Java FileInputStream

last modified January 10, 2023

Java FileInputStream tutorial shows how to use FileInputStream class to read files in Java.

Java FileInputStream

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

Java FileInputStream 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.

Java FileInputStream close

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.

Java FileInputStream read

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.

Java FileInputStream reading characters

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

package com.zetcode;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;

public class FileInputStreamEx {

    public static void main(String[] args) throws FileNotFoundException, IOException {

        String fileName = "/home/janbodnar/tmp/smallfile.txt";

        try (FileInputStream fis = new FileInputStream(fileName)) {

            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.

Java FileInputStream 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.

package com.zetcode;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;

public class FileInputStreamEx2 {

    public static void main(String[] args) throws FileNotFoundException, IOException {
        
        String fileName = "/home/janbodnar/tmp/smallfile.txt";

        try (FileInputStream fis = new FileInputStream(fileName)) {

            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.

Java FileInputStream reading file by text chunks

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

package com.zetcode;

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

public class FileInputStreamEx3 {

    public static void main(String[] args) throws FileNotFoundException, IOException {

        String fileName = "/home/janbodnar/tmp/bigfile.txt";

        try (FileInputStream fis = new FileInputStream(fileName)) {

            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.

Java FileInputStream with 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.

package com.zetcode;

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

public class FileInputStreamEx4 {

    public static void main(String[] args) throws FileNotFoundException, IOException {

        String fileName = "/home/janbodnar/tmp/bigfile.txt";

        try (BufferedReader br = new BufferedReader(new InputStreamReader(
                new FileInputStream(fileName), 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.

In this article, we have presented the Java FileInputStream class.

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