Lecture 20: More on Strings

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Lecture 20 More on Strings
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Graphics vs Text

• Weve spent a lot of time doing graphics
• Wrote some fun games
• learned the basics of programming
• There are a lot of practical programs that arent
  graphical
• Word Processors
• IDEs like Eclipse (its kind of a combination)
• Your Java Compiler
• Lots of scientific programs just output text

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Strings

• Weve dealt with at least four classes for
  dealing with text
• Text
• an objectdraw object, draws some text on the
  screen, allows it to be moved around
• JTextField
• a GUI object lets the user enter some text in a
  box
• Jlabel
• object for displaying small amount of text in
  GUIs
• String
• This is the most basic Java Object for dealing
  with text

String string This is a String
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What are Strings?

• String is a class in Java
• we know this already
• Behaves slightly differently from other objects
• Most types in Java are in one of 2 categories
• Object types
• Described by a class somewhere
• Created using a constructor and the new command
• Manipulated by calling methods
• Base types (int, boolean, double, etc)
• You can write literals
• 8, false, true, 45.98
• You can manipulate them with operators
• , -, , /, , , , etc.

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How have we used Strings?

• Define variables of type String
• Can assign values to String variables
• Can define functions that have String parameters
  or return types.
• Can specify String literals by writing things in
  quotes
• Can concatenate (connect) Strings with the
  operator
• Can determine if two Strings are equal using the
  equals() method
• Remember that we use equals(), not , with
  Strings

String string text.getText()
text.setText(This is some text.)
String string Your Mom
if (selectedItem.equals(Blue)) //do
something
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So what type is String?

• Its an object, but Java has special support
  built in, to make dealing with Strings easier
• Can use literals, like base types
• Can use operators, like base types
• Can use constructors, like object types
• Can use methods on Strings, like objects
• There are 50 methods in the String class.

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Whats inside Strings?

• Strings are made up of characters
• It turns out, inside Strings are arrays of
  characters
• Strings are kind of like the collection classes
  youre writing in Simon!
• Instead of NoisyButtons, they have characters
• New base type char
• A char is a single character
• Can declare variables of type char
• Can write literals with single quotes

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The char type

• Declaring a char variable
• Just use char as the type
• Writing a char literal
• Use single quotes around one character

private char letter
letter T
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The char type

• Computers represent characters as numbers
  internally
• This is mostly hidden from the user
• There are lots of different encodings that
  characters can be stored in
• ASCII
• 256 characters
• American Standard for Computer Information
  Interchange
• Unicode
• Up to 65,536 characters
• About 35,000 are in use
• First 256 are same as ASCII
• Supports lots of languages (Chinese, Japanese,
  etc)
• Other encodings
• There are tons for Japanese, all different, huge
  pain
• ISO-2022JP, EUC, Shift-JIS, JIS, etc.
• Encodings just determine what numbers (codes)
  represent particular characters on a computer
• Or, you could say that an encoding is a way for a
  computer to interpret some numbers known to be
  characters.

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ASCII

• Below are the numbers representing different
  ASCII characters

0 nul 1 soh 2 stx 3 etx 4 eot
5 enq 6 ack 7 bel 8 bs 9 ht
10 nl 11 vt 12 np 13 cr 14 so 15
si 16 dle 17 dc1 18 dc2 19 dc3 20
dc4 21 nak 22 syn 23 etb 24 can 25
em 26 sub 27 esc 28 fs 29 gs 30 rs
31 us 32 sp 33 ! 34 " 35
36 37 38 39 ' 40 (
41 ) 42 43 44 , 45 - 46
. 47 / 48 0 49 1 50 2 51 3
52 4 53 5 54 6 55 7 56 8
57 9 58 59 60 lt 61 62
gt 63 ? 64 _at_ 65 A 66 B 67 C
68 D 69 E 70 F 71 G 72 H
73 I 74 J 75 K 76 L 77 M 78
N 79 O 80 P 81 Q 82 R 83 S
84 T 85 U 86 V 87 W 88 X
89 Y 90 Z 91 92 \ 93 94
95 _ 96 97 a 98 b 99 c
100 d 101 e 102 f 103 g 104 h
105 i 106 j 107 k 108 l 109 m 110
n 111 o 112 p 113 q 114 r 115 s
116 t 117 u 118 v 119 w 120 x
121 y 122 z 123 124 125 126
127 del
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Comparing Characters

• Most encodings guarantee that charaters appear in
  order
• So you can compare them with operators like ints
• lt, gt, lt, gt, , and !
• Youre guaranteed that
• Uppercase letters are in alphabetical order
• So Z gt A
• Lowercase letters are in alphabetical order
• So z gt a
• Numbers are in numerical order
• So 9 gt 0, just like with numbers

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Getting characters from Strings

• You can get characters at particular indices in
  Strings
• Kind of like getting things from arrays
• Instead of , we use the charAt() method
• You can get the length of a String with the
  length() method.
• This returns the number of characters in the
  String
• Use these to iterate over all chars in a String

char c someString.charAt(5)
for (int i0 i lt someString.length() i)
char c someString.charAt(5) //do something
with c
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Checking Strings

• Suppose you had a program that took some input
• Say you have some JTextFields you want to only be
  numbers
• You can check this usng charAt().
• Heres a method to check that a String represents
  a valid number
• Can use this to tell if a String will be
  converted properly to a number.

// Checks whether a given string can be
interpreted as an integer i.e., // checks
that it is made up of characters that are digits
in the range // 0-9. Returns true if and
only if the string can be interpreted as //
an integer. private boolean isInteger(String
aString) boolean allNumeric true
for (int i 0 (i lt aString.length()
allNumeric) i) if
(aString.charAt(i) lt '0' aString.charAt(i) gt
'9') allNumeric false
return allNumeric
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String methods

• Well go through some of the String methods
• You can use these in your program to search
  through Strings in useful ways
• This is probably the kind of stuff we would have
  done first if this course werent mostly graphical

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indexOf()

• indexOf() returns the starting index of the first
  occurrence of s in the String its called on
• It returns -1 if it wasnt found
• Theres another version
• This will start searching at the index provided

public int indexOf(String s)
String courseName "Data Structures" int
position courseName.indexOf(r)
public int indexOf(String s, int start)
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indexOf()

• Whats the result of each of these?

String courseName "Data Structures"
courseName.indexOf("Struct", 2)
courseName.indexOf("Struct", 8)
courseName.indexOf("t", 2)
courseName.indexOf("t", 6)
courseName.indexOf("t", 7)
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Counting words

• You could write a method that counts the number
  of times a particular word occurs in a String
• What does this return for these calls?

public int wordCount(String text, String
word) int count 0 int pos
text.indexOf(word,0) while (pos gt 0)
count pos
text.indexOf(word,posword.length())
return count
wordCount("yabbadabbadoo","abba") wordCount("scoo
bydoobydoo","oo")
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Counting words

• You could write a method that counts the number
  of times a particular word occurs in a String
• How about this one?
• How would you modify this to get all of the
  bobs?

public int wordCount(String text, String
word) int count 0 int pos
text.indexOf(word,0) while (pos gt 0)
count pos
text.indexOf(word,posword.length())
return count
wordCount("bubbabobobbrain","bob")
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Case-insensitivity

• You can use two methods in Java to convert
  Strings to upper or lowercase
• So you could modify our previous method to count
  words regardless of case by adding just two lines

public String toLowerCase() public String
toUpperCase()
private int substringCounter( String text,
String word) int count 0
text text.toLowerCase() word
word.toLowerCase() int pos
text.indexOf(word,0) while (
pos gt 0 ) count pos
text.indexOf(word,posword.length())
return count
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Immutability

• Note that in that method, we had to assign the
  result of toUpperCase() and toLowerCase() to the
  variables
• Java Strings are immutable
• You cant change them with methods
• You can only make new Strings and assign them
  back to the variables
• Thats what these methods are doing.

private int substringCounter( String text,
String word) int count 0
text text.toLowerCase() word
word.toLowerCase() //etc.
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Cutting Strings up

• You already know we can put Strings together
  using the operator
• If you want to take Strings apart, use the
  substring() method
• For example, this will put 2 Strikes in
  strikesOnly

public String substring(int startIndex, int
indexBeyond)
String countText "3 Balls, 2 Strikes, 2
Outs" String strikesOnly strikesOnly
countText.substring(9,18)
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Some more String methods
public boolean startsWith(String s) //
true only if this string starts with s
public boolean endsWith(String s)
// true only if this string ends with s
public boolean equals(String s) // true
only if this string has same sequence of chars as
s public boolean
equalsIgnoreCase(String s) // true only
if this string has same sequence of chars as s
// except capital lower case letters
considered the same
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Some more String methods
public int lastIndexOf(String s) public
int lastIndexOf(String s, int startIndex)
// return index of last occurrence of s
(occurring at or // before
startIndex) in this string, and -1 if no match.
public String replace(char oldChar, char
newChar) // Returns a new string
resulting from replacing all //
occurrences of oldChar in this string with
newChar. public String trim()
// Eliminates all leading and trailing spaces.
public int compareTo(String s)
// Returns negative int if string before s
in case-sensitive // dictionary
order // returns 0 if equal
// returns positive int if string after s in
case-sensitive // dictionary
order. public char charAt(int index)
// Returns the character at the specified
index.