jgs CSE 205 Object-Oriented Programming and Data Structures Lecture 24: Streams and Exceptions Dr. Javier Gonzalez-Sanchez [email protected] javiergs.engineering.asu.edu | javiergs.com PERALTA 230U Office Hours: By appointment 

jgs Streams 

Javier Gonzalez-Sanchez | CSE205 | Fall 2021 | 3 jgs Where we are … Data Structures Files 

Javier Gonzalez-Sanchez | CSE205 | Fall 2021 | 4 jgs I/O Streams § An I/O Stream represents an input source or an output destination 

Javier Gonzalez-Sanchez | CSE205 | Fall 2021 | 5 jgs Hierarchy 

Javier Gonzalez-Sanchez | CSE205 | Fall 2021 | 6 jgs File Stream (binary data) import java.io.FileInputStream; import java.io.FileOutputStream; import java.io.IOException; public class CopyBytes { public static void main(String[] args) { try { FileInputStream in = new FileInputStream(”myfile.txt"); FileOutputStream out =new FileOutputStream("outagain.txt"); int c; while ((c = in.read()) != -1) { out.write(c); } in.close(); out.close(); } catch (IOException e) { System.out.println(“Failure!”); } } } 

Javier Gonzalez-Sanchez | CSE205 | Fall 2021 | 7 jgs File Stream (char data) import java.io.FileReader; import java.io.FileWriter; import java.io.IOException; public class CopyCharacters { public static void main(String[] args) { try { FileReader inputStream = new FileReader(”myFile.txt"); FileWriter outputStream = new FileWriter(”output.txt"); int c; while ((c = inputStream.read()) != -1) { outputStream.write(c); } inputStream.close(); outputStream.close(); } catch (IOException e) { System.out.println(“Failure!”); } } } 

Javier Gonzalez-Sanchez | CSE205 | Fall 2021 | 8 jgs File Stream (string data) import java.io.FileReader; import java.io.FileWriter; import java.io.BufferedReader; import java.io.PrintWriter; import java.io.IOException; public class CopyLines { public static void main(String[] args) { try { FileReader input = new FileReader(”myFile.txt"); FileWriter output = new FileWriter(”output.txt"); BufferedReader inputStream = new BufferedReader(input); PrintWriter outputStream = new PrintWriter(output); String l; while ((l = inputStream.readLine()) != null) { outputStream.println(l); } inputStream.close(); outputStream.close(); } catch (IOException e) { System.out.println(“Failure!”); } } } 

jgs Exceptions try – catch statement 

Javier Gonzalez-Sanchez | CSE205 | Fall 2021 | 10 jgs Exceptions § When executing Java code, different errors can occur due to wrong input, or other unforeseeable things. § When an error occurs, Java will normally stop and generate an error message. The technical term for this is: Java will throw an exception (an error). § The try statement allows you to define a block of code to be tested for errors while it is being executed. § The catch statement allows you to define a block of code to be executed, if an error occurs in the try block. § The try and catch keywords come in pairs. 

Javier Gonzalez-Sanchez | CSE205 | Fall 2021 | 11 jgs try-catch Statements try { // Block of code to try } catch(Exception e) { // Block of code to handle errors } 

Javier Gonzalez-Sanchez | CSE205 | Fall 2021 | 12 jgs Example 

Javier Gonzalez-Sanchez | CSE205 | Fall 2021 | 13 jgs Exceptions There are many exception types available in Java: § ArithmeticException, § FileNotFoundException, § ArrayIndexOutOfBoundsException, § SecurityException, § etc. § The parent of all is the class Exception 

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Javier Gonzalez-Sanchez | CSE205 | Fall 2021 | 15 jgs Example 

Javier Gonzalez-Sanchez | CSE205 | Fall 2021 | 16 jgs finally Statement § The finally statement lets you execute code, after try...catch, regardless of the result 

Javier Gonzalez-Sanchez | CSE205 | Fall 2021 | 17 jgs Example 

jgs throw 

Javier Gonzalez-Sanchez | CSE205 | Fall 2021 | 19 jgs Definition § The throw statement allows you to create a custom error 

Javier Gonzalez-Sanchez | CSE205 | Fall 2021 | 20 jgs Example 

Javier Gonzalez-Sanchez | CSE205 | Fall 2021 | 21 jgs Example 

Javier Gonzalez-Sanchez | CSE205 | Fall 2021 | 22 jgs Questions 

jgs CSE 205 Object-Oriented Programming and Data Structures Javier Gonzalez-Sanchez, Ph.D. [email protected] Fall 2021 Copyright. These slides can only be used as study material for the class CSE205 at Arizona State University. They cannot be distributed or used for another purpose.