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:

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

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