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Java InputStream tutorial

Java InputStream tutorial shows how to work with InputStream class in Java.

Java stream is a flow of data from a source or into a destination. A good metaphor for Java streams is water flowing from a tap into a bathtub and later into a drainage. InputStream and OutputStream are abstractions over low-level access to data, such as C file pointers.

Java InputStream

InputStream is a source for reading data. A stream can represent various kinds of sources, including disk files, devices, other programs, and memory arrays.

Streams support many different types of data, including simple bytes, primitive data types, localized characters, and objects.

Java InputStream subclasses

InputStream is an abstract class; it is a superclass for all classes representing an input stream of bytes, including AudioInputStream, ByteArrayInputStream, FileInputStream, FilterInputStream, ObjectInputStream, PipedInputStream, and SequenceInputStream.

Java InputStream close

The FileInputStream's close() method closes the 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 InputStream read

InputStream reads bytes with the following read methods :

Java InputStream read text

The following example shows how to read a text file with InputStream.

thermopylae.txt
The Battle of Thermopylae was fought between an alliance of Greek city-states, 
led by King Leonidas of Sparta, and the Persian Empire of Xerxes I over the 
course of three days, during the second Persian invasion of Greece. 

In the example, we use this text file.

JavaInputStreamText.java
package com.zetcode;

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

public class JavaInputStreamText {

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

        String fileName = "src/resources/thermopylae.txt";

        try (InputStream fis = new FileInputStream(fileName);
                InputStreamReader isr = new InputStreamReader(fis,
                        StandardCharsets.UTF_8);
                BufferedReader br = new BufferedReader(isr)) {

            br.lines().forEach(line -> System.out.println(line));
        }
    }
}

The text file is read with FileInputStream, InputStreamReader, and BufferedReader.

try (InputStream fis = new FileInputStream(fileName);

FileInputStream is a specialization of the InputStream for reading bytes from a file.

InputStreamReader isr = new InputStreamReader(fis,
                        StandardCharsets.UTF_8);

InputStreamReader is a bridge from byte streams to character streams: it reads bytes and decodes them into characters using a specified charset.

BufferedReader br = new BufferedReader(isr)) {

BufferedReader reads text from a character-input stream, buffering characters for efficient reading of characters, arrays, and lines.

br.lines().forEach(line -> System.out.println(line));

The data is read by lines from a buffered reader.

Java InputStream read bytes

The read methods of InputStream read bytes.

JavaInputStreamBytes.java
package com.zetcode;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;

public class JavaInputStreamBytes {

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

        String fileName = "src/resources/ball.png";

        try (InputStream is = new FileInputStream(fileName)) {
            
            byte[] buffer = new byte[is.available()];
            is.read(buffer);
            
            int i = 0;
            
            for (byte b: buffer) {
                
                if (i % 10 == 0) {
                    System.out.println();
                }
                
                System.out.printf("%02x ", b);
                
                i++;
            }
        }
        
        System.out.println();
    }
}

The example reads bytes from a PNG image and prints the bytes in hexadecimal format to the console.

try (InputStream is = new FileInputStream(fileName)) {

We use FileInputStream to read bytes from an image file.

byte[] buffer = new byte[is.available()];
is.read(buffer);

With the read() method, we read the bytes into the array of bytes.

int i = 0;

for (byte b: buffer) {
    
    if (i % 10 == 0) {
        System.out.println();
    }
    
    System.out.printf("%02x ", b);
    
    i++;
}

We go through the array and print the bytes to the console in hexadecimal format.

89 50 4e 47 0d 0a 1a 0a 00 00 
00 0d 49 48 44 52 00 00 00 0a 
00 00 00 0a 08 06 00 00 00 8d 
32 cf bd 00 00 00 04 73 42 49 
54 08 08 08 08 7c 08 64 88 00 
00 00 09 70 48 59 73 00 00 0d 
d7 00 00 0d d7 01 42 28 9b 78 
00 00 00 19 74 45 58 74 53 6f 
...

This is a partial sample output of the example.

Java InputStream read from URL

InputStream allows to read data from a URL source.

JavaInputStreamURL.java
package com.zetcode;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URL;

public class JavaInputStreamURL {

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

        String webSite = "http://www.something.com";
        URL url = new URL(webSite);

        try (InputStream is = url.openStream();
                BufferedReader br = new BufferedReader(
                        new InputStreamReader(is))) {

            br.lines().forEach(System.out::println);            
        }
    }
}

The example opens an InputStream to a web page and reads its data.

try (InputStream is = url.openStream();

An InputStream to a URL is created with openStream() method.

<html><head><title>Something.</title></head>
<body>Something.</body>
</html>

This is the output.

Java InputStream read deserialized data

ObjectInputStream reads serialized data previously written using ObjectOutputStream.

Country.java
package com.zetcode;

import java.io.Serializable;

public class Country implements Serializable {
    
    static final long serialVersionUID = 42L;
    
    private String name;
    private int population;

    public Country(String name, int population) {
        this.name = name;
        this.population = population;
    }
    
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getPopulation() {
        return population;
    }

    public void setPopulation(int population) {
        this.population = population;
    }
}

This is the Country bean. We are going to serialize and deserialize a list of countries.

JavaObjectOutputStreamEx.java
package com.zetcode;

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.io.OutputStream;
import java.util.ArrayList;
import java.util.List;

public class JavaObjectOutputStreamEx {

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

        String fileName = "src/resources/myfile.dat";

        try (OutputStream fis = new FileOutputStream(fileName);
                ObjectOutputStream out = new ObjectOutputStream(fis)) {

            List<Country> countries = new ArrayList<>();
            countries.add(new Country("Slovakia", 5429000));
            countries.add(new Country("Norway", 5271000));
            countries.add(new Country("Croatia", 4225000));
            countries.add(new Country("Russia", 143439000));

            out.writeObject(countries);
        }
    }
}

The example serializes a list of objects.

out.writeObject(countries);

A list of countries is written to the ObjectOutputStream.

JavaInputStreamObjects.java
package com.zetcode;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.ObjectInputStream;
import java.util.List;

public class JavaInputStreamObjects {

    public static void main(String[] args) throws IOException, ClassNotFoundException {
        
        String fileName = "src/resources/myfile.dat";
        
        try (InputStream fis = new FileInputStream(fileName);
                ObjectInputStream oin = new ObjectInputStream(fis)) {
            
            List<Country> countries = (List<Country>) oin.readObject();
            
            countries.forEach(System.out::println);
        }
    }
}

We use the ObjectInputStream to read serialized data.

Java InputStream read sequence of streams

SequenceInputStream represents a sequence of input streams. It allows to read from multiple of ordered streams.

JavaInputStreamSequence.java
package com.zetcode;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.SequenceInputStream;

public class JavaInputStreamSequence {

    public static void main(String[] args) throws IOException {
        
        String fileName1 = "src/resources/myfile.txt";
        String fileName2 = "src/resources/myfile1.txt";
        String fileName3 = "src/resources/myfile2.txt";

        try (InputStream is1 = new FileInputStream(fileName1);
                InputStream is2 = new FileInputStream(fileName2);
                InputStream is3 = new FileInputStream(fileName3);
                SequenceInputStream sis1 = new SequenceInputStream(is1, is2);
                SequenceInputStream sis = new SequenceInputStream(sis1, is3)) {

            int b = sis.read();

            while (b != -1) {
            
                System.out.printf("%c", b);
                b = sis.read();
            }

            System.out.println();
        }
    }
}

The example reads from three FileInputStreams.

try (InputStream is1 = new FileInputStream(fileName1);
        InputStream is2 = new FileInputStream(fileName2);
        InputStream is3 = new FileInputStream(fileName3);
        SequenceInputStream sis1 = new SequenceInputStream(is1, is2);
        SequenceInputStream sis = new SequenceInputStream(sis1, is3)) {

We define three input streams and these streams are placed into SequenceInputStreams.

int b = sis.read();

while (b != -1) {

    System.out.printf("%c", b);
    b = sis.read();
}

We read the data from the streams with read().

In this tutorial, we have presented the Java InputStream class. You might also be interested in the related tutorials: Java InputStreamReader tutorial, Java FileOutputStream tutorial, Java FileInputStream tutorial, Java file time, Java FileWriter tutorial, Java append to file, Reading text files in Java,, and Java tutorial.