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CSE 114 – Computer Science I
Strings, I/O, and Methods
Bonneville Salt Flats, Utah
Java character data
• char
• Type for a single character
• Each char is a 2-byte number (Unicode)
char symbol = '7';
System.out.println(symbol);
System.out.println((int)symbol);
• Unicode character set
Output?
– ‘0’ (48) … ‘9’ (57)
– ‘A’ (65) … ‘Z’ (90)
– ‘a’ (97) … ‘z’ (122)
7
55
String
• A class in Java API
– http://java.sun.com/javase/6/docs/api/java/lang/String.html
• Used for text:
String s1 = "Rudie Can't";
String s2 = s1 + " Fail";
System.out.println(s2);
Output?
Rudie Can’t Fail
More about Strings
• Each character is stored at an index
String sentence = "Charlie Don't Surf";
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
C h a r l i e
D o n ' t
S u r f
• The String class (from J2SE) has methods to process strings.
System.out.println("charAt(6) is " + sentence.charAt(6));
System.out.println(sentence.toUpperCase());
System.out.println(sentence);
System.out.println(sentence.substring(0,7) +
sentence.substring(14));
charAt(6) is e
CHARLIE DON'T SURF
Charlie Don't Surf
CharlieSurf
Output to Console window
by println methods
Strings are immutable!
• There are no methods to change them once they
have been created
• So how can I change a String?
1. make a new one
2. assign the new one to the old variable
String word = "Hello";
word.substring(0,4);
System.out.println(word);
word = word.substring(0, 4); Output?
System.out.println(word);
Hello
Hell
String functions
• “+” used for building new Strings. Ex:
String s = "If Music ";
s = s + "Could Talk";
int mins = 4, secs = 36;
String t = s + " (" + mins + ":" + secs + ")";
System.out.println(t);
Output?
If Music Could Talk (4:36)
Useful String functions
•
•
•
•
•
•
charAt
equals
equalsIgnoreCase
compareTo
startsWith
endsWith
•
•
•
•
•
•
•
indexOf
lastIndexOf
replace
substring
toLowerCase
toUpperCase
trim
s.equals(t)
• returns true if s and t have same letters
• false otherwise
Special Characters
• '\n' – newline
• '\t' – tab
• '\"' – quotation mark
• Ex, how can we print <img src="./pic.jpg" />
String s = "<img src=\"./pic.jpg\" />";
System.out.println(s);
How can we get user input?
• API methods
• One option: from console window
Scanner input = new Scanner(System.in);
System.out.print("Enter text: ");
String text = input.nextLine();
• Another option: use a dialog
text = JOptionPane.showInputDialog("Enter text: ");
How do we provide output?
• API methods
• One option: to console window (duh)
int num = 5;
System.out.print("Print five: " + num);
• Another option: use a dialog
int num = 5;
String text = "Print five: " + num;
JOptionPane.showMessageDialog(null, text);
How about reading/writing from/to a file?
• We use objects called streams
• Java as different types of streams
–
–
–
–
text
object
byte
etc.
• Let’s see how to use a text stream
– in Java, use BufferedReader/PrintWriter
Writing to a text file
• To use java.io.PrintWriter
1. open Stream to file
2. write text one line a a time using println(…)
3. close stream when done (i.e. end of file)
Text File Writing Example
try
{
File file = new File("Output.txt");
Writer writer = new FileWriter(file);
PrintWriter out = new PrintWriter(writer);
out.println("Janie");
out.println("Jones");
out.close();
}
catch(IOException ioe)
{
System.out.println("File not Found");
}
Reading from a text file
• To use java.io.BufferedReader
1. open Stream to file
2. read text one line a time using readLine()
• readLine returns null if no more text to read
3. close stream when done (i.e. end of file)
Text File Reading Example
try
{
File file = new File("Output.txt");
Reader reader = new FileReader(file);
BufferedReader in = new BufferedReader(reader);
String inputLine = in.readLine();
System.out.println(inputLine);
Output?
inputLine = in.readLine();
System.out.println(inputLine);
Janie
}
Jones
catch(IOException ioe)
{
System.out.println("File not Found");
}
Import statements
• Makes existing classes available to your program
• What classes?
– API classes
• Put at top of file using the class
• Ex:
// MAKE Scanner AVAILABLE
import java.util.Scanner;
//
Make all java.io classes available
import java.io.*;
What we’ve learned so far
• Simple program structure
• Declaring and initializing variables
• Performing simple calculations
• String manipulation
• User I/O
• File I/O
import java.util.Scanner;
public class ChangeMaker
{
public static void main(String[] args)
{
int change, rem, qs, ds, ns, ps;
System.out.print("Input change amount (1-99): ");
Scanner input = new Scanner(System.in);
change = input.nextInt();
qs = change / 25;
rem = change % 25;
ds = rem / 10;
rem = rem % 10;
ns = rem / 5;
rem = rem % 5;
ps = rem;
System.out.println(qs + " quarters");
System.out.println(ds + " dimes");
System.out.println(ns + " nickels");
System.out.println(ps + " pennies");
} // main
} // class ChangeMaker
Remember our
program?
What will we add next?
• Decision making
– if-else statements
• Relational Operators
• Logical Operators
– switch statements
• Iteration – efficient way of coding repetitive tasks
– do…while statements
– while statements
– for statements
But First!
• Can you name a method we’ve used so far?
• What is a method?
• Can we define our own methods?
• Why should we write methods?
• What’s the point?
Why write methods?
• To shorten your programs
– avoid writing identical code twice or more
• To modularize your programs
– fully tested methods can be trusted
• To make your programs more:
–
–
–
–
–
–
–
readable
reusable
testable
debuggable
extensible
adaptable
etc.
Rule of Thumb
• If you have to perform some operation in more
than one place inside your program, make a new
method to implement this operation and have other
parts of the program use it
• Ex: printing to the console
Method Header
• Describes how to use a method. Includes:
– return type
– name of method
– method arguments
• Note: documentation should describe method behavior
Output?
String s = "Rock the Casbah";
Rock
String t = s.substring(0, 4);
System.out.println(t);
• How do we know how to use substring? What’s the header?
public String substring(int beginIndex, int endIndex)
Static methods
• Remember the main method header?
public static void main(String[] args)
• What does static mean?
– associates a method with a particular class name
– any method can call a static method either:
• directly from within same class
OR
• using class name from outside class
Calling static methods directly example
public class StaticCallerWithin
{
public static void main(String[] args)
{
String song = getSongName();
System.out.println(song);
Output?
}
Straight to Hell
public static String getSongName()
{
return "Straight to Hell";
}
}
Calling external static methods example
public class StaticCallerFromOutside
{
public static void main(String[] args)
{
System.out.print("Random Number from 1-100: ");
double randomNum = Math.random();
System.out.print(randomNum*100 + 1);
}
}
• What’s the method header for Math.random?
public static double random()
Static Variables
• We can share variables among static methods
– global variables
• How?
– Declare a static variable outside of all methods
Static Variable Example
public class MyProgram
{
static String myGlobalSong = "Jimmy Jazz";
public static void main(String[] args)
{
System.out.println(myGlobalSong);
changeSong("Spanish Bombs");
Output?
System.out.println(myGlobalSong);
Jimmy Jazz
}
Spanish Bombs
public static void changeSong(String newSong)
{
myGlobalSong = newSong;
}
}
Call-by-value
• Note:
– method arguments are copies of the original data
• Consequence?
– methods cannot assign (‘=’) new values to arguments
and affect the original passed variables
• Why?
– changing argument values changes the copy, not the
original
Java Primitives Example
public static void main(String[] args)
{
Output?
int a = 5;
int b = 5;
changeNums(a, b);
System.out.println(a);
System.out.println(b);
}
public static void changeNums(int x, int y)
{
x = 0;
y = 0;
}
5
5
Java Objects (Strings) Example
public static void main(String[] args)
{
String a = "Hateful";
String b = "Career Opportunities";
changeStrings(a, b);
Output?
System.out.println(a);
Hateful
System.out.println(b);
Career Opportunities
}
public static void changeString(String x, String y)
{
x = "The Magnificent Seven";
y = "The Magnificent Seven";
}
• NOTE: When you pass an object to a method, you are passing a copy of the
object’s address
How can methods change local variables?
• By assigning returned values
• Ex, in the String class:
Output?
Hello
Hell
– substring method returns a new String
String s = "Hello";
s.substring(0, 4);
System.out.println(s);
s = s.substring(0, 4);
System.out.println(s);