TCSS 305 Stepp

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TCSS 305 Stepp

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Title: TCSS 305 Stepp

1
TCSS 305 (Stepp)

Unit 0
File I/O, Arrays

2
Interactive Programs and File I/O with Scanner
and PrintStream
3
Interactive programs

We have written several programs that print
output to the console.
It is also possible to read text input from the
console.
The user running the program types the input into
the console.
We can capture the input and use it as data in
our program.
A program that processes input from the user is
called an interactive program.
Interactive programs can be challenging
Computers and users think in very different ways.
Users tend to misbehave!

4
Input and System.in

We have now seen code that communicates with
objects.
Example objects DrawingPanel, Graphics, Color
When we print text output to the console, we
communicate with an object named System.out .
We call the println (or print) method of the
System.out object to print a message to the
console.
The object that holds the user's console input is
named System.in . But it is not easy to use...

5
Scanner

Since System.in is not easy to use by itself, we
will use a second object of a type named Scanner
to help.
Once we construct the Scanner object, we can ask
it to read various kinds of input from the
console.
Constructing a Scanner object to read console
input
Scanner ltnamegt new Scanner(System.in)
Example
Scanner console new Scanner(System.in)

6
Scanner as data source

Think of a Scanner like a faucet or showerhead
that can be attached to a source of 'water'
(data). In our case, the source of that data is
System.in .
Like a faucet must be connected to a water
source,the Scanner must be connected to a data
sourceby writing (System.in) when constructing
it.
The Scanner requires a file named Scanner.java
that must be saved into the same folder as your
program.
Scanner.java is available on the course web page.

7
Scanner methods

Methods of Scanner that we will use in this
chapter
Each of these methods causes your program to
pause until the user has typed input and pressed
Enter, then it returns the typed value to your
program.
(You may wish to review return values from last
chapter.)

8
Example Scanner usage

public class ReadSomeInput
public static void main(String args)
System.out.print("How old are you? ")
int age
Scanner console new Scanner(System.in)
age console.nextInt()
System.out.println("Wow, you're " age)
System.out.println("That's quite old!")
Output (user input underlined)
How old are you? 14
Wow, you're 14
That's quite old!

9
Scanning tokens

token A unit of user input. Tokens are
separated by whitespace (spaces, tabs, new
lines).
Example If the user types the following
23 3.14 John Smith "Hello world"
45.2 19
The tokens in the input are the following, and
can be interpreted as the given types
Token Type(s)
1. 23 int, double, String
2. 3.14 double, String
3. John String
4. Smith String
5. "Hello String
6. world" String
7. 45.2 double, String
8. 19 int, double, String

10
Consuming input

When the Scanner's methods are called, the
Scanner reads and returns the next input value to
our program.
If the type of the token isn't compatible with
the type we requested, the program crashes.
Imagine the Scanner as having an invisible cursor
that moves through all the user input.
As the scanner reads each input value, it
advances forward through the user input until it
has passed the given token.
This is called consuming the input token.
Key code consumed unconsumed last
token
double pi console.nextDouble() //
3.14
23\t3.14 John Smith\t"Hello world"\n\t\t45.2 19

11
Consume input example

Example If the following input from the user has
been typed,
23 3.14 John Smith "Hello world"
45.2 19
The Scanner views it as a linear stream of data,
like the following
23\t3.14 John Smith\t"Hello world"\n\t\t45.2
19\n
The Scanner positions its 'cursor' at the start
of the user input
23\t3.14 John Smith\t"Hello world"\n\t\t45.2
19\n
As we call the various next methods on the
Scanner, the scanner moves forward
int x console.nextInt()
// 23
23\t3.14 John Smith\t"Hello world"\n\t\t45.2
19\n
double pi console.nextDouble()
// 3.14
23\t3.14 John Smith\t"Hello world"\n\t\t45.2
19\n
String word console.next()
// "John"

12
Line-based input

The Scanner's nextLine method consumes and
returns an entire line of input as a String.
The Scanner moves its cursor from its current
position until it sees a \n new line character,
and returns all text that was found.
The new line character is consumed but not
returned.
Example
23 3.14 John Smith "Hello world"
45.2 19
String line1 console.nextLine()
23\t3.14 John Smith\t"Hello world"\n\t\t45.2
19\n
String line2 console.nextLine()
23\t3.14 John Smith\t"Hello world"\n\t\t45.2
19\n

13
Mixing line-based with tokens

It is not generally recommended to use nextLine
in combination with the other next__ methods,
because confusing results occur.
23 3.14Joe "Hello world" 45.2 19
int n console.nextInt()
// 2323\t3.14\nJoe\t"Hello world"\n\t\t45.2
19\n
double x console.nextDouble()
// 3.1423\t3.14\nJoe\t"Hello world"\n\t\t45.2
19\n
// User intends to grab the John Smith "Hello
world" line// but instead receives an empty
line!String line console.nextLine()
// ""23\t3.14\nJoe\t"Hello
world"\n\t\t45.2 19\n
// Calling nextLine again will get the line we
wanted.String line2 console.nextLine() //
"Joe\t\"Hello world\""23\t3.14\nJoe\t"Hello
world"\n\t\t45.2 19\n

14
Line-and-token example

Here's another example of the confusing behavior
Scanner console new Scanner(System.in)
System.out.print("Enter your age ")
int age console.nextInt()
System.out.print("Now enter your name ")
String name console.nextLine()
System.out.println(name " is " age " years
old.")
Log of execution (user input underlined)
Enter your age 12
Now enter your name Marty Stepp
is 12 years old.
Why?
User's overall input 12\nMarty Stepp
After nextInt() 12\nMarty Stepp
After nextLine() 12\nMarty Stepp

15
File objects

Java's File class represents files on the
user'shard drive.
Creating a File object does not actually create
that file on your hard drive.
When we want to read data out of a file, we
create a File object representing that file and
open it with a Scanner.
Creating a Scanner for a File, general syntax
Scanner ltnamegt new Scanner(new File("ltfile
namegt"))
Example
Scanner input new Scanner(new
File("numbers.txt"))
The File class is in Java's java.io package, so
we must write this at the top of our program
import java.io.

16
Exceptions, throwing

exception A Java object that represents a
program error.
checked exception An error that Java forces us
to handle in our program. Otherwise the program
will not compile.
Java forces us to specify what our program
should do to handle potential failures when
opening a file.
throws clause Keywords that can be added to the
header of our methods to explain to Java that we
plan NOT to handle file input failures. (We'll
just let the program crash in such a case.)
Throws clause, general syntax
public static lttypegt ltnamegt(ltparamsgt) throws
lttypegt
Example
public static void main(String args)
throws
FileNotFoundException

17
Sample file input program

import java.io.
// Displays each number in the given file,
// and displays their sum at the end.
public class Echo2
public static void main(String args)
throws
FileNotFoundException
Scanner input new Scanner(new
File("numbers.dat"))
double sum 0.0
for (int i 1 i lt 5 i)
double next input.nextDouble()
System.out.println("number " i "
" next)
sum next
System.out.println("Sum " sum)
Input File numbers.dat Output
308.2 14.9 7.4 number 1 308.2

18
Testing before reading

The Scanner has useful methods for testing to see
what the next input token will be
You can call these methods as a condition of an
if statement or loop.

19
Example test before read

import java.io.
// Displays each number in the given file,
// and displays their sum at the end.
public class Echo3
public static void main(String args)
throws
FileNotFoundException
Scanner input new Scanner(new
File("numbers.dat"))
double sum 0.0
while (input.hasNextDouble())
double next input.nextDouble()
System.out.println("number " i "
" next)
sum next
System.out.println("Sum " sum)
Input File numbers.dat Output
308.2 14.9 7.4 number 1 308.2

20
Files and input cursor

Recall that a Scanner views all input as a stream
of characters, which it processes with its input
cursor
308.2 14.9 7.4\n2.8\n\n\n3.9 4.7 -15.4\n2.8\n
Each call to next, nextInt, nextDouble advances
the cursor to the end of the current token
(whitespace-separated)
input.nextDouble()
308.2 14.9 7.4\n2.8\n\n\n3.9 4.7 -15.4\n2.8\n
input.nextDouble()
308.2 14.9 7.4\n2.8\n\n\n3.9 4.7 -15.4\n2.8\n

21
File names

Relative path "readme.txt" or "input/readme.txt"
Absolute path "C\\Documents\\smith\\hw6\\input\\
readme.txt"
In TextPad editor (and most editors), when you
construct a File object with just a file name,
Java assumes you mean a file in the current
directory.
Scanner input new Scanner(new
File("readme.txt"))
If our program is in the folder C\Documents and
Settings\johnson\hw6, that is where Java will
look for readme.txt.
In DrJava, when you construct a File object with
just a relative path, Java assumes you mean a
file in the directory where DrJava is installed
on your hard drive.
DrJava does not find your file unless you put the
input file in the same folder as the DrJava
program (on Mac, usually /Applications), or
specify its absolute path. Argh!

22
Line-by-line processing

Scanners have a method named nextLine that
returns text from the input cursor's current
position forward to the nearest \n new line
character.
You can use nextLine to break up a file's
contents into each line and examine the lines
individually.
Reading a file line-by-line, general syntax
Scanner input new Scanner(new File("ltfile
namegt"))
while (input.hasNextLine())
String line input.nextLine()
ltprocess this line...gt

23
Processing one line

Often the contents of each line are themselves
complex, so we want to tokenize each individual
line using its own Scanner.
Example file contents
Susan 12.5 8.1 7.6 3.2
Brad 4.0 11.6 6.5 2.7 12
Jennifer 8.0 8.0 8.0 8.0 7.5
A Scanner can be constructed to tokenize a
particular String, such as one line of an input
file.
Scanner ltnamegt new Scanner(ltStringgt)
Processing complex input, general syntax
Scanner input new Scanner(new File("ltfile
namegt"))
while (input.hasNextLine())
String line input.nextLine()
Scanner lineScan new Scanner(line)
ltprocess this line...gt

24
Line-based input, cont'd.

Often we have to write programs that scan through
a file, finding the appropriate piece of data,
and process only that piece
Enter a name Brad
Brad worked 36.8 hours (7.36 hours/day)
Often we have files with a particular token of
importance on each line, followed by more data
hours.txt
Susan 12.5 8.1 7.6 3.2
Brad 4.0 11.6 6.5 2.7 12
Jennifer 8.0 8.0 8.0 8.0 7.5

25
Searching a file for a line

Recall reading a file line-by-line, general
syntax
Scanner input new Scanner(new File("ltfile
namegt"))
while (input.hasNextLine())
String line input.nextLine()
Scanner lineScan new Scanner(line)
ltprocess this line...gt
For this program
Scanner input new Scanner(new
File("hours.txt"))
while (input.hasNextLine())
String line input.nextLine()
Scanner lineScan new Scanner(line)
String name lineScan.next()
... if this is the name we want, add up their
hours

26
Prompting for a file name

Instead of writing the file's name into the
program as a String, we can ask the user to tell
us the file name to use.
Here is one case where we may wish to use
nextLine on a console Scanner, because a file
name might have spaces in it.
Scanner console new Scanner(System.in)
System.out.print("Type a file name to use ")
String filename console.nextLine()
// open the file
Scanner input new Scanner(new File(filename))

27
File not found

What if the user types a file name that does not
exist?
We can use the exists() method of the File object
to make sure that file exists and can be read.
Scanner console new Scanner(System.in)
System.out.print("Type a file name to use ")
String filename console.nextLine()
File file new File(filename)
while (!file.exists())
System.out.println("File not found!")
System.out.print("Type a file name to use
")
String filename console.nextLine()
file new File(filename)
Scanner input new Scanner(file)
Output
Type a file name to use hourz.text
File not found!
Type a file name to use h0urz.txt

28
Complex multi-line records

Sometimes the data in the file consists of
'records', each of which is a group of
information that may occupy multiple lines
grades.dat
Erica Kane
3 2.8 4 3.9 3 3.1
Greenlee Smythe
3 3.9 3 4.0 4 3.9
Ryan Laveree
2 4.0 3 3.6 4 3.8 1 2.8
Adam Chandler
3 3.0 4 2.9 3 3.2 2 2.5
Adam Chandler, Jr
4 1.5 5 1.9
How can we process one or all of these records?

29
Output to files

Java has an object named PrintStream (in the
java.io package) that allows you to print output
into a destination, which can be a file.
System.out is also a PrintStream, so any methods
you have used on System.out (such as print,
println) will work on every PrintStream.
Printing into an output file, general syntax
PrintStream ltnamegt new PrintStream(
new FileOutputStream("ltfile namegt"))
...
Example
PrintStream output new PrintStream(
new FileOutputStream("output.txt"))
output.println("Hello, file!")

30
Arrays
31
Too many values...

How would we implement a program such as this?
How many numbers 5
Number 1 2
Number 2 7
Number 3 17
Number 4 -1
Number 5 64
Here are the numbers you typed
Number 1 2
Number 2 7
Number 3 17
Number 4 -1
Number 5 64
We don't know how many int variables we should
declare in advance... it could be very many!

32
Arrays

array A single variable that can store many
values of the same type.
element One value in an array.
index A 0-based integer, which is used to access
an element from an array.
We usually draw arrays as horizontal grids.
An array of integers

33
Array declaration

Declaring/initializing an array, general syntax
lttypegt ltnamegt new lttypegt ltlengthgt
The length of the array is specified between
brackets.
Example
int numbers new int10
Example
double grades new double6

34
Accessing array elements

Assigning a value to an array element, general
syntax
ltarray namegt ltindexgt ltvaluegt
Example
numbers0 27
numbers3 -1
When arrays are initially constructed, every
element has a 0-equivalent value.
int 0
double 0.0
boolean false
char '\0' (the "null character")
String or other object null (null means "no
object")

35
Arrays and for loops

Arrays are very commonly used with for loops that
pass over each element and process it in some
way
Example (print each element of an array)
for (int i 0 i lt 10 i)
System.out.print(numbersi " ")
Output
27 0 0 -1 0 0 0 0 0 0
Example (store squared numbers in an array)
for (int i 0 i lt 10 i)
numbersi i i

36
The .length field

An array has a field named .length that returns
its total number of elements.
This is preferred to be used instead of
hard-coding the array's length as a literal
value. (Why?)
Example
for (int i 0 i lt numbers.length i)
System.out.print(numbersi " ")
Output
0 1 4 9 16 25 36 49 64 81

37
Array initialization shortcut

Quick array initialization, general syntax
lttypegt ltnamegt ltvaluegt, ltvaluegt, ...,
ltvaluegt
Example
int numbers 12, 49, -2, 26, 5, 17, -6, 84,
72, 3
This syntax is useful when you know in advance
what the array's elements will be.

38
Array practice problem

What are the contents of the array a after the
following code?
int a 2, 5, 1, 6, 14, 7, 9
for (int i 1 i lt a.length i)
ai ai - 1

39
Limitations of arrays

An array cannot be directly printed by println
Example
int numbers new int10
System.out.println(numbers)
Output
I_at_1cb52ae
Arrays cannot be compared correctly with or
equals
Example
int numbers1 new int10
int numbers2 new int10
System.out.println(numbers1 numbers2)
Output
false

40
More array limitations

An array's size cannot be modified after it is
created.
The .length field is read-only you cannot change
it.
Example
int numbers new int10
numbers.length 20 // does not compile!
You can instead create a new larger array and
copy the elements over into it.
It is illegal to access an array element with an
index that is less than 0 or greater than its
.length - 1.
If you do so, your program will crash with an
exception.
Example
int numbers new int10
System.out.println(numbers10) // kaboom!

41
Why are arrays useful?

We can use arrays to store a large amount of data
without declaring many variables.
Example Read in a file of 1000 numbers, then
print out the numbers in reverse order.
Arrays can help us to group related data into
elements.
Example For a given school exam, open a file
full of exam scores and count up how many
students got each score from 0 through 100.
Arrays let us hold on to data and access it in
random order.
Example Read a file full of babies' names, store
each name's data as an element in a large array,
and then examine it later to find names that the
user types.

42
Arrays as parameters

An array can be passed as a parameter to a
method.
Example
int iq new int3
iq0 126
iq1 167
iq2 95
int max getMaxValue(iq)
System.out.println("Max " max)
...
public static int getMaxValue(int array)
int max array0
for (int i 1 i lt array.length i)
if (arrayi gt max)
max arrayi
return max

43
Arrays as parameters, contd.

When an array is passed as a parameter, it is
passed as a reference (similar to objects).
If the method modifies elements of its array
parameter, the changes will also be seen in the
original array.
Example
int iq new int3
iq0 126
iq1 167
iq2 95
destroy(iq)
System.out.println(array0 " " array1)
...
public static void destroy(int array)
for (int i 0 i lt array.length i)
arrayi 0 // ha ha!
Output
0 0

44
Array practice problems

Write a method named print that accepts an array
of integers as its parameter and prints the
elements of the array in the following format
7, -2, 46, 3, 55
Write a method named equal that accepts 2 arrays
of integers as its parameters and returns whether
those two arrays contain exactly the same
elements in the same order.

45
String methods with arrays

These String methods return arrays
String s "long book"

46
Arrays of objects

Recall when you construct an array of primitive
values like ints, the elements' values are all
initialized to 0.
What is the equivalent of 0 for objects?
When you construct an array of objects (such as
Strings), each element initially stores a special
reference value called null.
null means 'no object'
Your program will crash if you try to call
methods on a null reference.
String words new String5

47
The dreaded 'null pointer'