Java IO Streams

Introduction

When developing Java applications, you'll often need to interact with various input/output devices. Input-Output devices are part of the underlying hardware and operating system. Direct interaction with I/O devices using native implementations can increase maintenance complexity when devices or operating systems change.

Solution

Java provides the java.io package API to interact with various input-output devices from Java programs in a platform-independent way.

Key Features

• Multiple Classes for Multiple IO Devices: The IO API provides different classes for different I/O devices
• Easy Device Switching: Change devices by creating objects of different classes; the implementation remains the same
• Exception Handling: All methods and some constructors throw java.io.IOException (checked exception)

Exception Handling Required

Since IOException is a checked exception, you must handle it using try-catch blocks or declare it with the throws keyword.

Standard Streams

The JVM automatically initializes some input-output streams at startup. These are available in the System class:

Reference	Device	Description
System.in	Keyboard	Default input device
System.out	Console	Default output device
System.err	Console	Error output device

Special Characters

When reading from the keyboard, pressing ENTER adds extra ASCII values to the buffer:

• 13 for \r (carriage return - moves cursor to beginning of line)
• 10 for \n (line feed - moves cursor to next line)

You need to ignore these values when creating strings from keyboard input.

Stream Types

A stream is a sequence of bits used for data transfer.

Classification by Operation

Input Streams

Used to read data from various input devices like keyboard, file, network, etc.

Output Streams

Used to write data to various output devices like monitor, file, network, etc.

Classification by Data Type

Byte Stream

Used to read or write byte (ASCII value) data.

Character Stream

Used to read or write character data.

Byte Input Streams

Byte Input Streams are used to read byte data from various input devices.

Abstract Base Class

InputStream is an abstract class and serves as the superclass for all Input Byte Streams.

Byte Input Stream Classes

Available Classes:

• ByteArrayInputStream
• BufferedInputStream
• DataInputStream
• FileInputStream
• ObjectInputStream

Byte Output Streams

Byte Output Streams are used to write byte data to various output devices.

Abstract Base Class

OutputStream is an abstract class and serves as the superclass for all Output Byte Streams.

Byte Output Stream Classes

Available Classes:

• ByteArrayOutputStream
• BufferedOutputStream
• DataOutputStream
• FileOutputStream
• ObjectOutputStream

Character Input Streams

Character Input Streams are used to read character data from various input devices.

Abstract Base Class

Reader is an abstract class and serves as the superclass for all Character Input Streams.

Character Input Stream Classes

Available Classes:

• FileReader
• BufferedReader
• CharArrayReader
• InputStreamReader

Character Output Streams

Character Output Streams are used to write character data to various output devices.

Abstract Base Class

Writer is an abstract class and serves as the superclass for all Character Output Streams.

Character Output Stream Classes

Available Classes:

• FileWriter
• BufferedWriter
• CharArrayWriter
• OutputStreamWriter
• PrintWriter

Stream Conversion

InputStreamReader

No Inheritance Relationship

There is no inheritance relation between InputStream and Reader classes.

InputStreamReader acts as a bridge from byte streams to character streams. It reads bytes and decodes them into characters.

InputStream is = System.in;
Reader reader = new InputStreamReader(is);

OutputStreamWriter

No Inheritance Relationship

There is no inheritance relation between OutputStream and Writer classes.

OutputStreamWriter converts OutputStream objects to Writer objects.

Important

When using Writer objects, you must invoke the flush() method to send data to the output device.

OutputStream os = new FileOutputStream("output.txt");
Writer writer = new OutputStreamWriter(os);
writer.write("Hello");
writer.flush(); // Required!

Buffered Streams

Buffered streams improve performance by reducing the number of I/O operations.

BufferedInputStream

BufferedInputStream(InputStream ins)

BufferedReader

BufferedReader(Reader rd)

Performance Benefit

Buffered streams read data in chunks, reducing the number of expensive I/O operations.

File Operations

Reading from Files

FileInputStream (Byte Stream)

FileInputStream(File f)
FileInputStream(String fname)

FileReader (Character Stream)

FileReader(File f)
FileReader(String fname)

Writing to Files

FileOutputStream (Byte Stream)

FileOutputStream(File f)
FileOutputStream(String fName)
FileOutputStream(File f, boolean append)
FileOutputStream(String fName, boolean append)

FileWriter (Character Stream)

FileWriter(File f)
FileWriter(String fName)
FileWriter(File f, boolean append)
FileWriter(String fName, boolean append)

Append Mode

The boolean append parameter determines whether data is appended to existing file content (true) or overwrites it (false).

Code Examples

Example 1: Reading from Keyboard (Byte Stream)

import java.io.*;

class Lab812 {
    public static void main(String args[]) {
        InputStream is = null;
        BufferedInputStream bis = null;
        try {
            is = System.in;
            bis = new BufferedInputStream(is);
            System.out.println("Enter Data");
            while(true) {
                int mydata = bis.read();
                if(mydata == 13)
                    break;
                char ch = (char) mydata;
                System.out.print(ch);
            }
        } catch(Exception ex) {
            System.err.println(ex);
        } finally {
            try {
                if(bis != null) bis.close();
            } catch(Exception x) {}
        }
    }
}

Example 2: Reading from File (Byte Stream)

import java.io.*;

class Lab813 {
    public static void main(String args[]) {
        InputStream is = null;
        BufferedInputStream bis = null;
        try {
            is = new FileInputStream("D:\\myfiles\\myjlc.txt");
            bis = new BufferedInputStream(is);
            while(true) {
                int mydata = bis.read();
                if(mydata == -1)
                    break;
                char ch = (char) mydata;
                System.out.print(ch);
            }
        } catch(Exception ex) {
            System.err.println(ex);
        } finally {
            try {
                if(is != null) is.close();
                if(bis != null) bis.close();
            } catch(Exception x) {}
        }
    }
}

EOF Detection

The read() method returns -1 when the end of file is reached.

Example 3: Reading from Keyboard (Character Stream)

import java.io.*;

class Lab814 {
    public static void main(String args[]) {
        Reader reader = null;
        BufferedReader br = null;
        try {
            reader = new InputStreamReader(System.in);
            br = new BufferedReader(reader);
            
            System.out.println("Enter CID");
            String cid = br.readLine();
            
            System.out.println("Enter Cname");
            String cname = br.readLine();
            
            System.out.println("Enter Email");
            String email = br.readLine();
            
            System.out.println(cid + "\t" + cname + "\t" + email);
        } catch(Exception ex) {
            System.err.println(ex);
        } finally {
            try {
                if(reader != null) reader.close();
                if(br != null) br.close();
            } catch(Exception x) {}
        }
    }
}

Example 4: Reading from File (Character Stream)

import java.io.*;

class Lab815 {
    public static void main(String args[]) {
        Reader reader = null;
        BufferedReader br = null;
        try {
            reader = new FileReader("D:\\myfiles\\myjlc.txt");
            br = new BufferedReader(reader);
            while(true) {
                String mydata = br.readLine();
                if(mydata == null)
                    break;
                System.out.println(mydata);
            }
        } catch(Exception ex) {
            System.err.println(ex);
        } finally {
            try {
                if(reader != null) reader.close();
                if(br != null) br.close();
            } catch(Exception x) {}
        }
    }
}

Example 5: Copying Files (Byte Stream)

import java.io.*;

class Lab816 {
    public static void main(String args[]) {
        FileInputStream fis = null;
        FileOutputStream fos = null;
        try {
            fis = new FileInputStream("D:\\myfiles\\myjlc1.txt");
            // Append mode: true
            fos = new FileOutputStream("D:\\myfiles\\hello.txt", true);
            // Overwrite mode: false
            // fos = new FileOutputStream("D:\\myfiles\\hello.txt", false);
            
            while(true) {
                int asc = fis.read();
                if(asc == -1) break;
                fos.write(asc);
            }
            System.err.println("Done!!!");
        } catch(Exception ex) {
            System.err.println(ex);
        } finally {
            try {
                if(fis != null) fis.close();
                if(fos != null) fos.close();
            } catch(Exception x) {}
        }
    }
}

Example 6: Copying Files (Character Stream)

import java.io.*;

class Lab817 {
    public static void main(String args[]) {
        FileReader fr = null;
        BufferedReader br = null;
        FileWriter fw = null;
        BufferedWriter bw = null;
        try {
            fr = new FileReader("D:\\myfiles\\myjlc1.txt");
            br = new BufferedReader(fr);
            fw = new FileWriter("D:\\myfiles\\hai.txt", true);
            bw = new BufferedWriter(fw);
            
            while(true) {
                String mydata = br.readLine();
                System.out.println(mydata);
                if(mydata == null) break;
                bw.write(mydata);
                bw.newLine();
            }
            bw.flush();
            System.err.println("Done!!!");
        } catch(Exception ex) {
            System.err.println(ex);
        } finally {
            try {
                if(fr != null) fr.close();
                if(br != null) br.close();
                if(fw != null) fw.close();
                if(bw != null) bw.close();
            } catch(Exception x) {}
        }
    }
}

File Class

The File class in the java.io package provides various methods to perform operations on files and directories.

File Object Representation

A File object represents a file or directory on the file system.

Example 7: Listing Root Drives

import java.io.*;

class Lab818 {
    public static void main(String args[]) {
        try {
            File[] myfiles = File.listRoots();
            System.out.println(myfiles.length);
            
            for(File myfile : myfiles) {
                long total = myfile.getTotalSpace() / 1024 / 1024 / 1024;
                long free = myfile.getFreeSpace() / 1024 / 1024 / 1024;
                long usable = myfile.getUsableSpace() / 1024 / 1024 / 1024;
                
                System.out.println("\t" + myfile + "\t" + total + 
                                   "\t" + free + "\t" + usable);
            }
            System.out.println("Done!!!");
        } catch(Exception ex) {
            System.err.println(ex);
        }
    }
}

Example 8: Listing Directory Contents

import java.io.*;

class Lab819 {
    public static void main(String args[]) {
        try {
            File file = new File("D:\\MyJLC2.0");
            File myfiles[] = file.listFiles();
            System.out.println("Total Items: " + myfiles.length);
            
            for(File myfile : myfiles) {
                String fname = myfile.getName();
                boolean isDir = myfile.isDirectory();
                boolean isFile = myfile.isFile();
                boolean isHid = myfile.isHidden();
                
                System.out.println("\t" + fname + "\t" + isDir + 
                                   "\t" + isFile + "\t" + isHid);
            }
            System.out.println("Done!!!");
        } catch(Exception ex) {
            System.err.println(ex);
        }
    }
}

Example 9: Custom File Filter

import java.io.*;

class MyFilter implements FileFilter {
    String str;
    
    public MyFilter(String str) {
        this.str = str;
    }
    
    public boolean accept(File myfile) {
        return myfile.getName().endsWith(str);
    }
}

class Lab820 {
    public static void main(String args[]) {
        try {
            File file = new File("D:\\MyJLC2.0\\Java FSD-B1\\Core Java\\Module-8");
            
            // List Java files
            System.out.println("=== Java Files ===");
            File[] javaFiles = file.listFiles(new MyFilter("java"));
            System.out.println("Total Items: " + javaFiles.length);
            for(File myfile : javaFiles) {
                System.out.println("\t" + myfile.getName());
            }
            
            // List class files
            System.out.println("=== Class Files ===");
            File[] classFiles = file.listFiles(new MyFilter("class"));
            System.out.println("Total Items: " + classFiles.length);
            for(File myfile : classFiles) {
                System.out.println("\t" + myfile.getName());
            }
            
            System.out.println("Done!!!");
        } catch(Exception ex) {
            System.err.println(ex);
        }
    }
}

Example 10: Creating and Deleting Directories

import java.io.*;

class Lab821 {
    public static void main(String args[]) {
        try {
            File myfile = new File("D:\\MyJLC2.0\\DSA");
            System.out.println("\t" + myfile);
            
            boolean flag = myfile.exists();
            System.out.println("\tDSA exists? " + flag);
            
            boolean x = myfile.mkdirs();
            System.out.println("\tDSA Created? " + x);
            
            flag = myfile.exists();
            System.out.println("\tDSA exists? " + flag);
            
            x = myfile.mkdirs();
            System.out.println("\tDSA Created? " + x);
            
            boolean y = myfile.delete();
            System.out.println("\tDSA deleted? " + y);
            
            System.out.println("Done!!!");
        } catch(Exception ex) {
            System.err.println(ex);
        }
    }
}

Example 11: Creating and Deleting Files

import java.io.*;

class Lab822 {
    public static void main(String args[]) {
        try {
            File myfile = new File("D:\\MyJLC2.0\\Hello.java");
            System.out.println("\t" + myfile);
            
            boolean flag = myfile.exists();
            System.out.println("\tHello.java exists? " + flag);
            
            boolean x = myfile.createNewFile();
            System.out.println("\tHello.java Created? " + x);
            
            flag = myfile.exists();
            System.out.println("\tHello.java exists? " + flag);
            
            x = myfile.createNewFile();
            System.out.println("\tHello.java Created? " + x);
            
            boolean y = myfile.delete();
            System.out.println("\tHello.java deleted? " + y);
            
            System.out.println("Done!!!");
        } catch(Exception ex) {
            System.err.println(ex);
        }
    }
}

Example 12: Working with File Paths

import java.io.*;

class Lab823 {
    public static void main(String args[]) {
        try {
            File mypath = new File("D:\\MyJLC2.0");
            File mydir = new File(mypath, "DSA");
            File myfile = new File(mydir, "Hello.java");
            
            System.out.println("\t" + mypath.exists());
            System.out.println("\t" + mydir.exists());
            System.out.println("\t" + myfile.exists());
            
            if(!mydir.exists()) {
                mydir.mkdirs();
            }
            if(!myfile.exists()) {
                myfile.createNewFile();
            }
            
            System.out.println("\tAfter Creating");
            System.out.println("\t" + mypath.exists());
            System.out.println("\t" + mydir.exists());
            System.out.println("\t" + myfile.exists());
            
            System.out.println("Done!!!");
        } catch(Exception ex) {
            System.err.println(ex);
        }
    }
}

Example 13: File Properties and Permissions

import java.io.*;

class Lab824 {
    public static void main(String args[]) {
        try {
            File mypath = new File("D:\\MyJLC2.0");
            File mydir = new File(mypath, "DSA");
            File myfile = new File(mydir, "Hello.java");
            
            System.out.println("\t" + myfile.exists());
            System.out.println("\t" + myfile.getParent());
            System.out.println("\t" + myfile.getCanonicalPath());
            System.out.println("\t" + myfile.getAbsolutePath());
            System.out.println("\t" + myfile.getPath());
            System.out.println("\t" + myfile.canRead());
            System.out.println("\t" + myfile.canWrite());
            System.out.println("\t" + myfile.lastModified());
            System.out.println("\t" + myfile.length());
            
            myfile.setReadOnly();
            System.out.println("\t" + myfile.canRead());
            System.out.println("\t" + myfile.canWrite());
            
            System.out.println("Done!!!");
        } catch(Exception ex) {
            System.err.println(ex);
        }
    }
}

Serialization

Serialization is the process of saving an object's state into a file.

Serialization Process (2 Steps)

1. Reading data from the object
2. Writing data to the file

Deserialization Process (3 Steps)

1. Reading data from the file
2. Creating an object on the remote machine
3. Initializing the object with data from the file

Required Classes

• ObjectOutputStream: Output Byte Stream for serializing objects
• ObjectInputStream: Input Byte Stream for deserializing objects

Key Methods

Class	Method	Description
ObjectOutputStream	void writeObject(Object)	Saves object state to file
ObjectInputStream	Object readObject()	Creates object from file

Use Cases for Serialization

1. Secure Storage: Store object data in a secured format accessible only by authorized Java applications
2. Memory Management: Manage memory required for Java applications
3. Network Transfer: Transfer object data between machines through a network

Serialization Requirements

Important

By default, objects are NOT eligible for serialization.

To serialize a class:

1. The class must implement the java.io.Serializable marker interface
2. If you try to serialize an object without implementing Serializable, a java.io.NotSerializableException will be thrown

serialVersionUID

The serialVersionUID specifies class version information during serialization and is used during deserialization.

Deserialization Behavior

When deserializing an object:

• Static variables get the value defined in the class (static members are not serialized with the object)
• The same .class file used during serialization must be available
• Constructor is NOT invoked during deserialization

Example 14: Basic Serialization

import java.io.*;

class Customer implements Serializable {
    int cid;
    String cname;
    String email;
    long phone;
    
    Customer() {}
    
    Customer(int cid, String cname, String email, long phone) {
        this.cid = cid;
        this.cname = cname;
        this.email = email;
        this.phone = phone;
    }
    
    public String toString() {
        return "[" + cid + "," + cname + "," + email + "," + phone + "]";
    }
}

class Lab825 {
    public static void main(String args[]) throws IOException {
        // serializeCustomer();
        deserializeCustomer();
    }
    
    static void serializeCustomer() throws IOException {
        ObjectOutputStream oos = null;
        FileOutputStream fos = null;
        try {
            fos = new FileOutputStream("mycustomers1.txt");
            oos = new ObjectOutputStream(fos);
            Customer mycust = new Customer(101, "sri", "sri@jlc", 
                                           12345, "sri123", "sri123");
            oos.writeObject(mycust);
            System.err.println("Object is Serialized!!!");
        } catch(Exception ex) {
            System.err.println("ooooooops!!" + ex);
        } finally {
            if(fos != null) fos.close();
            if(oos != null) oos.close();
        }
    }
    
    static void deserializeCustomer() throws IOException {
        ObjectInputStream ois = null;
        FileInputStream fis = null;
        try {
            fis = new FileInputStream("mycustomers3.txt");
            ois = new ObjectInputStream(fis);
            Object obj = ois.readObject();
            Customer mycust = (Customer) obj;
            System.err.println("here the customer=> " + mycust);
            System.err.println("Object is De-Serialized!!!");
        } catch(Exception ex) {
            System.err.println("ooooooops!!" + ex);
        } finally {
            if(fis != null) fis.close();
            if(ois != null) ois.close();
        }
    }
}

Result

When deserializing, username and password will be null because they are marked as transient.

Externalization

Externalization is the process of customizing the serialization process.

Key Differences

Feature	Serializable	Externalizable
Interface Type	Marker interface	Has abstract methods
Control	Automatic	Manual
Methods	None	writeExternal(), readExternal()
Constructor	Not required	Public no-arg constructor required
Performance	Slower	Faster (selective serialization)

Externalizable Interface

The java.io.Externalizable interface contains two abstract methods:

void writeExternal(ObjectOutput out) throws IOException
void readExternal(ObjectInput in) throws IOException, ClassNotFoundException

Method Behavior

• writeExternal(): Called during serialization

  • Receives ObjectOutput stream as parameter
  • Define which fields to serialize

• readExternal(): Called during deserialization

  • Receives ObjectInput stream as parameter
  • Define how to deserialize fields

Required Constructor

Your class MUST have a public no-argument constructor for Externalization to work.

Example 17: Externalization

import java.io.*;

class Customer implements Externalizable {
    int cid;
    String cname;
    String email;
    long phone;
    String username;
    String password;
    
    public Customer() {}
    
    Customer(int cid, String cname, String email, long phone, 
             String username, String password) {
        this.cid = cid;
        this.cname = cname;
        this.email = email;
        this.phone = phone;
        this.username = username;
        this.password = password;
    }
    
    public String toString() {
        return "[" + cid + "," + cname + "," + email + "," + 
               phone + "," + username + "," + password + "]";
    }
    
    public void writeExternal(ObjectOutput myoutput) throws IOException {
        myoutput.writeInt(this.cid + 100);
        myoutput.writeObject(this.cname);
        myoutput.writeObject(this.email);
        myoutput.writeLong(this.phone);
    }
    
    public void readExternal(ObjectInput myinput) 
            throws IOException, ClassNotFoundException {
        this.cid = myinput.readInt() - 100;
        this.cname = myinput.readObject().toString();
        this.email = myinput.readObject().toString();
        this.phone = myinput.readLong();
    }
}

class Lab828 {
    public static void main(String args[]) throws IOException {
        // serializeCustomer();
        deserializeCustomer();
    }
    
    static void serializeCustomer() throws IOException {
        ObjectOutputStream oos = null;
        FileOutputStream fos = null;
        try {
            fos = new FileOutputStream("mycustomers3.txt");
            oos = new ObjectOutputStream(fos);
            Customer mycust = new Customer(101, "sri", "sri@jlc", 
                                           12345, "sri123", "sri123");
            oos.writeObject(mycust);
            System.out.println("Object is Serialized!!!");
        } catch(Exception ex) {
            System.err.println("ooooooops!!" + ex);
        } finally {
            if(fos != null) fos.close();
            if(oos != null) oos.close();
        }
    }
    
    static void deserializeCustomer() throws IOException {
        ObjectInputStream ois = null;
        FileInputStream fis = null;
        try {
            fis = new FileInputStream("mycustomers3.txt");
            ois = new ObjectInputStream(fis);
            Object obj = ois.readObject();
            Customer mycust = (Customer) obj;
            System.err.println("here the customer=> " + mycust);
            System.err.println("Object is De-Serialized!!!");
        } catch(Exception ex) {
            System.err.println("ooooooops!!" + ex);
        } finally {
            if(fis != null) fis.close();
            if(ois != null) ois.close();
        }
    }
}

Custom Logic

In the example above, cid is stored with +100 offset and retrieved with -100 offset. The username and password fields are not serialized at all.

Practice Questions

Assignment Questions

1. What is Stream?
2. What are the types of Streams available in Java?
3. What are the differences between Byte Streams and Character Streams?
4. What are the classes defined to manage Byte Streams?
5. What are the classes defined to manage Character Streams?
6. What is the use of InputStreamReader class?
7. What is the difference between flush() and close() method?
8. What is the use of boolean parameter in FileWriter(File, boolean) constructor?
9. How to find the total number of root drives available in your file system?
10. How to find the number of characters available in a file?
11. How to find if a file is available or not?
12. How to display the list of files and folders available in a directory?
13. What is Serialization?
14. What is the need of implementing Serializable interface in Serialization?
15. What is De-Serialization?
16. What is the use of serialVersionUID?
17. Which class object will be used for Serialization?
18. Which class object will be used for De-Serialization?
19. How to serialize individual properties of the class?
20. How to stop serialization of particular fields?
21. Can I serialize static fields of the class?
22. What is transient keyword?
23. What is Externalization? What is the interface need to implement for Externalization?
24. What are the methods I need to override in case of Externalization?
25. What is the difference between Serialization and Externalization?

Multiple Choice Questions

Q1. Which package contains classes for input & output operations?

Options:

• A) java.util
• B) java.lang
• C) java.io ✓
• D) All of the mentioned

Answer: C) java.io

Q2. Which class is NOT a member of java.io package?

Options:

• A) StringBuffer ✓
• B) StringReader
• C) Writer
• D) File

Answer: A) StringBuffer (belongs to java.lang package)

Q3. Which interface is NOT a member of java.io package?

Options:

• A) DataInput
• B) ObjectInput
• C) ObjectFilter ✓
• D) FileFilter

Answer: C) ObjectFilter

Q4. Which class is NOT related to input/output stream?

Options:

• A) File ✓
• B) Writer
• C) InputStream
• D) Reader

Answer: A) File (represents file paths, not a stream)

Q5. What is specified by a File object?

Options:

• A) a file in disk
• B) directory path ✓
• C) directory in disk
• D) None of the mentioned

Answer: B) directory path

Q6. Legal statements to create a new file (Select all that apply)

Options:

• A) file.create();
• B) FileOutputStream fos = new FileOutputStream(file); ✓
• C) FileWriter out = new FileWriter(file); ✓
• D) FileInputStream fis = new FileInputStream(file);
• E) RandomAccessFile raf = new RandomAccessFile(file); ✓

Answer: B, C, E

Q7. What is used to perform all I/O operations in Java?

Options:

• A) Streams ✓
• B) Variables
• C) Classes
• D) Methods

Answer: A) Streams

Q8. Which is a type of stream in Java?

Options:

• A) Integer stream
• B) Short stream
• C) Byte stream ✓
• D) Long stream

Answer: C) Byte stream

Q9. Which classes are used by Byte streams?

Options:

• A) InputStream ✓
• B) InputOutputStream
• C) Reader
• D) All of the mentioned

Answer: A) InputStream

Q10. Which exception is thrown by read() method?

Options:

• A) IOException ✓
• B) InterruptedException
• C) SystemException
• D) SystemInputException

Answer: A) IOException

Q11. Which method reads a string from input stream?

Options:

• A) get()
• B) getLine()
• C) read()
• D) readLine() ✓

Answer: D) readLine()

Q12. Which class reads characters from console?

Options:

• A) BufferedReader ✓
• B) StringReader
• C) BufferedStreamReader
• D) InputStreamReader

Answer: A) BufferedReader

Q13. Which class reads from a file?

Options:

• A) InputStream
• B) BufferedInputStream
• C) FileInputStream ✓
• D) BufferedFileInputStream

Answer: C) FileInputStream

Q14. What value is returned by read() when EOF is encountered?

Options:

• A) 0
• B) 1
• C) -1 ✓
• D) null

Answer: C) -1

Q15. Correct statement to create BufferedWriter?

Options:

• A) BufferedWriter bw = new BufferedWriter(new File("out.dat"));
• B) BufferedWriter bw = new BufferedWriter(new FileWriter("out.dat")); ✓
• C) BufferedWriter bw = new BufferedWriter("out.dat");
• D) BufferedWriter bw = new BufferedWriter(new FileOutputStream("out.dat"));

Answer: B

Q16. Java Stream classes are categorized into:

Options:

• A) Byte and Character Stream Classes ✓
• B) Stream and String Classes
• C) String and Character Stream Classes
• D) Buffer and Character Stream Classes

Answer: A) Byte and Character Stream Classes

Q17. Which class converts byte data to character data?

Options:

• A) InputStreamReader ✓
• B) OutputStreamReader
• C) StringStreamReader
• D) PrintStreamReader

Answer: A) InputStreamReader

Q18. What is the process of writing object state to byte stream?

Options:

• A) Serialization ✓
• B) Externalization
• C) File Filtering
• D) All of the mentioned

Answer: A) Serialization

Q19. Which interface controls serialization/deserialization?

Options:

• A) Serializable
• B) Externalization ✓
• C) FileFilter
• D) ObjectInput

Answer: B) Externalization (Note: Externalizable is the correct interface name)

Q20. Which method writes object to stream?

Options:

• A) write()
• B) write()
• C) streamWrite()
• D) writeObject() ✓

Answer: D) writeObject()

Q21. Code Output: obj.getName() where obj = new File("/java/system")

Options:

• A) Java
• B) System ✓
• C) java/system
• D) /java/system

Answer: B) System

Q22. Code Output: obj.getAbsolutePath() where obj = new File("/java/system")

Options:

• A) Java
• B) System
• C) java/system
• D) /java/system ✓

Answer: D) /java/system

Q23. Code Output with input "abcqfgh"

BufferedReader obj = new BufferedReader(new InputStreamReader(System.in));
do {
    c = (char) obj.read();
    System.out.print(c);
} while(c != 'q');

Options:

• A) abcqfgh
• B) abc
• C) abcq ✓
• D) abcqfgh

Answer: C) abcq

Q24. What does obj.available() return?

Options:

• A) true
• B) false
• C) prints number of bytes in file ✓
• D) prints number of characters in the file

Answer: C) prints number of bytes in file

Q25. Code Output: System.out.println(arr instanceof Serializable)

int arr[] = new int[1];

Options:

• A) true ✓
• B) false
• C) Compile time error
• D) Runtime error

Answer: A) true (arrays are serializable in Java)

Summary

Key Takeaways

Best Practices

Resource Management

• Always close streams in a finally block
• Use try-with-resources (Java 7+) for automatic resource management
• Close streams in reverse order of opening

Exception Handling

• Handle IOException properly
• Use specific exception types when possible
• Log errors for debugging

Performance

• Use buffered streams for better performance
• Choose appropriate stream types (byte vs character)
• Consider using NIO for large files

Serialization

• Implement Serializable for default behavior
• Use transient for sensitive data
• Implement Externalizable for custom control
• Always provide serialVersionUID

Common Pitfalls

Watch Out!

• Forgetting to flush() when using Writer
• Not closing streams (resource leaks)
• Incorrect deserialization order
• Missing no-arg constructor with Externalizable
• Attempting to serialize non-Serializable objects

Explicit Property Serialization

You can serialize individual properties explicitly using corresponding writeX() and readX() methods:

Order Matters

You must deserialize data in the same order it was serialized.

Serialization and Inheritance

When serializing objects with inherited properties:

Serializable Superclass

✅ If the superclass implements java.io.Serializable:

• Inherited properties WILL be serialized

Non-Serializable Superclass

❌ If the superclass does NOT implement java.io.Serializable:

• Inherited properties will NOT be serialized
• During deserialization:
  • Inherited properties get default values defined in the class
  • Default constructor of the non-serializable superclass will be invoked
  • If default constructor is not available, an exception will be thrown

Example 15: Serialization with Inheritance

import java.io.*;

class User implements Serializable {
    String username;
    String password;
    
    User() {}
    
    User(String username, String password) {
        this.username = username;
        this.password = password;
    }
    
    public String toString() {
        return "," + username + "," + password;
    }
}

class Customer extends User {
    int cid;
    String cname;
    String email;
    long phone;
    
    Customer() {}
    
    Customer(int cid, String cname, String email, long phone) {
        this.cid = cid;
        this.cname = cname;
        this.email = email;
        this.phone = phone;
    }
    
    Customer(int cid, String cname, String email, long phone, 
             String username, String password) {
        this.cid = cid;
        this.cname = cname;
        this.email = email;
        this.phone = phone;
        this.username = username;
        this.password = password;
    }
    
    public String toString() {
        return "[" + cid + "," + cname + "," + email + "," + 
               phone + super.toString() + "]";
    }
}

class Lab826 {
    public static void main(String args[]) throws IOException {
        // serializeCustomer();
        deserializeCustomer();
    }
    
    static void serializeCustomer() throws IOException {
        ObjectOutputStream oos = null;
        FileOutputStream fos = null;
        try {
            fos = new FileOutputStream("mycustomers2.txt");
            oos = new ObjectOutputStream(fos);
            Customer mycust = new Customer(101, "sri", "sri@jlc", 
                                           12345, "sri123", "sri123");
            oos.writeObject(mycust);
            System.err.println("Object is Serialized!!!");
        } catch(Exception ex) {
            System.err.println("ooooooops!!" + ex);
        } finally {
            if(fos != null) fos.close();
            if(oos != null) oos.close();
        }
    }
    
    static void deserializeCustomer() throws IOException {
        ObjectInputStream ois = null;
        FileInputStream fis = null;
        try {
            fis = new FileInputStream("mycustomers2.txt");
            ois = new ObjectInputStream(fis);
            Object obj = ois.readObject();
            Customer mycust = (Customer) obj;
            System.err.println("here the customer=> " + mycust);
            System.err.println("Object is De-Serialized!!!");
        } catch(Exception ex) {
            System.err.println("ooooooops!!" + ex);
        } finally {
            if(fis != null) fis.close();
            if(ois != null) ois.close();
        }
    }
}

The transient Keyword

The transient keyword prevents specific instance variables from being serialized.

Key Points

• Used with instance variables
• Applies during serialization process
• By default, all instance variables are serialized
• Variables marked as transient will NOT be serialized
• During deserialization, transient variables get default values based on their data type

Use Case

Use transient for sensitive data (like passwords) or data that shouldn't be persisted (like cached values).

Lab827: Demonstrating the transient Keyword

This example demonstrates how the transient keyword prevents specific instance variables from being serialized.

Customer Class with Transient Fields

import java.io.*;

class Customer implements Serializable {
    int cid;
    String cname;
    String email;
    long phone;
    transient String username;  // Will NOT be serialized
    transient String password;  // Will NOT be serialized
    
    Customer() {}
    
    Customer(int cid, String cname, String email, long phone, 
             String username, String password) {
        this.cid = cid;
        this.cname = cname;
        this.email = email;
        this.phone = phone;
        this.username = username;
        this.password = password;
    }
    
    public String toString() {
        return "[" + cid + "," + cname + "," + email + "," + 
               phone + "," + username + "," + password + "]";
    }
}

Transient Fields

The username and password fields are marked as transient, which means:

• They will not be saved during serialization
• They will be null after deserialization
• This is useful for sensitive data or temporary values

Main Lab827 Class

class Lab827 {
    public static void main(String args[]) throws IOException {
        // Uncomment to serialize
        // serializeCustomer();
        
        // Uncomment to deserialize
        deserializeCustomer();
    }
    
    /**
     * Serializes a Customer object to a file.
     * Note: transient fields (username, password) will NOT be saved.
     */
    static void serializeCustomer() throws IOException {
        ObjectOutputStream oos = null;
        FileOutputStream fos = null;
        try {
            fos = new FileOutputStream("mycustomers3.txt");
            oos = new ObjectOutputStream(fos);
            
            // Create customer with all fields including transient ones
            Customer mycust = new Customer(101, "sri", "sri@jlc", 
                                           12345, "sri123", "sri123");
            
            // Serialize the object
            oos.writeObject(mycust);
            
            System.err.println(" Object is Serialized !!! ");
        } catch(Exception ex) {
            System.err.println("ooooooops!!" + ex);
        } finally {
            if(fos != null)
                fos.close();
            if(oos != null)
                oos.close();
        }
    }
    
    /**
     * Deserializes a Customer object from a file.
     * Note: transient fields will be null after deserialization.
     */
    static void deserializeCustomer() throws IOException {
        ObjectInputStream ois = null;
        FileInputStream fis = null;
        try {
            fis = new FileInputStream("mycustomers3.txt");
            ois = new ObjectInputStream(fis);
            
            // Read the object
            Object obj = ois.readObject();
            Customer mycust = (Customer) obj;
            
            // Print the customer - username and password will be null
            System.err.println("here the customer=> " + mycust);
            System.err.println(" Object is De-Serialized !!! ");
        } catch(Exception ex) {
            System.err.println("ooooooops!!" + ex);
        } finally {
            if(fis != null)
                fis.close();
            if(ois != null)
                ois.close();
        }
    }
}

Execution Flow

Expected Output

During Serialization

Object is Serialized !!!

During Deserialization

here the customer=> [101,sri,sri@jlc,12345,null,null]
Object is De-Serialized !!!

Transient Field Behavior

Notice in the output that username and password are null after deserialization, even though they were set to "sri123" during object creation. This is because they are marked as transient.

Key Observations

Field	Marked as transient?	Serialized?	Value after Deserialization
cid	❌ No	✅ Yes	Original value (101)
cname	❌ No	✅ Yes	Original value ("sri")
email	❌ No	✅ Yes	Original value ("sri@jlc")
phone	❌ No	✅ Yes	Original value (12345)
username	✅ Yes	❌ No	null
password	✅ Yes	❌ No	null

Use Cases for transient

When to use transient

1. Sensitive Data: Passwords, credit card numbers, SSN
2. Derived Values: Calculated fields that can be recomputed
3. Temporary State: Cache, session data, thread references
4. Non-serializable References: Objects that cannot or should not be serialized
5. Large Data: Binary data or references to external resources

Best Practices

Resource Management

Always close streams in the finally block to prevent resource leaks:

finally {
    if(fis != null) fis.close();
    if(ois != null) ois.close();
}

Modern Alternative (Java 7+)

Use try-with-resources for automatic resource management:

try (FileOutputStream fos = new FileOutputStream("mycustomers3.txt");
     ObjectOutputStream oos = new ObjectOutputStream(fos)) {
    oos.writeObject(mycust);
    System.err.println("Object is Serialized !!!");
}

Comparison with Other Approaches

Approach	Implementation	Control Level	Use Case
Default	Implement Serializable	All fields serialized	Simple objects, no sensitive data
transient	Mark specific fields	Exclude specific fields	Some sensitive/temporary fields
Externalization	Implement Externalizable	Complete control	Complex custom serialization logic