1. Files The UI text pane window is transient:
Typing large amounts of input into the text pane is a pain!
It would be nice to be able to save the output of the program easily.
Large amounts of text belong in files
How can you read from a file?
How can you write to a file?
Easy!
Files behave very like the UI text pane:
next, nextInt etc read from a file, (via a Scanner object)
print, println, printf write to a file (via a PrintStream object)

2. Files: An overview Enter numbers: 40 60 30 50 done UI.nextInt();
UI Window
UI.nextInt();
UI.println();
A real file: “myfile.txt”
next();
My Program :
int a =??.nextInt();
??.println(a*5); }
Scanner
object
File object
PrintStream object
println();

3. Scanner
Scanner: a class in Java that allows a program to read user input from a source (a file, input window, a String, …)
New concept? Not really!
UI.next() and other next methods are actually a Scanner working in the background
while(UI.hasNextInt()){ double amt = UI.nextDouble() }
Most UI next methods work same as Scanner next methods, but a very few subtle differences…see Java documentation
Scanner scan = new Scanner(myfile);
while(scan.hasNextInt()){
double amt = scan.nextDouble(); }

4. Reading using Scanner:
/** Read lines from a file and print them to UI text pane. */
public void readFile(){
File myfile = new File(“input.txt”);
Scanner scan = new Scanner(myfile);
UI.println(“ input.txt ”);
while (scan.hasNext()){
String line= scan.nextLine();
UI.println(line); }
UI.println(“ end of input.txt ”);
Almost right, but compiler complains!!!
Dealing with files may “raise exceptions”
Missing bits to handle exceptions

5. Files: handling exceptions
If a piece of code might raise an exception:
Have to enclose it in a
try { … }
catch (IOException e) { … }
public void readFile(){
File myfile = new File(“input.txt”);
try {
Scanner scan = new Scanner(myfile);
while (scan.hasNext()){
String line = scan.nextLine();
UI.println(line);
UI.println(“ end of input.txt ”);
catch (IOException e) { UI.println(“File failure: ” + e); }
what to do
what to do if it goes wrong

6. Reading from files: example 2
84 Isaac Newton
48 James Clerk Maxwell
66 Marie Curie
62 Aristotle
/** Finds oldest person in file of ages and names. */ public void printOldest(){ try { Scanner scan = new Scanner(new File(“names.txt")); String oldest = ""; int maxAge = 0; while (scan.hasNextInt()){ int age = scan.nextInt(); String name = scan.nextLine(); if (age > maxAge) { maxAge = age; oldest = name; } UI.printf(“Oldest is %s (%d)%n”, oldest, maxAge); } catch (IOException e) { UI.printf(“File failure: %s%n”, e); }
no need to prompt user!
Copes with multi-token names

7. Testing end of file Unlike UI text pane, can test for end of file:
/** Finds oldest person in file of names and ages. */
public void printOldest(){
try {
Scanner scan = new Scanner(new File(“names.txt"));
String oldest = “”;
int maxAge = 0;
while (scan.hasNext()){
String name = scan.next();
int age = scan.nextInt();
if (age > maxAge) {
maxAge = age;
oldest = name; }
UI.println(“Oldest is %s (%d)%n”, oldest, maxAge);
} catch (IOException e) { UI.printf(“File failure: %s%n”, e); }
Newton 84 Maxwell
48 Curie 66
Aristotle 62
False when at end of file
name first; age second
Note: name must be
just one token!!

8. A common simple pattern
File with one entity per line, described by multiple values:
while (sc.hasNext() ){
String type = sc.next();
double cost = sc.nextDouble();
int wheels = sc.nextInt();
String colour = sc.next();
String make = sc.next()
if (wheels > 4) { …. }
else { …
bicycle green Giant
truck black Isuzu
car red Toyota …
Read all the values
into variables
process the values in
the variables

9. Handling multiple items on line
Sometimes you can tell where the line ends:
/**Sums counts of items in file of items and counts*/
public void printItemCounts(){
try {
Scanner scan = new Scanner(new File(“data.txt"));
while (scan.hasNext()){
String item = scan.next();
int lineTot = 0;
while (scan.hasNextInt()) {
lineTot = lineTot + scan.nextInt(); }
UI.println(item + “: ” + lineTot);
} catch (IOException e) { UI.printf(“File failure: %s\n”, e); }
biscuits
cake 3 5 2
fruit
beans
biscuits: 88
cake: 10
fruit: 235
beans: 33

10. Reading files line by line
Sometimes can’t tell where ends of lines are:
may need to read a line at a time, then process:
/**Adds up numbers on each line of a file */
public void addCounts(){
try {
Scanner scan = new Scanner(new File(“data.txt"));
while (scan.hasNext()){
String line = scan.nextLine();
Scanner lineSc = new Scanner(line);
int lineTot = 0;
while (lineSc.hasNextInt()) {
lineTot = lineTot + lineSc.nextInt(); }
UI.println(lineTot);
} catch (IOException e) { UI.printf(“File failure: %s\n”, e); }
3 5 2
Wrapping a Scanner
around a String,
Lets you “read” values
from the String