This package contains the input/output facilities (i.e classes) that are used to read input or write output
Stream
A stream is a collection of memory cells.
A stream is connected to a data source or a device like a file, keyboard, network computer, monitor, printer
A stream is the mediator between the datasource/device and a program, i.e the program reads data from the stream or the program writes the data to the stream.
Inputstream.
The stream from which the program reads the data is called inputstream
Outputstream.
The stream to which the program writes data is called outputstream
Streams in java.
Streams in java are objects of the relevant stream classes.
For example
BufferedInputStream is a stream class
BufferedOutputStream is a stream class
BufferedInputStream bis;
BufferedOutputStream bout;
bis and bout are objects (stream objects).
All stream classes are a part of "java.io" package
Types of streams
There are two types
(i) Byte streams
(ii)Character streams
Byte streams are used to read and write binary data (8 bits data)
Character streams are used to read and write character data (supports reading and writing 16 bits character data)
The super classes of Byte stream category
InputStream and OutputStream
The super classes of character stream category
Reader and Writer
few class names
BufferedReader
BufferedWriter
FileReader
FileWriter
BufferedInputStream
FileInputStream
FileOutputStream
The input stream "System.in" is connected to k/b.
the "InputStreamReader" class. Object of this class is used convert bytes to characters.
the "BufferedReader" class. This is the subclass of "Reader" used to read from buffer.
the "read()" method .
This method is a member of "InputStream" and "Reader" classes.
This is used to read one character from the stream and return an int.
This method throws IOException
when the end of stream is reached this method returns -1
Example
import java.io.*;
class ReadDemo
{
public static void main(String args[]) throws Exception
{
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
System.out.println("Enter few characters (CTRL-C to stop)");
int i=0;
do
{
i = br.read();
if(i != -1)
Sop((char)i);
} while(i != -1);
}
}
Example
import java.io.*;
class ReadLineDemo
{
public static void main(String args[]) throws Exception
{
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
int num1=0,num2=0;
String temp="";
System.out.println("Enter first number");
temp = br.readLine();
num1 = Integer.parseInt(temp);
System.out.println("Enter other number");
temp = br.readLine();
num2 = Integer.parseInt(temp);
Sop(num1+num2);
System.out.println("Enter few characters (CTRL-C to stop)");
int i=0;
do
{
i = br.read();
if(i != -1)
Sop((char)i);
} while(i != -1);
}
}
The "FileInputStream" and "FileOutputStream" classes
These are used in working with files.
import java.io.*;
class FileCopy
{
public static void main(String args[]) throws Exception
{
FileInputStream in = new FileInputStream("test.txt");
FileOutputStream out = new FileOutputStream("test2.txt");
int i=0;
do
{
i = in.read();
if(i != -1)
out.write(i);
} while(i!=-1);
System.out.println("file copied...");
}
}
}
The "DataInputStream" and "DataOutputStream" classes
These are used to read and write primitive data types like int, char and float etc
DataInputStream class contains methods like readInt(),readByte() etc for reading int s , byte s
DataOutputStream class contains methods like writeInt(), writeByte() etc for writing int s, byte s import java.io.*;
class DIS
{
public static void main(String args[]) throws Exception
{
DataInputStream in = new DataInputStream(System.in);
DataOutputStream out = new DataOutputStream(System.out);
System.out.println("Enter a number ");
int x= in.readInt();
out.writeInt(x);
}
}
Object's state.
An object's state is the data of instance variables of an object
For example
Student s1 = new Student();
s1.sno=10;
s1.sname="srini";
the values of "sno" and "sname" (10 and srini) of "s1" are together called as object's state
Serialization.
It is the process of saving an object's state to a storage
The "java.io.Serializable" interface
This interface has no methods. interfaces which contain no methods are called marker interfaces.
only objects those classes which implement "Serializable" interface can be serialized
the "java.io.ObjectInputStream" class and "java.io.ObjectOutputStream" class
These classes contain the methods used to read a serialized object and write an object's state toa storage
Example.
ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("student.ser"));
ObjectInputStream in = new ObjectInputStream(new FileInputStream("student.ser"));
the "readObject()" and "writeObject()" methods of the "ObjectInputStream" and "ObjectOutputStream" classes
These methods are used to read an object and write an object
Example.
Student s1= new Student();
s1.sno=10; s1.sname="Ravi";
out.writeObject(s1);
Student s = (Student) in.readObject();
assuming that "out" and "in" are objects of "ObjectOutputStream" and "ObjectInputStream" classes
import java.io.*;
class Student implements Serializable
{
int studentNo;
String studentName;
Student(){}
Student(int studentNo,String studentName)
{
this.studentNo = studentNo;
this.studentName = studentName;
}
void printStudent()
{
System.out.println(studentNo+":"+studentName); }
}
class SerializeDemo
{
public static void main(String args[]) throws Exception
{
ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("student.ser"));
Student s1 = new Student(10,"Ravi");
out.writeObject(s1);
System.out.println("object Serialzed..");
System.out.println("read that object from another program...");
}
}
import java.io.*;
class ReadSerializedObject
{
public static void main(String args[]) throws Exception
{
ObjectInputStream in = new ObjectInputStream(newFileInputStream("student.ser"));
Student s1 = (Student) in.readObject();
s1.printStudent();
}
}
The "transient" keyword
if we declare a variable using this keyword, then that variable will not serialized
ex
class Student
{
int sno;
transient int marks; // not serialized }
static variables are NOT SERIALIZED if a class implements "Serializable" interface then all its subclasses automatically become serializable
ex
class A implements Serializable
{ }
class B extends A { }
class C extends B { }
class B and C automatically become serializable
the "NotSerializableException" class
if a class contains an object of other class that is NOT IMPLEMENTING the "Serializable" interface and if we try to serialize the source object that we get an exception called "NotSerializableException" at runtime
ex.
class A { }
class B implements Serializable
{
A a1 = new A(); // exception A a2; // no exception only reference transient A a3=new A(); // no exception
//because transient }
The "java.io.Externalizable" interface
This interface extends the "Serializable" interface and declares two methods "readExternal()" and "writeExternal()"
This interface and its methods are used to control serialization. i.e we can write custom code in "readExternal()" and "writeExternal()" methods which are automatically executed when "readObject()" and "writeObject()" are executed.
class A implements Externalizable
{
public void readExternal() throws IOException,ClassNotFoundException
{
some code
}
public void writeExternal() throws IOException,ClassNotFoundException
{
some code
}
}
A a1 = new A();
out.writeObject(a1);
A temp = (A) in.readObject();
the "java.io.PrintWriter" class
This is a subclass of "Writer"
This is a convenient class for writing output
ex.
PrintWriter pw = new PrintWriter(System.out,true);
pw.println("hello");
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