1. 1 String Class The primitive data types provide only the char for dealing with alpha-numeric data.

2. 2 String Class The problem arises when we are required to maintain and process data that, by its nature, exceeds a single character

3. 3 String Class One Way to resolve this problem is to create our own string of characters

4. 4 String Class While this is within our ability, maintaining such structures would be burdensome We can quickly see that every developer would begin to write their own solutions to handle a series of related characters

5. 5 String Class Well, we have: A common problem in handling an array/string of characters A set of generic functions that will be used to manipulate these characters…

6. 6 String Class What can be done to provide a practical solution ? Bundle all of these common functions that manipulate a string of related characters Create a String class !!!

7. 7 String Class Java provides two classes, the String and the StringBuffer to handle a series of characters or words

8. 8 Our Objective: Examine Java’s String class and StringBuffer Class We will work with Java Docs to understand the methods within each class We will look at the actual Java code to analyze HOW the String methods are implemented We will implement these classes in our projects

9. 9 What is Available OnLine: Class Lecture Notes This Powerpoint Presentation Java’s String class code StringExamples.java StringConstructors.java StringBufferConstructors.java StringBufferCapLen.java StringBufferInsert.java

10. 10 The String and StringBuffer classes provide a means for maintaining alpha-numeric data such as words or other text.

11. 11 We will examine these classes and their methods. We will also look into the code for these classes to get a better idea of how to design and develop useful classes for ourselves.

12. 12 The String and StringBuffer Classes are defined in the following Java API: java.lang.String java.lang.StringBuffer

13. 13 Strings: The Java String is a class and as such it has constructors and methods Actually, the String class has 9 constructor methods !!!

14. 14 There are literal Strings like “Hello World” that Java knows how to handle and there are the + and += operators that can be used (in their overloaded state) to concatenate strings with numbers or with other strings

15. 15 We will learn how to: Declare and use string literals Instantiate Strings by way of the various class constructors Convert strings to and from numbers

16. 16 Work with the immutable property of a String Use Various String methods

17. 17 Literal Strings: These kind of string objects are not instantiated as the Java compiler simply treats them as a reference to a String object that is stored in the runtime stack. They are really unreferenced variables

18. 18 Example: // string literal using a method int litLength = 0; litLength = "My String Literal".length(); System.out.println("length of \"My String Literal\" is: " + litLength); // Answer is 17

19. 19 You can also explicitly declare an instance of the String class (an object of): // declare string literal String myString = "Hello World";

20. 20 TPS There are many ways to create an instance of a String Write out as many different ways you can think of… Lets look at Java Docs list of constructors…..\..\..\Desktop\JAVA DOCS.lnk..\..\..\Desktop\JAVA DOCS.lnk

21. 21 TPS String MyString1 = new String(); String myString2 = "Hello World"; String myString3 = new String("Hello Wabbit") ; String myString4 = new String(myString2);

22. 22 String’s Immutability: We can initialize a String object / variable using a literal or by a using the return of a String method For example, we can initialize an empty string one of 2 ways: String myString = “ “ ; // instantiates a //string object with no value, empty string

23. 23 String myString = new String(); // instantiates a string object with no // value, empty string An uninitialized String: String myString; // myString is set to null NOTE: null and empty string are very different !!!

24. 24 You may execute String methods on an EMPTY string but if you attempt to Execute them against a NULL String, you will get a runtime error (NULL OBJECT REFERENCE) Another way to initialize a String: String myString = new String(“Hello Wabbit”) ; // invokes string constructor

25. 25 A String, once constructed, can not be modified (there are no modifier methods, sets, in the String class there are only accessor, gets, methods) NONE of the String methods MODIFY the private attributes of the String !!! This means that the String class is immutable

26. 26 The only way to “change” a String is to change the REFERENCE of the string with a new series of characters Example: String myString3 = new String("The Jets Stink !!!"); System.out.println(myString3); myString3 = ("The Giants Rule !!!"); System.out.println(myString3);

27. 27 The place in memory, reference point associated with the variable, String instance/ object, has been modified to point to the new String “The Giants Rule !!!” Java’s garbage collector ( explicitly called with System.gc(0); ) will destroy the old literal “The Jets Stink !!!”

28. 28 You can (copy constructor) have ONE String variable (object) COPIED into another variable: // have 1 string get a COPY of another string // different references or memory locations String myString4 = new String(myString3); System.out.println(myString4); myString4 = ("Changed MyString4"); // now strings 3 and 4 are different System.out.println(myString4); System.out.println(myString3);

29. 29 These variables have two different memory locations or references You can have TWO String variables (objects) refer to the same string object:

30. 30 // have 2 strings REFER to the same String //object reference // SHARED reference or memory location String myString5 = myString4; System.out.println(myString5); myString5 = (“Yankees Stink"); // now strings 4 and 5 refer to different // references System.out.println(myString5); System.out.println(myString4);

31. 31 String Class Methods: Review the Java API java.lang.String to review the various constructors and methods We will review some of the more common String methods but is up to the Student to understand and get familiar with the other methods as you are responsible for knowing about and how to use ANY published method !!!

32. 32 TPS Using the StringExamples.Java as a guide, create a program that uses the String class Create String class objects using the different methods Copy 1 String to another, print out the values Then, look through the Java Doc and work with some of the other methods

33. 33 String Class Methods: We will also examine the actual Java String class code to see how some of these methods are implemented We will look at a few of the String class constructors Lets look at the String Class Subset (handout) and review how some of the String methods are written…

34. 34 Common Methods: length() charAt() substring() concat()

35. 35 Common Methods: compareTo() compareToIgnoreCase() equals() equalsIgnoreCase()

36. 36 Common Methods: indexOf() lastIndexOf() trim() replace()

37. 37 Common Methods: toUpperCase() toLowerCase()

38. 38 length( ) returns the number of characters in the string System.out.println("The length of myString5 is: " + myString5.length()); // ANS 13 The string is “Yankees Stink”

39. 39 charAt( ) Returns a character of a String // length minus 2 gets next to last char as // "charat" starts at element ZERO and // NOT 1 char c = myString5.charAt(myString5.length() - 2); System.out.println(" the next to last character is: " + c); //n The last example returns ‘n’ and NOT ‘i’ because charAt(0) starts at position ZERO And NOT ONE

40. 40 If we were to just say: char c = myString5.charAt (myString5.length()); We would get an IndexOutOfBounds exception error (more on exception handling in a later Lecture)

41. 41 substring( ) overloaded methods returns portions of a string (as a string) // substring String myString6 = myString5.substring(3); System.out.println(myString6); // kees Stink

42. 42 myString6 = myString5.substring(8, 12); // [from, to) positions // to position is EXCLUSIVE System.out.println(myString6); // Stin "hamburger".substring(4, 8) returns "urge" "smiles".substring(1, 5) returns "mile" Note in the second substring method, the from position is INCLUSIVE and the to position is EXCLUSIVE (as it represents the position FOLLOWING the substring)

43. 43 TPS Work with the string methods just reviewed  Length  charAt  substring Ask the user for a word Display the length of the word Display the middlemost character of the word Split the word into 2 separate strings and display each new string

44. 44 Concatenation: The concat( ) method snaps together 2 strings and works exactly the same way as the overloaded operator+ method

45. 45 // concat String myString7 = "Millburn "; String myString8 = "High School"; String myString9 = myString7.concat(myString8); System.out.println(myString9); // Millburn High School String myString10 = myString7 + myString8; System.out.println(myString10); //Millburn High School

46. 46 Both produce the same results You can also use the operator+= overloaded operator, however it is MOST inefficient String S = “2*2”; S += “=4”; S is now “2*2 = 4” however it is inefficient because the RVALUE is a temp string that is used to reassign variable S’s reference point to now refer to “2*2=4”

47. 47 You can also concatenate Strings and numbers as long as Java understands you are working with strings String S = “Year “; S += 2002; Results in S being = to “Year 2002”

48. 48 HOWEVER, if you write: String S = “Year “; S += ‘ ‘ + 2003; The compiler thinks you are performing math on ‘ ‘ and 2003 and will in effect perform S += (‘ ‘ + 2003) which will find the ASCII or Unicode for a space, which is 32, and add that to 2003 giving S the value “Year 2035”

49. 49 If you had modified the code to concatenate a space AS A STRING S += “ “ + 2003; Then you would get the intended result as “ “ is understood as a string IMPORTANT NOTE:String concatenation with + (operator+) actually invokes the toString( ) method for the particular class and converts any numbers to Strings

50. 50 Finding characters and substrings: indexOf( char c) returns the position of the first occurance of the character c in the string Remember that indicies begin at ZERO There are several overloaded indexOf methods A NOT FOUND condition results in a –1

51. 51 Examples: int pos = myString7.indexOf('b'); // Millburn System.out.println(pos); // 4 int pos = myString7.indexOf('x'); System.out.println(pos); // -1 You can also start searching form any point within the string

52. 52 Example: pos = myString8.indexOf('h', 4); // High School System.out.println(pos); // 7 Here, you search for an ‘h’ but start your search after element 4

53. 53 You can perform a search starting at the end of the string Example: // High School pos = myString8.lastIndexOf('h', 8); System.out.println(pos); // 7 pos = myString8.lastIndexOf('h', 2); System.out.println(pos); // -1

54. 54 Here, the second parameter is the STARTING POINT from the END of the string There methods are also overloaded to allow a search for a specific SUBSTRING The methods are similar except that instead of a character variable or literal you add in a string literal or a String variable

55. 55 int pos = myString7.indexOf('bur'); // Millburn System.out.println(pos); // 4 pos = myString8.indexOf('hoo', 4); // High School System.out.println(pos); // 7 String myStr = “ch” pos = myString8.lastIndexOf(mySr, 8); System.out.println(pos); // 6

56. 56 TPS Work with the string methods just reviewed  concat  +=  +  indexOf Read in a series of words from a file (4 or 5 words) Add these words to a single string Ask the user to search for a character and display each position the character is found

57. 57 Comparisons: You CAN NOT use relational operators ( == != >=, etc) to compare Strings (unlike C++ which overloads those operators o work with strings, Java however does not allow for operators to be overloaded)

58. 58 Comparisons: AS previously discussed, these operators, when applied against objects, compare REFERENCES to those objects (their memory address) and NOT THEIR STATE (values of their attributes)

59. 59 Comparisons: Relational Operators work on primitive data types because there they compare the VALUES in these variables

60. 60 Comparisons: To compare the STATE of a String, use the following methods: equals( ) equalsIgnoreCase() compareTo()

61. 61 Comparisons: equals() and equalsIgnoreCase() are boolean methods and return simply TRUE or FALSE True if the strings have the same length and characters (case matters) otherwise it returns false

62. 62 Examples: boolean TF = myString7.equals(myString8); System.out.println(TF); // FALSE TF = myString7.equals("MIllburn "); System.out.println(TF); // FALSE TF = mStr.equalsIgnoreCase("Millburn "); System.out.println(TF); // TRUE

63. 63 What will happen here: String s; Boolean TF = s.equals(“OK”);

64. 64 What will happen here: String s; Boolean TF = s.equals(“OK”); Answer is a runtime error as s has yet to be instantiated, so it is NULL Object Reference

65. 65 One method for first checking to see if an object is indeed instantiated (resident in memory) you can do the following: Boolean TF = ( s != null && s.equals(“OK”));

66. 66 I guess I lied when I said that you can not use relational operators You can but they only EVALUATE THE REFERENCE (memory) Here we are first checking to see if the string object s is resident, if it is not then the first part of the if is FALSE, and because of short circuit evaluation, the second part of the if is NEVER evaluated Cool, huh !!!

67. 67 CompareTo() returns an integer that describes the RESULT of the comparison S1.compareTo(s2) RETURNS: A negative integer if s1 PRECEEDES s2 ZERO if they are equal Positive integer if s1 POSTCEDES s2 (comes later)

68. 68 He comparison begins at the first character and proceeds LEFT to RIGHT until different characters are encountered in corresponding positions OR until one of the strings end When a difference occurs, the method returns the difference of the Unicodes (ASCII values) of the different characters

69. 69 The string with the “SMALLER” character, the one with he lower ASCII code, is deemed smaller With strings of different lengths, the method returns the difference in lengths, so he shorter string is deemed SMALLER Remember that UPPER CASE letters come BEFORE lower case letters

70. 70 This process is called LEXICOGRAPHIC ORDERING

71. 71 Examples: String s = "ABC"; int result = s.compareTo("abc"); System.out.println(result);

72. 72 Examples: String s = "ABC"; int result = s.compareTo("abc"); System.out.println(result); //NEGATIVE 32, ABC smaller than abc result = s.compareTo("ABCD"); System.out.println(result);

73. 73 Examples: result = s.compareTo("ABCD"); System.out.println(result); //NEGATIVE 1, ABC smaller than ABCD result = s.compareTo("Amuh"); System.out.println(result);

74. 74 Examples: result = s.compareTo("Amuh"); System.out.println(result); //NEGATIVE 43 s = "adset"; result = s.compareTo("ADSET"); System.out.println(result);

75. 75 Examples: s = "adset"; result = s.compareTo("ADSET"); System.out.println(result); //POSITIVE 32, adset GT ADSET result = s.compareTo("aDsetf"); System.out.println(result);

76. 76 Examples: result = s.compareTo("aDsetf"); System.out.println(result); //NEGATIVE 32

77. 77 There is also a compareToIgnoreCase( ) that works in the same manner except it ignores case differences

78. 78 Remember that Strings are OBJECTS and their variable names are REFERENCES to those objects Therefore using the == to compare Strings will merely compare their REFERENCES

79. 79 String i = new String("21"); String y = new String("21"); // compares REFERENCES if (i == y) System.out.println("The Strings are EQUAL !!!"); else System.out.println("The Strings are NOT EQUAL !!!"); // compares STATE if (i.equals(y)) System.out.println("The Strings are EQUAL !!!"); else System.out.println("The Strings are NOT EQUAL !!!");

80. 80 String i = new String("21"); String y = new String("21"); // compares REFERENCES i = y; if (i == y) System.out.println("The Strings are EQUAL !!!"); else System.out.println("The Strings are NOT EQUAL !!!"); RETURNS TRUE

81. 81 TPS Try this out by writing code that evaluates 2 strings for equality using the = = and then usin g the String methods See that = = compares references and the String methods compare STATE

82. 82 Conversions: toUpperCase() returns a new string in all UPPER CASE toLowerCase() returns a new string in all lower case replace(c1, c2) builds and returns a new string in which all occurances of rthe character c1 are REPLACED with c2

83. 83 Conversions: trim() builds and returns a new string in which all the “whitespace” characters (spaces, tabs, newline) are trimmed from the beginning and the end of the string

84. 84 Examples: String s1 = “ modify This onE “ String s2; s2 = s1.trim(); s2 = s1.toUpperCase();

85. 85 Examples: String s1 = “ modify This onE “ String s2; s2 = s1.trim(); // s2 = “modify This onE” s2 = s1.toUpperCase(); // s2 = “ MODIFY THIS ONE “

86. 86 Examples: String s1 = “ modify This onE “ String s2; s2 = s1.toLowerCase(); s2 = s1.replace(‘o’, ‘x’);

87. 87 Examples: String s1 = “ modify This onE “ String s2; s2 = s1.toLowerCase(); // s2 = “ modify this one “ s2 = s1.replace(‘o’, ‘x’); // s2 = “ mxdify This xnE “

88. 88 Remember that String s1 REMAINS UNCHANGED !!! What does this code do ? S1.trim();

89. 89 Remember that String s1 REMAINS UNCHANGED !!! What does this code do ? S1.trim(); // ANSWER is nothing as there is no placeholder, string, to accept the returned string from the trim method See handouts 1 and 2 for Code examples using String

90. 90 TPS Work with the string methods just reviewed  trim  toUpperCase  toLowerCase  replace Read in a series of words from a file (4 or 5 words) Remove whitespace Convert every other word to UC & LC Replace all vowels with X

91. 91 Converting Strings to Numbers and Numbers into Strings: There are 2 ways to convert an int into a String of digits (including a minus sign iff)

92. 92 1.use the static method (remember what a static method is/ means ?) toString() of the Integer class(as mentioned before, we will discuss the Integer as well as the ofther Number classes in the second semester) int n = -123; String s1 = Integer.toString(n); // s1 now has the value “-123”

93. 93 Now, notice that we did not create an instance of the Integer class ! Well, I thought that in order to work with the methods of a class you have to create an instance of that class ?

94. 94 The answer, obviously, is that Integer’s toString() method is a STATIC method Static methods can be executed without having an instance (object) of that class declared as there is only one version of the static method for ANY instance of the particular class

95. 95 Remember, however, that STATIC mathods MAY NOT CALL non Static Methods as non Static methods are tied to an instance of the class (object)

96. 96 The Integer class (called a wrapper class as it “wraps” around the int primitive data type) is part of the java.lang package that is AUTOMATICALLY imported into all programs

97. 97 2. Use the Static method of the String class valueOf() int n = -123; String s2 = String.valueOf(n); // s2 now has the value “-123”

98. 98 Even though we said there are 2 ways, can you think of a third ?

99. 99 Even though we said there are 2 ways, can you think of a third ? How about using concatenation !!! int n = -123; String s2 = “” + n; // s2 now has the value “-123”

100. 100 You can perform the same type of operations on doubles (there is also a Double wrapper class) double n = 123.5; String s1 = Double.toString(n); // s1 now has the value “123.5”

101. 101 However, with doubles the resulting String value may not display cleanly, depending on the value in the double To properly present the double in a String use a specified FORMAT There is a class called DecimalFormat

102. 102 Create an instance of this class that describes the desired format Then use that format for the conversion into a string Example:

103. 103 import java.text.DecimalFormat; DecimalFormat moneyFormat = new DecimalFormat("0.00"); double totalSales = 123.5; String s1 = moneyFormat.format(totalSales); System.out.println("total Sales Are $" + s1); // 123.50

104. 104 Converting a String into an Integer can be accomplished by using the parseInt( ) method of the Integer class Example: String s2 = “-123”; int n = Integer.parseInt(s2); // n = -123

105. 105 The same works for doubles String s2 = “123.56”; double n = Double.parseDouble(s2); // n = 123.56 These methods throw NumberFormatExceprion error if the string is not in the proper format

106. 106 TPS Work with the various ways to convert a number into a string Then try converting a string into a number Work with ALL the different ways to accomplish this

107. 107 Character Methods: These methods can help to identify a specific character (in a String) is a digit, a letter or something else The Character wrapper has several static boolean methods They all take a char as an argument and return TRUE or FALSE

108. 108 Example: Char c = ‘2’; Boolean anwr = Character.isDigit( c ); char c1 = Character.toUpperCase(‘a’); char c2 = Character.toLowerCase(‘A’);

109. 109 Char c = ‘2’; Boolean anwr = Character.isDigit( c ); // TRUE char c1 = Character.toUpperCase(‘a’); // c1 = ‘A’ char c2 = Character.toLowerCase(‘A’); // c1 = ‘a’

110. 110 Java API String Search: TPS Find the method that compares PORTIONS of 2 string objects for equality

111. 111 Java API String Search: TPS Find the method that compares PORTIONS of 2 string objects for equality ANS regionMatches( )

112. 112 S3.regionMatches( 0, s4, 0,5); 0 is the starting index of the string s3 s4 is the comparison string 0 the starting index of the comparison string s4 5 is the number of characters to compare between the 2 strings Returns TRUE or FALSE

113. 113 S3.regionMatches(true, 0, s4, 0,5); Overloaded method, first argument, if true IGNORES case

114. 114 TPS Find the method(s) that tests a string to see if it starts with or ends with a particular set of characters

115. 115 TPS Find the method(s) that tests a string to see if it starts with or ends with a particular set of characters ANS startsWith( ) endsWith( )

116. 116 String s3 = “started”; s3.startsWith(“st”); If s3 starts with the letters “st” returns TRUE

117. 117 s3.startsWith(“st”, 2); If s3 starts AT INDEX 2 (3 rd element) with the letters “st” returns TRUE S3.endsWith(“ed”); If s3 ends with the letters “ed” returns TRUE

118. 118 StringBuffer Class: Unlike the Sring class, the StringBuffer class in NOT immutable as there are modifier, set, methods that allow you to alter the private attributes (state) of an instance of the SringBuffer class StringBuffer attributes’ STATE is dynamic

119. 119 StringBuffer stores the characters of the string along with a capacity This class has THREE constructors B1 = new StringBuffer(); empty string with a default capacity of 16 characters

120. 120 B1 = new StringBuffer( 10 ); empty string with an initial capacity of 10 characters

121. 121 Example: StringBuffer mySB = new StringBuffer(“Kerry Collins Rules”); char c = mySB.charAt(0); mySB = setCharAt(0, Chatacter.toLower( c ) ); mySB is now = kerry Collins

122. 122 Unlike the String class, which would have necessitated a NEW reference, StringBuffer maintains the original reference and only 1 character is modified More efficient to use than String class when you will be constantly modifying the state of a “string”

123. 123 Some Methods of this class are : length( ) capacity( ) setLength( ) ensureCapacity( ) charAt( )

124. 124 Some Methods of this class are : setCharAt( ) getChars( ) reverse( ) append( ) insert( ) delete( )

125. 125 StringBuffer sb = new StringBuffer(“Hello there”); sb.charAt(4); sb.setCharAt(6, ‘T’);

126. 126 StringBuffer sb = new StringBuffer(“Hello there”); sb.charAt(4); // returns the character at the index specified ANS = ‘o’ sb.setCharAt(6, ‘T’); // sets the char at the index sb now = “Hello There”

127. 127 int a = 7; sb.append(“ “); sb.append(a);

128. 128 int a = 7; sb.append(“ “); sb.append(a); // sb now = “Hello There 7”

129. 129 See handouts 3, 4, and 5 for Code examples using StringBuffer Review the Java API java.lang.StringBuffer to review the various constructors and methods

130. 130 Projects... Backwords Letters and Digits Occurrences Name Flip Encode / Decode Palindrome Count Words Dictionary Find Names GREP

131. 131...Projects: Text Analysis Using Classes Add A StringBuffer vs String project to examine efficiency NOTE: Read ALL projects, find common elements, plan, create a CLASS of common methods !!!

132. 132 TEST IS THE DAY AFTER THE PROJECT IS DUE !!!