What is InputStream & Output Stream? Why and when do we use them?

The goal of InputStream and OutputStream is to abstract different ways to input and output: whether the stream is a file, a web page, or the screen shouldn't matter. All that matters is that you receive information from the stream (or send information into that stream.)

InputStream is used for many things that you read from.

OutputStream is used for many things that you write to.

Here's some sample code. It assumes the InputStream instr and OutputStream osstr have already been created:

int i;

while ((i = instr.read()) != -1) {
    osstr.write(i);
}

instr.close();
osstr.close();

---

InputStream is used for reading, OutputStream for writing. They are connected as decorators to one another such that you can read/write all different types of data from all different types of sources.

For example, you can write primitive data to a file:

File file = new File("C:/text.bin");
file.createNewFile();
DataOutputStream stream = new DataOutputStream(new FileOutputStream(file));
stream.writeBoolean(true);
stream.writeInt(1234);
stream.close();

To read the written contents:

File file = new File("C:/text.bin");
DataInputStream stream = new DataInputStream(new FileInputStream(file));
boolean isTrue = stream.readBoolean();
int value = stream.readInt();
stream.close();
System.out.printlin(isTrue + " " + value);

You can use other types of streams to enhance the reading/writing. For example, you can introduce a buffer for efficiency:

DataInputStream stream = new DataInputStream(
    new BufferedInputStream(new FileInputStream(file)));

You can write other data such as objects:

MyClass myObject = new MyClass(); // MyClass have to implement Serializable
ObjectOutputStream stream = new ObjectOutputStream(
    new FileOutputStream("C:/text.obj"));
stream.writeObject(myObject);
stream.close();

You can read from other different input sources:

byte[] test = new byte[] {0, 0, 1, 0, 0, 0, 1, 1, 8, 9};
DataInputStream stream = new DataInputStream(new ByteArrayInputStream(test));
int value0 = stream.readInt();
int value1 = stream.readInt();
byte value2 = stream.readByte();
byte value3 = stream.readByte();
stream.close();
System.out.println(value0 + " " + value1 + " " + value2 + " " + value3);

For most input streams there is an output stream, also. You can define your own streams to reading/writing special things and there are complex streams for reading complex things (for example there are Streams for reading/writing ZIP format).

---

From the Java Tutorial:

A stream is a sequence of data.

A program uses an input stream to read data from a source, one item at a time:

https://i.stack.imgur.com/7IT8V.gif

A program uses an output stream to write data to a destination, one item at time:

https://i.stack.imgur.com/qzDzU.gif

The data source and data destination pictured above can be anything that holds, generates, or consumes data. Obviously this includes disk files, but a source or destination can also be another program, a peripheral device, a network socket, or an array.

Sample code from oracle tutorial:

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;

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

        FileInputStream in = null;
        FileOutputStream out = null;

        try {
            in = new FileInputStream("xanadu.txt");
            out = new FileOutputStream("outagain.txt");
            int c;

            while ((c = in.read()) != -1) {
                out.write(c);
            }
        } finally {
            if (in != null) {
                in.close();
            }
            if (out != null) {
                out.close();
            }
        }
    }
}

This program uses byte streams to copy xanadu.txt file to outagain.txt , by writing one byte at a time

Have a look at this SE question to know more details about advanced Character streams, which are wrappers on top of Byte Streams :

byte stream and character stream

---

you read from an InputStream and write to an OutputStream.

for example, say you want to copy a file. You would create a FileInputStream to read from the source file and a FileOutputStream to write to the new file.

If your data is a character stream, you could use a FileReader instead of an InputStream and a FileWriter instead of an OutputStream if you prefer.

InputStream input = ... // many different types
OutputStream output = ... // many different types

byte[] buffer = new byte[1024];
int n = 0;
while ((n = input.read(buffer)) != -1)
    output.write(buffer, 0, n);

input.close();
output.close();

---

OutputStream is an abstract class that represents writing output. There are many different OutputStream classes, and they write out to certain things (like the screen, or Files, or byte arrays, or network connections, or etc). InputStream classes access the same things, but they read data in from them.

Here is a good basic example of using FileOutputStream and FileInputStream to write data to a file, then read it back in.

---

A stream is a continuous flow of liquid, air, or gas.

Java stream is a flow of data from a source into a destination. The source or destination can be a disk, memory, socket, or other programs. The data can be bytes, characters, or objects. The same applies for C# or C++ streams. A good metaphor for Java streams is water flowing from a tap into a bathtub and later into a drainage.

The data represents the static part of the stream; the read and write methods the dynamic part of the stream.

InputStream represents a flow of data from the source, the OutputStream represents a flow of data into the destination. Finally, InputStream and OutputStream are abstractions over low-level access to data, such as C file pointers.

---

Stream: In laymen terms stream is data , most generic stream is binary representation of data.

Input Stream : If you are reading data from a file or any other source , stream used is input stream. In a simpler terms input stream acts as a channel to read data.

Output Stream : If you want to read and process data from a source (file etc) you first need to save the data , the mean to store data is output stream .

---

An output stream is generally related to some data destination like a file or a network etc.In java output stream is a destination where data is eventually written and it ends

import java.io.printstream;

class PPrint {
    static PPrintStream oout = new PPrintStream();
}

class PPrintStream {
    void print(String str) { 
        System.out.println(str)
    }
}

class outputstreamDemo {
    public static void main(String args[]) {
        System.out.println("hello world");
        System.out.prinln("this is output stream demo");
    }
}

---

For one kind of InputStream, you can think of it as a "representation" of a data source, like a file. For example:

FileInputStream fileInputStream = new FileInputStream("/path/to/file/abc.txt");

fileInputStream represents the data in this path, which you can use read method to read bytes from the file.

For the other kind of InputStream, they take in another inputStream and do further processing, like decompression. For example:

GZIPInputStream gzipInputStream = new GZIPInputStream(fileInputStream);

gzipInputStream will treat the fileInputStream as a compressed data source. When you use the read(buffer, 0, buffer.length) method, it will decompress part of the gzip file into the buffer you provide.

The reason why we use InputStream because as the data in the source becomes larger and larger, say we have 500GB data in the source file, we don't want to hold everything in the memory (expensive machine; not friendly for GC allocation), and we want to get some result faster (reading the whole file may take a long time).

The same thing for OutputStream. We can start moving some result to the destination without waiting for the whole thing to finish, plus less memory consumption.

If you want more explanations and examples, you have check these summaries: InputStream, OutputStream, How To Use InputStream, How To Use OutputStream

---

In continue to the great other answers, in my simple words:

Stream - like mentioned @Sher Mohammad is data.

Input stream - for example is to get input – data – from the file. The case is when I have a file (the user upload a file – input) – and I want to read what we have there.

Output Stream – is the vice versa. For example – you are generating an excel file, and output it to some place.

The “how to write” to the file, is defined at the sender (the excel workbook class) not at the file output stream.

See here example in this context.

try (OutputStream fileOut = new FileOutputStream("xssf-align.xlsx")) {
    wb.write(fileOut);
}
wb.close();

---