1 Exceptions Exception handling – Exception Indication of problem during execution – E.g., divide by zero – Chained exceptions Uses of exception handling – Process exceptions from program components – Handle exceptions in a uniform manner in large projects – Remove error-handling code from “main line” of execution

1 Exceptions A method detects an error and throws an exception – Exception handler processes the error – Uncaught exceptions yield adverse effects Might terminate program execution

1 Try-Catch Code that could generate errors put in try blocks – Code for error handling enclosed in a catch clause – The finally clause always executes Termination model of exception handling – The block in which the exception occurs expires throws clause specifies exceptions method throws

1 Files Long-term storage of large amounts of data Persistent data exists after termination of program Files stored on secondary storage devices – Magnetic disks – Optical disks – Magnetic tapes Sequential and random access files

1 Data Hierarchy Smallest data item in a computer is a bit – Bit can be either 0 or 1 – Bit short for “binary digit” Programmers work with higher level data items – Decimal digits: (0-9) – Letters: (A-Z and a-z) – Special symbols: (e.g., %, &, *, (, ), -, +, “, :, ?, /, etc.) – Java uses Unicode characters composed of 2 bytes A byte is 8 bits long Fields (Java instance variables) – Composed of characters or bytes – Conveys meaning

1 Data Hierarchy Data hierarchy – Data items in a computer form a hierarchy Progresses from bits, to characters, to fields, etc. Records – Composed of several fields – Implemented as a class in Java – See Fig for example File is a group of related records – One field in each record is a record key Record key is a unique identifier for a record – Sequential file Records stored in order by record key

1 Data Hierarchy RandyRed J u d y Green SallyBlack TomBlue JudyGreen IrisOrange File Record Field Byte (ASCII character J) Bit

1 Files and Streams Java views a file as a stream of bytes (Fig. 17.2) – File ends with end-of-file marker or a specific byte number – File as a stream of bytes associated with an object Java also associates streams with devices – System.in, System.out, and System.err – Streams can be redirected

1 Files and Streams File processing with classes in package java.io – FileInputStream for byte-based input from a file – FileOutputStream for byte-based output to a file – FileReader for character-based input from a file – FileWriter for character-based output to a file

1 Files and Streams Java views a file as a stream of bytes (Fig. 17.2) – File ends with end-of-file marker or a specific byte number – File as a stream of bytes associated with an object Java also associates streams with devices – System.in, System.out, and System.err – Streams can be redirected

1 Files and Streams File processing with classes in package java.io – FileInputStream for byte-based input from a file – FileOutputStream for byte-based output to a file – FileReader for character-based input from a file – FileWriter for character-based output to a file

1 Java’s view of a file of n bytes n-1 end-of-file marker 67

1 Files and Streams Buffering – Improves performance of I/O – Copies each output to a region of memory called a buffer – Entire buffer output to disk at once One long disk access takes less time than many smaller ones – BufferedOutputStream buffers file output – BufferedInputStream buffers file input

1 Class File – Provides useful information about a file or directory – Does not open files or process files Fig lists some useful File methods