Java FileInputStream

last modified March 5, 2024

In this article we show how to use FileInputStream class to read files in Java.

FileInputStream reads input bytes from a file in a file system.

The constructors

These are FileInputStream constructors:

• FileInputStream(File file) — creates a file input stream to read from a File object.
• FileInputStream(String name) — creates a file input stream to read from the specified file name.
• FileInputStream(FileDescriptor fdObj) — creates a file input read from the specified file descriptor.

The close method

The FileInputStream's close method closes the file input stream and releases any system resources associated with this stream. In our examples we use try-with-resources statement, which ensures that each resource is closed at the end of the statement.

The read methods

FileInputStream reads bytes with the following read methods :

• read(byte[] b) — reads up to b.length bytes of data from this input stream into an array of bytes.
• read(byte[] b, int off, int len) — reads up to len bytes of data from this input stream into an array of bytes.
• read — reads one byte from the file input stream.

Reading characters

The following example uses FileInputStream to read three characters from a file.

sky blue notice word

In the small text file we have four English words.

import java.io.FileInputStream;

void main() throws Exception {

    String fname = "smallfile.txt";

    try (var fis = new FileInputStream(fname)) {

        char c1 = (char) fis.read();
        char c2 = (char) fis.read();
        char c3 = (char) fis.read();

        System.out.println(c1);
        System.out.println(c2);
        System.out.println(c3);
    }
}

The code example reads three characters with read.

char c1 = (char) fis.read();

We read a character with read and cast the value into a char.

System.out.println(c1);

The character is printed to the console.

Reading file by characters

The read method returns -1 if the end of the file is reached. With a while loop we can read a whole file character by character. Note that this way is not very efficient.

sky blue notice word

We have four English words in the text file.

import java.io.FileInputStream;

void main() throws Exception {
    
    String fname = "smallfile.txt";

    try (var fis = new FileInputStream(fname)) {

        int i; 
        
        while ((i = fis.read()) != -1) {
            System.out.print((char) i);
        }
    }        
    
    System.out.println();
}

The example reads the contents of a file and writes it to the terminal.

while ((i = fis.read()) != -1) {
    System.out.print((char) i);
}

In a while loop we read a character from a FileInputStream until the read method returns -1.

---

If the file is a text file and contains non-latin characters, we wrap the FileInputStream into an InputStreamReader. The InputStreamReader accepts the character encoding as the second parameter.

sky blue notice буква čerešňa

We have three English words, one Russian, and one Slovak.

import java.io.FileInputStream;
import java.io.InputStreamReader;
import java.nio.charset.StandardCharsets;

void main() throws Exception {

    String fname = "smallfile2.txt";

    try (var fis = new InputStreamReader(new FileInputStream(fname),
            StandardCharsets.UTF_8)) {

        int i;

        while ((i = fis.read()) != -1) {
            System.out.print((char) i);
        }
    }

    System.out.println();
}

The example reads from a small file character by character. The file also contains non-latin characters.

$ java Main.java
sky blue notice буква čerešňa

Reading file by text chunks

It is more efficient to read a file by data chunks; for instance 1024 bytes in each method call.

import java.io.FileInputStream;
import java.nio.charset.StandardCharsets;

void main() throws Exception {

    String fname = "bigfile.txt";

    try (var fis = new FileInputStream(fname)) {

        int i = 0;

        do {

            byte[] buf = new byte[1024];
            i = fis.read(buf);
            
            String value = new String(buf, StandardCharsets.UTF_8);
            System.out.print(value);

        } while (i != -1);
    }
}

In this example we read a file by data chunks

byte[] buf = new byte[1024];

We read data from a file into this array of bytes.

i = fis.read(buf);

The read method reads up to b.length bytes of data from this the stream into the provided array of bytes.

String value = new String(buf, StandardCharsets.UTF_8);

From the array of bytes, we create a String. We specify the charset used.

Using BufferedReader

Reading is more efficient with BufferedReader. BufferedReader reads text from a character-input stream, buffering characters so as to provide for the efficient reading of characters.

import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.InputStreamReader;
import java.nio.charset.StandardCharsets;

void main() throws Exception {

    String fname = "bigfile.txt";

    try (var br = new BufferedReader(new InputStreamReader(
            new FileInputStream(fname), StandardCharsets.UTF_8));) {

        String line;
        
        while ((line = br.readLine()) != null) {
            
            System.out.println(line);
        }
    }

    System.out.println();
}

The example reads a big file using buffering technique for greater efficiency.

while ((line = br.readLine()) != null) {

The readLine method reads a line of text from a buffer.

Source

Java FileInputStream - language reference

In this article we have presented the Java FileInputStream class.

Author

My name is Jan Bodnar and I am a passionate programmer with many years of programming experience. I have been writing programming articles since 2007. So far, I have written over 1400 articles and 8 e-books. I have over eight years of experience in teaching programming.

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