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

Question

Someone explain to me what InputStream and OutputStream are?

I am confused about the use cases for both InputStream and

Answer 1

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).

Answer 2

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