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Week 116 — How can one read the contents of a file from inside a Java program?

Question of the Week #116
How can one read the contents of a file from inside a Java program?
9 Replies
Eric McIntyre
Eric McIntyre•4w ago
The InputStream and Reader classes allow reading binary and textual data from various sources including the network or files. These can be obtained using the Files.newInputStream and Files.newBufferedReader methods. 
// count the number of lines in a file assuming it's encoded with ASCII using line feeds (\n)
int lineCount = 0;
try(InputStream is = new BufferedInputStream(Files.newInputStream(Path.of("someFile.txt")))) {
  //process the file byte-per-byte
  int readByte;
  while((readByte = is.read()) != -1) {
    // do something with each byte - in this case check whether the byte is a line break and increment the line counter
    if((readByte & 0x80) != 0) {
      throw new IllegalStateException("not ASCII");
    }
    if(readByte == '\n') {
      lineCount++;
    }
  }
}
// count the number of lines in a file assuming it's encoded with ASCII using line feeds (\n)
int lineCount = 0;
try(InputStream is = new BufferedInputStream(Files.newInputStream(Path.of("someFile.txt")))) {
  //process the file byte-per-byte
  int readByte;
  while((readByte = is.read()) != -1) {
    // do something with each byte - in this case check whether the byte is a line break and increment the line counter
    if((readByte & 0x80) != 0) {
      throw new IllegalStateException("not ASCII");
    }
    if(readByte == '\n') {
      lineCount++;
    }
  }
}
try(InputStream is = Files.newInputStream(Path.of("someFile.txt"));
    OutputStream os = Files.newOutputStream(Path.of("target.txt"))) {
  // It's also possible to put all data from an InputStream to an OutputStream
  is.transferTo(os);
}
try(InputStream is = Files.newInputStream(Path.of("someFile.txt"));
    OutputStream os = Files.newOutputStream(Path.of("target.txt"))) {
  // It's also possible to put all data from an InputStream to an OutputStream
  is.transferTo(os);
}
try(BufferedReader br = Files.newBufferedReader(Path.of("someFile.txt"))){
  String line;
  while((line = br.readLine()) != null){
    System.out.println(line);
  }
}
try(BufferedReader br = Files.newBufferedReader(Path.of("someFile.txt"))){
  String line;
  while((line = br.readLine()) != null){
    System.out.println(line);
  }
}
Eric McIntyre
Eric McIntyre•4w ago
In addition to that, the Files class provides many other methods for accessing files. One of these is the readAllLines method which returns a List containing all lines in the file: 
List<String> lines = Files.readAllLines(Path.of("someFile.txt"));
System.out.println(lines.size() + " lines");
System.out.println(lines);
List<String> lines = Files.readAllLines(Path.of("someFile.txt"));
System.out.println(lines.size() + " lines");
System.out.println(lines);
This is also possible using a Stream: 
try(Stream<String> lineStream = Files.lines(Path.of("someFile.txt"))) {
  long nonEmptyLineCount = lineStream
    .filter(line -> !line.isEmpty())
    .count();
  System.out.println(nonEmptyLineCount);
}
try(Stream<String> lineStream = Files.lines(Path.of("someFile.txt"))) {
  long nonEmptyLineCount = lineStream
    .filter(line -> !line.isEmpty())
    .count();
  System.out.println(nonEmptyLineCount);
}
To read an entire file into memory, the Files.readString and Files.readAllBytes methods can be used: 
byte[] contentAsBytes = Files.readAllBytes(Path.of("someFile.txt"));
String contentAsString = Files.readString(Path.of("someFile.txt"));
byte[] contentAsBytes = Files.readAllBytes(Path.of("someFile.txt"));
String contentAsString = Files.readString(Path.of("someFile.txt"));
The Files.copy method allows copying a file and Files.move can be used to move a file: 
Files.copy(Path.of("someFile.txt"), Path.of("target.txt"));
Files.move(Path.of("someFile.txt"), Path.of("target2.txt"));
Files.copy(Path.of("someFile.txt"), Path.of("target.txt"));
Files.move(Path.of("someFile.txt"), Path.of("target2.txt"));
📖 Sample answer from dan1st
Eric McIntyre
Eric McIntyre•4w ago
To read a file we need the File object as and pass that file to good old Scanner which will traverse that file as it would to any normal console input. Since we might be passing a file that doesn't exist so we also need to handle the exception import java.io.*; import java.util.Scanner; public class Main { public static void main(String[] args) throws FileNotFoundException { File file = new File("src\temp.txt"); Scanner sc = new Scanner(file); while (sc.hasNext()){ System.out.println(sc.nextLine()); } } }
Submission from lunatic69noob
Eric McIntyre
Eric McIntyre•4w ago
import java.io.*; public class FileReaderExample { public static void main(String[] args) { try (BufferedReader reader = new BufferedReader(new FileReader("example.txt"))) { String line; while ((line = reader.readLine()) != null) { System.out.println(line); } } catch (IOException e) { e.printStackTrace(); } } }
Submission from chillkillsayeh
Eric McIntyre
Eric McIntyre•4w ago
you can use serialization and de-serialization mechanism to read and write data in to a file from java program. it directly write contents of an object into a file using FileInputStream , FileOutputStreams , ObjectInputStream and ObjectOutputStream Eg: in case of an employee object
Eric McIntyre
Eric McIntyre•4w ago
No description
No description
Eric McIntyre
Eric McIntyre•4w ago
this reads and store object data in file in a serialized way
Submission from nyx.enigmatic
Eric McIntyre
Eric McIntyre•4w ago
How can one read the contents of a file from inside a Java program? In this short tutorial we review multiples ways to read the content of a file with Java. I am using Java 21 for the code examples but the code is compatible with old java versions. If you are continue using Java 8 in 2025, please consider updating your Java version, this 2025 year we have a new Java LTS release, Java 25. I will start with my favorite one, reading the content of a .txt file with Java. Imagine that we have a txt file with the next content, I included the line number of each line in the file: 
1.The angel from my nightmare
2.The shadow in the background of the morgue
3.The unsuspecting victim of darkness in the valley
4.We can live like Jack and Sally, if we want
5.Where you can always find me
6.And we'll have Halloween on Christmas
7.And, in the night, we'll wish this never ends
8.We'll wish this never ends.
1.The angel from my nightmare
2.The shadow in the background of the morgue
3.The unsuspecting victim of darkness in the valley
4.We can live like Jack and Sally, if we want
5.Where you can always find me
6.And we'll have Halloween on Christmas
7.And, in the night, we'll wish this never ends
8.We'll wish this never ends.
Files.readAllLines The next is the Java program to read the content of the file line by line: 
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.List;

public class Example1 {
    public static void main(String[] args) {
        String filePath = "data.txt";

        try {
            List<String> lines = Files.readAllLines(Paths.get(filePath));

            for (String line : lines) {
                System.out.println(line);
            }
        } catch (IOException e) {
            System.out.println("An error ocurred reading the file: " + e.getMessage());
        }
    }
}
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.List;

public class Example1 {
    public static void main(String[] args) {
        String filePath = "data.txt";

        try {
            List<String> lines = Files.readAllLines(Paths.get(filePath));

            for (String line : lines) {
                System.out.println(line);
            }
        } catch (IOException e) {
            System.out.println("An error ocurred reading the file: " + e.getMessage());
        }
    }
}
If you are using Java 11+, you can execute the command java Example1.java to compile and execute the program in one step, the nex is the output code: 
1.The angel from my nightmare
2.The shadow in the background of the morgue
3.The unsuspecting victim of darkness in the valley
4.We can live like Jack and Sally, if we want
5.Where you can always find me
6.And we'll have Halloween on Christmas
7.And, in the night, we'll wish this never ends
8.We'll wish this never ends.
1.The angel from my nightmare
2.The shadow in the background of the morgue
3.The unsuspecting victim of darkness in the valley
4.We can live like Jack and Sally, if we want
5.Where you can always find me
6.And we'll have Halloween on Christmas
7.And, in the night, we'll wish this never ends
8.We'll wish this never ends.
The code is simple, we use the Files.readAllLines method to read all the lines of the file and store them in a List<String>. Then we iterate over the list and print each line. BufferedReader The next code example shows how to read the content of a file using a BufferedReader we use the same .txt file. This pattern is more efficient for reading large files because with Files.readAllLines we are storing the whole file content in memory at once, and with BufferedReader we are reading line by line. 
import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;

public class Example2 {
 public static void main(String[] args) {
  String filePath = "data.txt";
  try (BufferedReader br = new BufferedReader(new FileReader(filePath))) {
   String line;
   while ((line = br.readLine()) != null) {
    System.out.println(line);
   }

  } catch (FileNotFoundException e) {
   System.out.println("The file cannot be found: " + e.getMessage());
  } catch (IOException e) {
   System.out.println("An error ocurred reading the file: " + e.getMessage());
  }
 }
}
import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;

public class Example2 {
 public static void main(String[] args) {
  String filePath = "data.txt";
  try (BufferedReader br = new BufferedReader(new FileReader(filePath))) {
   String line;
   while ((line = br.readLine()) != null) {
    System.out.println(line);
   }

  } catch (FileNotFoundException e) {
   System.out.println("The file cannot be found: " + e.getMessage());
  } catch (IOException e) {
   System.out.println("An error ocurred reading the file: " + e.getMessage());
  }
 }
}
when you execute the program, the output will be the same as the previous example: Files.readAllBytes This time we read the file content in bytes format, this pattern of saving the file content in a variable is very common used when saving the file content in a database or sending the content through a network: 
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;

public class Example3 {
    public static void main(String[] args) {        
        String filePath = "data.txt";
        
        try {
            byte[] contentBytes = Files.readAllBytes(Paths.get(filePath));
            
            for (byte b : contentBytes) {
                char letter = (char) b;
                System.out.println(letter);
            }
            
        } catch (IOException e) {
            System.out.println("An error ocurred reading the file: " + e.getMessage());
        }
    }
}
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;

public class Example3 {
    public static void main(String[] args) {        
        String filePath = "data.txt";
        
        try {
            byte[] contentBytes = Files.readAllBytes(Paths.get(filePath));
            
            for (byte b : contentBytes) {
                char letter = (char) b;
                System.out.println(letter);
            }
            
        } catch (IOException e) {
            System.out.println("An error ocurred reading the file: " + e.getMessage());
        }
    }
}
The output will be the content of the file in bytes format like the next: 
1
.
T
h
e

a
n
g
e
l

f
r
o
m
1
.
T
h
e

a
n
g
e
l

f
r
o
m
This time I am not pasting the whole output because it is too long, we are reading one character one by one. Remember that spaces and line breaks are also characters. At the end we have all the file content in a byte[] variable.
Eric McIntyre
Eric McIntyre•4w ago
FileInputStream 
import java.io.ByteArrayOutputStream;
import java.io.FileInputStream;
import java.io.IOException;

public class Example4 {
    public static void main(String[] args) {
        String filePath = "data.txt";

        int bufferSize = 100; // number of bytes

        try (FileInputStream fis = new FileInputStream(filePath);
                ByteArrayOutputStream baos = new ByteArrayOutputStream()) {
            byte[] buffer = new byte[bufferSize];
            int bytesRead;

            while ((bytesRead = fis.read(buffer)) != -1) {
                baos.write(buffer, 0, bytesRead);
                // do something with each chunk
            }

            byte[] fileContent = baos.toByteArray();
            System.out.println(new String(fileContent));

        } catch (IOException e) {
            System.out.println("An error ocurred reading the file: " + e.getMessage());
        }

    }
}
import java.io.ByteArrayOutputStream;
import java.io.FileInputStream;
import java.io.IOException;

public class Example4 {
    public static void main(String[] args) {
        String filePath = "data.txt";

        int bufferSize = 100; // number of bytes

        try (FileInputStream fis = new FileInputStream(filePath);
                ByteArrayOutputStream baos = new ByteArrayOutputStream()) {
            byte[] buffer = new byte[bufferSize];
            int bytesRead;

            while ((bytesRead = fis.read(buffer)) != -1) {
                baos.write(buffer, 0, bytesRead);
                // do something with each chunk
            }

            byte[] fileContent = baos.toByteArray();
            System.out.println(new String(fileContent));

        } catch (IOException e) {
            System.out.println("An error ocurred reading the file: " + e.getMessage());
        }

    }
}
The output will be the content of the file in chunks of bytes. This pattern is very efficient when you want to do something with each chunk of bytes, for example maybe are you uploading a big file like a video file to a Object Storage so you can upload the file in chunks rather than a big operation. These ways of reading a file in Java are the most common that I saw in my professional career, I know that there are other ways of reading a file, but to cover all of them in this tutorial will be too long.
⭐ Submission from pablohdz

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