CIS 234: Strings (click, scroll down)

Dr. Ralph D. WestfallApril, 2010

What Is a String? series of characters enclosed in double

quotes characters can include letters (lower case

and upper case), digits, special characters (spaces, punctuation marks, mathematical operators, "escape sequences," etc.)

"Kim" "137 Flower St." "a + b\n“ it is a data type but not a primitive

one

Strings in Memory memory is assigned when a string is

created if a string's value changes, its memory

location (and possibly length) changes "anonymous" strings have a memory

location but no variable nameSystem.out.println("Total: "); //anonymous

Strings in Memory - 2 each character in a string is

identified by an offset from its memory location

1st character is at address of string offset = 0

2nd character is at string address + 1 x 2 offset = 2 bytes (in Unicode)

3rd is at string address + 2 x 2, etc.

Exercise based on the spreadsheet

example, make up a variable name and address table and a memory diagram for: someone’s name their age how much money is in their pocket

String Class String is a class in Java, not a primitive

data type, so strings are declared with String (1st letter capitalized) unlike other classes, don't always have to

use newString myName = new String("Juan"); // okString yourName = "Viji"; // also ok

String Class Methods when creating a string, you create

an object of the String class can use String class methods with

strings you create, by attaching a method to a string with a dot

int size = myName.length();int size = "Joan of Arcadia".length();/* length method in String class

assigns # of characters in myName object to size */

String Class Comparisons with objects, == compares memory

locations, not actual values == wouldn't recognize exactly same string

values if in different memory locations in some situations, == test on exactly

the same strings may NOT be true to be safe, need to use String class

methods instead to compare values

String Class equals Method compares strings in same or different

memory locationsprivate String aName;private boolean test;aName = "Lee "; //includes spaces test = (aName.equals("Lee")) // false

"Lee " includes space characterstest = ("Lee".equals(aName)) // another

way

String compareTo Method compares two strings letter by

letter if identical, returns 0 otherwise returns numeric distance

between 1st nonmatching characters value is calculated as (1st – 2nd) if 1st is less, value is negative

String compareTo Method - 2String aName = "Li"; int distance =

(aName.compareTo("Le")); // i is ASCII 105, e is ASCII 101 aName ("Li) is 1st, "Le" is 2nd alphabetically, "Li" comes after "Lee"

result = "Li" minus "Le" therefore distance is positive (+4)

Practice

oneName = "Lee"; twoName = "Lidia";threeName = "Larry";test = (oneName.equals(twoName))

// test = ?num =

(threeName.compareTo(oneName));

// num= ?

String length Method

size = myEmail.length(); make sure that object you attach

length method to is a String, not an array of Strings possible to have an array (group with

same name) of strings length is also a method of Array class,

so would return size of array instead of string

String indexOf Method finds first location of a specific

character within a string 1st character has index of 0 if character not found, returns –1 (minus)

hisEmail = "Enfei@fish.net";at = hisEmail.indexOf('@'); // at = ?? can do this in Excel

too

String charAt Method returns character at offset

(integer) provided in argument

String twoName = "Lidia Papadakis";char firstLetter = twoName.charAt(0);char secondLetter =

twoName.charAt(1);

Practice

fourthName = "Hsing-Chen";num = fourthName.length(); // num = ?num2 = fourthName.indexOf ('i'); //num2

= ?myChar = fourthName.charAt(4);

//myChar= ?

startsWith, endsWith Methods compare start or end of string with

another string value return true or false

aString = "vegetate";test1 = aString.startsWith("b"); // ??test2 = aString.endsWith("ate"); // ??

String replace Method replaces all occurrences of 1

character with another cf. "Replace all" in Word, Excel, etc.

arguments are character to find, character to replace it withoneString = "some hot";twoString = oneString.replace('o', 'a');// twoString = ?

String substring Method returns part of another string arguments are starting position

(offset) and "boundary" position (end +1)oneString = "separate";twoString = oneString.substring(3, 7);// twoString = ?

//notes on slides 8 to 18

String Class Case Methods toUpperCase converts to capital

lettersaName = aName.toUpperCase(); used for "shouting" in Mouth class

toLowerCase converts to lower case used for "quiet" in Mouth class

Makes it possible to ignore case of inputs Makes ANA the same as Ana and ana

Numbers to Strings toString converts other data types

to stringslong bigNum = 1234243342;strBigNum = toString(bigNum);

// stringBigNum = "1234243342"

Strings to Numbers use data type "wrapper" class methods

wrapper classes make it possible to treat "primitive" data types like they were classes

int num = Integer.parseInt("135"); Integer class is wrapper for int data typedouble dNum = Double.valueOf("1.2343"); Double class is double data type wrapper

Review Questions only alphabetical letters can be in

Strings, not numerical ones: T or F? Strings are primitive variables: T or

F? why is the s capitalized in String?

where else are first letters capitalized? what punctuation marks enclose

values in a String?

Review Questions - 2 what is the recommended way to

check if two Strings are equal? int distance = ("J".compareTo("Lo")); how do we find the length of a String? how do we pick out parts of a String?

what are the arguments we need for this? who is richer—Scrooge McDuck or

Flintheart Glomgold—and why? (artist)

Appendix 1: StringBuffer Class problems with String class

every time you change a string's value, Java creates a new string

the length of a string is set when it is created, and can't be changed

StringBuffer class deals with these problems stored in same place, size can increase

StringBuffer Constructors 3 constructors (note signatures)

StringBuffer() creates an empty StringBuffer object with space for 16 characters

StringBuffer(int length) new object has space for # of characters in argument

StringBuffer(String s) creates object with string s characters plus space for 16 more

StringBuffer Methods append method adds new string to

end of previous one insert method puts characters in

between existing characters, starting at specified location

for both, if not enough space, Java will automatically expand the buffer object

Appendix 2: Inputting #s > 9

// (declarations omitted)newChar = (char) System.in.read();while (newChar >= '0' && newChar <= '9'){

inString = inString + newChar; // "adding" newChar = (char)System.in.read();}inNum = Integer.parseInt(inString);

Questions re Previous Slide why does code input newChar

before loop starts? why does code input newChar after

adding newChar to inString? what happens when user hits Enter

key?