Chapter 20 – Streams and Binary Input/Output Big Java Early Objects by Cay Horstmann Copyright © 2014 by John Wiley & Sons. All rights reserved.

Chapter Goals  To become familiar with text and binary file formats  To learn about encryption  To understand when to use sequential and random file access  To read and write objects using serialization Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 2

Contents  Readers, Writers, and Streams  Binary Input and Output  Random Access  Object Streams Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 3

20.1 Readers, Writers, and Streams  Two ways to store data:  Text format: human-readable form, as a sequence of characters  E.g. Integer 12,345 stored as characters '1' '2' '3' '4' '5'  More convenient for humans: easier to produce input and to check output  Readers and writers handle data in text form  Binary format: data items are represented in bytes  E.g. Integer 12,345 stored as sequence of four bytes  More compact and more efficient  Streams handle binary data Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 4

Java Classes for Input and Output Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 5

Text Data  Reader and Writer and their subclasses were designed to process text input and output  PrintWriter was used in Chapter 7  Scanner class is more convenient than Reader class  By default, these classes use the character encoding of the computer executing the program  OK, when only exchanging data with users from same country  Otherwise, good idea to use UTF-8 encoding: Scanner in = new Scanner(input, "UTF-8"); // Input can be a File or InputStream PrintWriter out = new PrintWriter(output, "UTF-8"); // Output can be a File or OutputStream Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 6

20.2 Binary Input and Output  Use InputStream and OutputStream and their subclasses to process binary input and output  To read: FileInputStream inputStream = new FileInputStream("input.bin");  To write: FileOutputStream outputStream = new FileOutputStream("output.bin");  System.out is a PrintStream object Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 7

Binary Input  Use read method of InputStream class to read a single byte  returns the next byte as an int between 0 and 255  or, the integer -1 at end of file InputStream in =...; int next = in.read(); if (next != -1) { Process next // a value between 0 and 255 } Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 8

Binary Output  Use write method of OutputStream class to write a single byte: OutputStream out =...; int value=...; // should be between 0 and 255 out.write(value);  When finished writing to the file, close it: out.close(); Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 9

An Encryption Program (1)  File encryption: To scramble file so that it is readable only to those who know the encryption method and secret keyword  To use Caesar cipher:  Choose encryption key – a number between 1 and 25 that indicates the shift to be used in encrypting each byte  Example: If the key is 3, replace A with D, B with E,...  To decrypt, use the negative of the encryption key Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 10

An Encryption Program (2)  Encryption: int next = in.read(); if (next == -1) } done = true; } else { int encrypted = encrypt(next); out.write(encrypted); } Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 11

CaesarCipher.java Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 12 Continued 1 import java.io.InputStream; 2 import java.io.OutputStream; 3 import java.io.IOException; 4 5 /** 6 This class encrypts files using the Caesar cipher. 7 For decryption, use an encryptor whose key is the 8 negative of the encryption key. 9 */ 10 public class CaesarCipher 11 { 12 private int key; /** 15 Constructs a cipher object with a given key. aKey the encryption key 17 */ 18 public CaesarCipher(int aKey) 19 { 20 key = aKey; 21 } 22

CaesarCipher.java (cont.) Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 13 Continued 23 /** 24 Encrypts the contents of a stream. in the input stream out the output stream 27 */ 28 public void encryptStream(InputStream in, OutputStream out) 29 throws IOException 30 { 31 boolean done = false; 32 while (!done) 33 { 34 int next = in.read(); 35 if (next == -1) 36 { 37 done = true; 38 } 39 else 40 { 41 int encrypted = encrypt(next); 42 out.write(encrypted); 43 } 44 } 45 } 46

CaesarCipher.java (cont.) Copyright © 2014 by John Wiley & Sons. All rights reserved. Page /** 48 Encrypts a value. b the value to encrypt (between 0 and 255) the encrypted value 51 */ 52 public int encrypt(int b) 53 { 54 return (b + key) % 256; 55 } 56 }

CaesarEncryptor.java Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 15 Continued 1 import java.io.File; 2 import java.io.FileInputStream; 3 import java.io.FileOutputStream; 4 import java.io.InputStream; 5 import java.io.IOException; 6 import java.io.OutputStream; 7 import java.util.Scanner; 8 9 /** 10 This program encrypts a file, using the Caesar cipher. 11 */ 12 public class CaesarEncryptor 13 { 14 public static void main(String[] args) 15 { 16 Scanner in = new Scanner(System.in); 17 try 18 { 19 System.out.print("Input file: "); 20 String inFile = in.next(); 21 System.out.print("Output file: "); 22 String outFile = in.next();

CaesarEncryptor.java (cont.) Copyright © 2014 by John Wiley & Sons. All rights reserved. Page System.out.print("Encryption key: "); 24 int key = in.nextInt(); InputStream inStream = new FileInputStream(inFile); 27 OutputStream outStream = new FileOutputStream(outFile); CaesarCipher cipher = new CaesarCipher(key); 30 cipher.encryptStream(inStream, outStream); inStream.close(); 33 outStream.close(); 34 } 35 catch (IOException exception) 36 { 37 System.out.println("Error processing file: " + exception); 38 } 39 } 40 }

20.3 Random Access  Sequential access: process file one byte at a time  Random access: access file at arbitrary locations  Only disk files support random access System.in and System.out do not  Each disk file has a special file pointer position Read or write at pointer position Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 17

RandomAccessFile Class  Open a file with open mode:  Reading only ( "r" )  Reading and writing ( "rw" ) RandomAccessFile f = new RandomAcessFile("bank.dat","rw");  To move the file pointer to a specific byte: f.seek(position);  To get the current position of the file pointer: long position = f.getFilePointer(); // of type "long" because files can be very large  To find the number of bytes in a file: long fileLength = f.length(); Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 18

Bank Account Program (1)  Use a random access file to store a set of bank accounts  Program lets you pick an account and deposit money into it  To manipulate a data set in a file, pay special attention to data formatting  Suppose we store the data as text Say account 1001 has a balance of $900, and account 1015 has a balance of 0: Want to deposit $100 into account 1001: Writing out the new value: Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 19

Bank Account Program (2)  Better way to manipulate a data set in a file:  Give each value a fixed size that is sufficiently large  Every record has the same size  Easy to skip quickly to a given record  To store numbers, it is easier to store them in binary format Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 20

Bank Account Program (3)  RandomAccessFile class stores binary data  readInt and writeInt methods read/write integers as four-byte quantities  readDouble and writeDouble methods use eight-byte quantities  To find out how many bank accounts are in the file: public int size() throws IOException { return (int) (file.length() / RECORD_SIZE); // RECORD_SIZE is 12 bytes: // 4 bytes for account number plus // 8 bytes for balance } Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 21

Bank Account Program (4)  To read the n th account in the file: public BankAccount read(int n) throws IOException { file.seek(n * RECORD_SIZE); int accountNumber = file.readInt(); double balance = file.readDouble(); return new BankAccount(accountNumber, balance); } Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 22

Bank Account Program (5)  To write the n th account in the file: public void write(int n, BankAccount account) throws IOException { file.seek(n * RECORD_SIZE); file.writeInt(account.getAccountNumber()); file.writeDouble(account.getBalance()); } Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 23

BankSimulator.java Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 24 Continued 1 import java.io.IOException; 2 import java.util.Scanner; 3 4 /** 5 This program demonstrates random access. You can access existing 6 accounts and deposit money, or create new accounts. The 7 accounts are saved in a random access file. 8 */ 9 public class BankSimulator 10 { 11 public static void main(String[] args) throws IOException 12 { 13 Scanner in = new Scanner(System.in); 14 BankData data = new BankData(); 15 try 16 { 17 data.open("bank.dat"); 18

BankSimulator.java (cont.) Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 25 Continued 19 boolean done = false; 20 while (!done) 21 { 22 System.out.print("Account number: "); 23 int accountNumber = in.nextInt(); 24 System.out.print("Amount to deposit: "); 25 double amount = in.nextDouble(); int position = data.find(accountNumber); 28 BankAccount account; 29 if (position >= 0) 30 { 31 account = data.read(position); 32 account.deposit(amount); 33 System.out.println("New balance: " + account.getBalance()); 34 }

BankSimulator.java (cont.) Copyright © 2014 by John Wiley & Sons. All rights reserved. Page else // Add account 36 { 37 account = new BankAccount(accountNumber, amount); 38 position = data.size(); 39 System.out.println("Adding new account."); 40 } 41 data.write(position, account); System.out.print("Done? (Y/N) "); 44 String input = in.next(); 45 if (input.equalsIgnoreCase("Y")) done = true; 46 } 47 } 48 finally 49 { 50 data.close(); 51 } 52 } 53 }

BankData.java Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 27 Continued 1 import java.io.IOException; 2 import java.io.RandomAccessFile; 3 4 /** 5 This class is a conduit to a random access file 6 containing bank account records. 7 */ 8 public class BankData 9 { 10 private RandomAccessFile file; public static final int INT_SIZE = 4; 13 public static final int DOUBLE_SIZE = 8; 14 public static final int RECORD_SIZE = INT_SIZE + DOUBLE_SIZE; /** 17 Constructs a BankData object that is not associated with a file. 18 */ 19 public BankData() 20 { 21 file = null; 22 } 23

BankData.java (cont.) Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 28 Continued 24 /** 25 Opens the data file. filename the name of the file containing bank 27 account information 28 */ 29 public void open(String filename) 30 throws IOException 31 { 32 if (file != null) { file.close(); } 33 file = new RandomAccessFile(filename, "rw"); 34 } /** 37 Gets the number of accounts in the file. the number of accounts 39 */ 40 public int size() 41 throws IOException 42 { 43 return (int) (file.length() / RECORD_SIZE); 44 } 45

BankData.java (cont.) Page 29 Continued 46 /** 47 Closes the data file. 48 */ 49 public void close() 50 throws IOException 51 { 52 if (file != null) { file.close(); } 53 file = null; 54 } /** 57 Reads a bank account record. n the index of the account in the data file a bank account object initialized with the file data 60 */ 61 public BankAccount read(int n) 62 throws IOException 63 { 64 file.seek(n * RECORD_SIZE); 65 int accountNumber = file.readInt(); 66 double balance = file.readDouble(); 67 return new BankAccount(accountNumber, balance); 68 } 69

BankData.java (cont.) Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 30 Continued 70 /** 71 Finds the position of a bank account with a given number accountNumber the number to find the position of the account with the given number, 74 or -1 if there is no such account 75 */ 76 public int find(int accountNumber) 77 throws IOException 78 { 79 for (int i = 0; i < size(); i++) 80 { 81 file.seek(i * RECORD_SIZE); 82 int a = file.readInt(); 83 if (a == accountNumber) {return i; } 84 // Found a match 85 } 86 return -1; // No match in the entire file 87 } 88

BankData.java (cont.) Copyright © 2014 by John Wiley & Sons. All rights reserved. Page /** 90 Writes a bank account record to the data file n the index of the account in the data file account the account to write 93 */ 94 public void write(int n, BankAccount account) 95 throws IOException 96 { 97 file.seek(n * RECORD_SIZE); 98 file.writeInt(account.getAccountNumber()); 99 file.writeDouble(account.getBalance()); 100 } 101 } Continued

BankData.java (cont.) Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 32 Program Run: Account number: 1001 Amount to deposit: 100 Adding new account. Done? (Y/N) N Account number: 1018 Amount to deposit: 200 Adding new account. Done? (Y/N) N Account number: 1001 Amount to deposit: 1000 New balance: Done? (Y/N) Y

20.4 Object Streams  ObjectOutputStream class can save entire objects to disk  ObjectInputStream class can read them back in  Use streams, not writers because objects are saved in binary format Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 33

Writing an Object to File  The object output stream saves all instance variables: BankAccount b =...; ObjectOutputStream out = new ObjectOutputStream( new FileOutputStream("bank.dat")); out.writeObject(b); Copyright © 2011 by John Wiley & Sons. All rights reserved. Page 34

Reading an Object From File  readObject method returns an Object reference  Need to remember the types of the objects that you saved and use a cast: ObjectInputStream in = new ObjectInputStream( new FileInputStream("bank.dat")); BankAccount b =(BankAccount) in.readObject();  readObject method can throw ClassNotFoundException  Checked exception ⇒ you must catch or declare it Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 35

Write and Read Array List  Write: ArrayList a = new ArrayList (); // Now add many BankAccount objects into a out.writeObject(a);  Read: ArrayList a = (ArrayList ) in.readObject(); Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 36

Serializable Interface  Objects that are written to an object stream must belong to a class that implements the Serializable interface: class BankAccount implements Serializable { … }  Serializable interface has no methods  Serialization: Process of saving objects to a stream  Each object is assigned a serial number on the stream  If the same object is saved twice, only serial number is written out the second time  When reading, duplicate serial numbers are restored as references to the same object Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 37

Bank.java Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 38 Continued

Bank.java (cont.) Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 39

SerialDemo.java Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 40 Continued

SerialDemo.java (cont.) Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 41 Continued

SerialDemo.java (cont.) Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 42

Summary: Java Class Hierarchy for Handling Input and Output  Streams access sequences of bytes. Readers and writers access sequences of characters. Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 43

Summary: Input and Output of Binary Data  Use FileInputStream and FileOutputStream classes to read and write binary data from and to disk files.  The InputStream.read method returns an integer, either -1 to indicate end of input, or a byte between 0 and 255.  The OutputStream.write method writes a single byte. Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 44

Summary: Random Access  In sequential file access, a file is processed one byte at a time.  Random access allows access at arbitrary locations in the file, without first reading the bytes preceding the access location.  A file pointer is a position in a random access file. Because files can be very large, the file pointer is of type long.  The RandomAccessFile class reads and writes numbers in binary form. Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 45

Summary: Object Streams  Use object streams to save and restore all instance variables of an object automatically.  Objects saved to an object stream must belong to classes that implement the Serializable interface. Copyright © 2014 by John Wiley & Sons. All rights reserved. Page 46