-input stream.

As described in the naming, this class uses a buffer. The default amount of data that is buffered is 8192 bytes, but it could be set to a custom size for performance reasons:

The file, or rather an instance of a File class, isn’t an appropriate data source for the BufferedReader , so we need to use a FileReader , which extends InputStreamReader . It is a convenience class for reading information from text files and isn’t necessarily suitable for reading a raw stream of bytes:

The initialization of a buffered reader was written using the try-with-resources syntax, specific to Java 7 or higher. If you’re using an older version, you should initialize the br variable before the try statement and close it in the finally block.

Here’s an example of the previous code without the try-with-resources syntax:

The code will loop through the lines of the provided file and stop when it meets the null line, which is the end of the file.

Don’t get confused as the null isn’t equal to an empty line and the file will be read until the end.

The lines Method

A BufferedReader class also has a lines method that returns a Stream . This stream contains lines that were read by the BufferedReader , as its elements.

You can easily convert this stream into a list if you need to:

Reading through this list is the same as reading through a Stream, which are covered in the next section:

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Java 8 Streams

If you’re already familiar with the Java 8 Streams, you can use them as a cleaner alternative to the legacy loop:

Here we’re using try-with-resources syntax once again, initializing a lines stream with the Files.lines() static helper method. The System.out::println method reference is used for the demo purposes, and you should replace it with whatever code you’ll be using to process your lines of text.

In addition to a clean API, streams are very useful when you want to apply multiple operations to the data or filter something out.

Let’s assume we have a task to print all of the lines that are found in a given text file and end with the «/» character. The lines should be transformed to the uppercase and sorted alphabetically.

By modifying our initial «Streams API» example we’ll get a very clean implementation:

The filter() method returns a stream consisting of the elements of this stream that match the given predicate. In our case we’re leaving only those that end with the «/».

The map() method returns a stream consisting of the results of applying the given function to the elements of this stream.

The toUpperCase() method of a String class helps us to achieve the desired result and is being used here as a method reference, just like the println call from our previous example.

The sorted() method returns a stream consisting of the elements of this stream, sorted according to the natural order. You’re also able to provide a custom Comparator , and in that case sorting will be performed according to it.

While the order of operations could be changed for the filter() , sorted() , and map() methods, the forEach() should be always placed in the end as it’s a terminal operation. It returns void and for that matter, nothing can be chained to it further.

Apache Commons

If you’re already using Apache Commons in your project, you might want to utilize the helper that reads all the lines from a file into a List :

Remember, that this approach reads all lines from the file into the lines list and only then the execution of the for loop starts. It might take a significant amount of time, and you should think twice before using it on large text files.

Conclusion

There are multiple ways of reading a file line by line in Java, and the selection of the appropriate approach is entirely a programmer’s decision. You should think of the size of the files you plan to process, performance requirements, code style and libraries that are already in the project. Make sure to test on some corner cases like huge, empty, or non-existent files, and you’ll be good to go with any of the provided examples.

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