CS 1054 Lecture 15 File Readers and Writers

CS 1054: Lecture 15 - File Readers and Writers

Text Input/Output Read from Your Java Classes Write to Input Text file Output Text file

Example: Averaging Student Scores Perform calculations

Perform Calculation with Java Program Java Class Input. txt Reads from Input file Performs Calculations Writes to Output file Output. txt

Steps to Input/Output • Input – Open file – Read data – Close file • Output – Open file – Write file – Close file

Sample File. Input. Output Class Import java. io. *; public class File. Input. Output { // To read from File private String input. File. Name = “input. txt”; private File. Input. Stream fis; private Buffered. Reader br; // To write to Files private String output. File. Name = “output. txt”; private File. Output. Stream fos; private Print. Stream ps; }

public File. Input. Output() { // Open files fis = new File. Input. Stream(input. File. Name); // For Input br = new Buffered. Reader (new Input. Stream. Reader (fis)); fos = new File. Output. Stream(output. File. Name); //For output ps = new Print. Stream(fos); //To perform required calculations read. File(); }

• Error message: “Unreported exception java. io. IOException must be caught or declared to be thrown” Add clause throws Exception ** For vs. while loop… ** why use read. Line() ** how to convert integers to characters?

public void perform. Calculation() throws Exception { String line = br. read. Line(); // line = Students|Scores line = br. read. Line(); // line = Student 1|89 line = br. read. Line(); // line = Student 2|92 line = br. read. Line(); // line = Student 3|65 line = br. read. Line(); // line = Student 4|70 line = br. read. Line(); // line = Student 5|13 lines = br. read. Line() // line = ? ? ? }

public void perform. Calculation() throws Exception { String line = br. read. Line(); while (line != null) // if not end of file { line = br. read. Line(); // read next line from file // The line has both Students and scores } }

Some Classes to Know String. Tokenizer – Allows you to break strings at specified delimiters (characters) String. Tokenizer st = new String. Tokenizer("this is a test"); while (st. has. More. Tokens()) { System. out. println(st. next. Token()); } prints the following output: This is a test

In our Example: Students|Scores => delimiter = ‘|’ String 1 =>Student. Id; String 2 =>Score;

import java. util. String. Tokenizer public void perform. Calculation() throws Exception { String line = br. read. Line(); while (line != null) // if not end of file { line = br. read. Line(); // read next line from file String. Tokenizer stk = new String. Tokernizer(line, “|”); String student. Id = stk. next. Token(); //Student. Id String score = stk. next. Token(); //Score } }

Integer Class • Provides Method to Convert ints to Strings and Strings to ints – static int parse. Int(String s) Note: It’s a static method String score = “ 89”; int Student. Score = Integer. parse. Int(score);

import java. lang. Integer; public void perform. Calculation() throws Exception { String line = br. read. Line(); while (line != null) // if not end of file { line = br. read. Line(); // read next line from file String. Tokenizer stk = new String. Tokernizer(line, “|”); String student. Id = stk. next. Token(); //Student. Id String score = stk. next. Token(); //Score // To convert String to an int student. Score = Integer. parse. Int(score); } }