1. Characters, String and Regular expressions

2. Characters char data type is used to represent a single character. Characters are stored in a computer memory using some form of encoding. Java uses Unicode, which includes ASCII, for representing char constants.

3. ASCII Encoding For example, character 'O' is 79 (row value 70 + col value 9 = 79). O 9 70

4. Unicode Encoding The Unicode Worldwide Character Standard (Unicode) supports the interchange, processing, and display of the written texts of diverse languages. Java uses the Unicode standard for representing char constants. char ch1 = 'X'; System.out.println(ch1); System.out.println( (int) ch1); X 88

5. Character Processing Declaration and initialization char ch1, ch2 = ‘X’; Type conversion between int and char. System.out.print("ASCII code of character X is " + (int) ' X ' ); System.out.print("Character with ASCII code 88 is " + (char)88 ); This comparison returns true because ASCII value of 'A' is 65 while that of 'c' is 99. ‘A’ < ‘c’

6. Strings A string is a sequence of characters that is treated as a single value. Instances of the String class are used to represent strings in Java.

7. String declaration Create a String object String ; =new String(“ ”); Create a String literal String ; = “<value of a string”;

8. Example String word1; word1 = new String(“Java”); OR String word1; word1 = “Java”;

9. Examples We can do this because String objects are immutable.

10. String constructor No-argument constructor One-argument constructor A String object One-argument constructor A char array Three-argument constructor A char array An integer specifies the starting position An integer specifies the number of characters to access

11. Review To compute how many characters the string myString contains, we use: a. myString.size b. myString.size() c. myString.length d. myString.length()

12. Review To compute how many characters the string myString contains, we use: a. myString.size b. myString.size() c. myString.length d. myString.length()

13. Review Java uses this to represent characters of diverse languages a. ASCII b. UNICODE c. EDBIC d. BINARY

14. Review Java uses this to represent characters of diverse languages a. ASCII b. UNICODE c. EDBIC d. BINARY

15. Accessing Individual Elements Individual characters in a String accessed with the charAt method. 012345 6 Sumatr a String name = "Sumatra"; name This variable refers to the whole string. name.charAt( 3 ) The method returns the character at position # 3.

16. Other Useful String Operators MethodMeaning compareTo Compares the two strings. str1.compareTo( str2 ) substring Extracts the a substring from a string. str1.substring( 1, 4 ) trim Removes the leading and trailing spaces. str1.trim( ) valueOf Converts a given primitive data value to a string. String.valueOf( 123.4565 ) startsWith Returns true if a string starts with a specified prefix string. str1.startsWith( str2 ) endsWith Returns true if a string ends with a specified suffix string. str1.endsWith( str2 ) length Return the length of the given string str1.length()

17. Compute Length of a string Method: length() Returns the length of a string Example: String strVar; strVar = new String(“Java”); int len = strVar.length();

18. Substring Method: Extract a substring from a given string by specifying the beginning and ending positions Example: String strVar, strSubStr; strVar = new String(“Exam after Easter”); strSubStr = strVar.substring(0,4);

19. Index position of a substring within another string Method: Find an index position of a substring within another string. Example: String strVar1 = “Google it”; String strVar2 = “Google”; int index; index = strVar1.indexOf(strVar2);

20. String concatenation Method: Create a new string from two strings by concatenating the two strings. Example: String strVar1 = “Google”; String strVar2 = “ Search Engine”; String sumStr; sumStr = strVar1.concat(strVar2);

21. Review What method is used to refer to individual character in a String a. getBytes b. indexOf c. getChars d. charAt

22. Review What method is used to refer to individual character in a String a. getBytes b. indexOf c. getChars d. charAt

23. Review To compare two strings in Java, we use a. == b. equals method c. != d. <>

24. Review To compare two strings in Java, we use a. == b. equals method c. != d. <>

25. String comparison Methods: equals equalsIgnoreCase compareTo compareToIgnoreCase

26. String comparison equals, and equalsIgnoreCase Example: String string1 =“COMPSCI”; String string2 = “compsci” boolean isEqual; isEqual = string1.equals(string2);

27. Common error Comparing references with == can lead to logic errors, because == compares the references to determine whether they refer to the same object, not whether two objects have the same contents. When two identical (but separate) objects are compared with ==, the result will be false. When comparing objects to determine whether they have the same contents, use method equals.

28. String comparison compareTo Example: String string1 =“Adam”; String string2 = “AdamA”; int compareResult; compareResult = string1.compareTo(string2);

29. String comparison - string1.compareTo(string2) Compares two strings lexicographically will return 0 if two strings are equal will return negative value if string1 is less than string 2 will return positive value if string1 is greater than string 2 The comparison is based on the Unicode value of each character in the strings

30. String comparison The comparison is based on the Unicode value of each character in the strings let k be the smallest index valid for both strings; compareTo returns the difference of the two character values at position k in the two string -- that is, the value: character at the position k of string 1 – character at the position k of string 2

31. regionMatches regionMatches(boolean ignoreCase, int toffset, String other, int ooffset, int len)String A substring of this String object is compared to a substring of the argument other. The result is true if these substrings represent character sequences that are the same, ignoring case if and only if ignoreCase is true.

32. The String Class is Immutable In Java a String object is immutable This means once a String object is created, it cannot be changed, such as replacing a character with another character or removing a character The String methods we have used so far do not change the original string. They created a new string from the original. For example, substring creates a new string from a given string.

33. Review If x.equals(y) is true, then x==y is always true a. True b. False

34. Review If x.equals(y), then x==y is always true a. True b. False

35. The StringBuffer Class In many string processing applications, we would like to change the contents of a string. In other words, we want it to be mutable. Manipulating the content of a string, such as replacing a character, appending a string with another string, deleting a portion of a string, and so on, may be accomplished by using the StringBuffer class.

36. StringBuffer Example StringBuffer word = new StringBuffer("Java"); word.setCharAt(0, 'D'); word.setCharAt(1, 'i'); Changing a string Java to Diva word : StringBuffer Java Before word : StringBuffer Diva After

37. Delete a substring from a StringBuffer object StringBuffer word = new StringBuffer(“CCourse”); word.delete(0,1);

38. Append a string StringBuffer word = new StringBuffer(“CS ”); word.append(“Course”);

39. Insert a string StringBuffer word = new StringBuffer(“MCS Course”); word.insert(4,“220”);

40. Convert from StringBuffer to String StringBuffer word = new StringBuffer(“Java”); word.setCharAt(0,’D’); word.setCharAt(1,’i’); System.out.println(word.toString());

41. Review Both the String and StringBuffer classes include the charAt and setCharAt methods a. True b. False

42. Review Both the String and StringBuffer classes include the charAt and setCharAt methods a. True b. False

43. Review What will be the value of str after the following statements are executed: String str; StringBuffer strBuf; str ="Decaffeinated"; strBuf = new StringBuffer(str.substring(2,7)); strBuf.setCharAt(1,'o'); strBuf.append('e'); str = strBuf.toString();

44. StringBuffer methods Method length Return StringBuffer length Method capacity Return StringBuffer capacity Method setLength Increase or decrease StringBuffer length Method ensureCapacity Set StringBuffer capacity Guarantee that StringBuffer has minimum capacity

45. Class StringTokenizer Tokenizer Partition String into individual substrings Use delimiter Typically whitespace characters (space, tab, newline, etc) Java offers java.util.StringTokenizer

46. Outline

47. Pattern Example Suppose students are assigned a three-digit code: The first digit represents the major (5 indicates computer science); The second digit represents either in-state (1), out-of- state (2), or international (3); The third digit indicates campus housing: On-campus dorms are numbered 1-7. Students living off-campus are represented by the digit 8. The 3-digit pattern to represent computer science majors living on-campus is 5[123][1-7] first character is 5 second character is 1, 2, or 3 third character is any digit between 1 and 7

48. Regular Expressions, Class Pattern and Class Matcher Regular expression Sequence of characters and symbols Useful for validating input and ensuring data format Facilitate the construction of a compiler Regular-expression operations in String Method matches Matches the contents of a String to regular expression Returns a boolean indicating whether the match succeeded

49. R egular Expressions, Class Pattern and Class Matcher Predefine character classes Escape sequence that represents a group of character Digit Numeric character Word character Any letter, digit, underscore Whitespace character Space, tab, carriage return, newline, form feed

50. Predefined character classes.

51. Regular Expressions Other patterns Square brackets ( [] ) Match characters that do not have a predefined character class E.g., [aeiou] matches a single character that is a vowel Dash ( - ) Ranges of characters E.g., [A-Z] matches a single uppercase letter ^ Not include the indicated characters E.g., [^Z] matches any character other than Z

52. Regular expression Quantifiers Plus (+) Match one or more occurrences E.g., A+ Matches AAA but not empty string Asterisk ( * ) Match zero or more occurrences E.g., A* Matches both AAA and empty string Others in Fig. 29.22

53. Quantifiers used in regular expressions.

54. Regular Expression Examples ExpressionDescription [013] A single digit 0, 1, or 3. [0-9][0-9] Any two-digit number from 00 to 99. [0-9&&[^4567]] A single digit that is 0, 1, 2, 3, 8, or 9. [a-z0-9] A single character that is either a lowercase letter or a digit. [a-zA-Z][a-zA- Z0-9_$]* A valid Java identifier consisting of alphanumeric characters, underscores, and dollar signs, with the first character being an alphabet. [wb](ad|eed) Matches wad, weed, bad, and beed.

55. Regular expression Replacing substrings and splitting strings String method replaceAll Replace text in a string with new text String method replaceFirst Replace the first occurrence of a pattern match String method split Divides string into several substrings

56. Regular expression Class Pattern Represents a regular expression Class Match Contains a regular-expression pattern and a CharSequence Interface CharSequence Allows read access to a sequence of characters String and StringBuffer implement CharSequence

57. Regular Expression Examples ExpressionDescription [013] A single digit 0, 1, or 3. [0-9][0-9] Any two-digit number from 00 to 99. [0-9&&[^4567]] A single digit that is 0, 1, 2, 3, 8, or 9. [a-z0-9] A single character that is either a lowercase letter or a digit. [a-zA-z][a-zA- Z0-9_$]* A valid Java identifier consisting of alphanumeric characters, underscores, and dollar signs, with the first character being an alphabet. [wb](ad|eed) Matches wad, weed, bad, and beed.

58. Matching Searches for 2 character pattern whose first character may be any uppercase letter between A and G, and whose second character may be any number except 4 Searches for a character pattern that may be any alphabet except p,q,r,s, or t A. str.matches(“[a-zA-Z]&&[^pqrst]”); B. str.matches(“[A-G][0-9 && [^4]]”);

59. Matching Searches for 2 character pattern whose first character may be any uppercase letter between A and G, and whose second character may be any number except 4 Searches for a character pattern that may be any alphabet except p,q,r,s, or t A. str.matches(“[a-zA-Z]&&[^pqrst]”); B. str.matches(“[A-G][0-9]&&[^4]”); B A

60. Review Which character sequence would be used to designate a character pattern of a fixed length of three digits A. 3[0-9] B. [0-9][0-9][0-9] C. [0-9]3

61. Review Which character sequence would be used to designate a character pattern of a fixed length of three digits A. 3[0-9] B. [0-9][0-9][0-9] C. [0-9]3

62. Review Choose the correct argument for the following code that searches for any number between 100 and 999 in a given string. str.matches(“ “); A. [0-9][0-9][1-9] B. [0-9][0-9][0-9] C. [1-9][0-9][0-9]

63. Review Choose the correct argument for the following code that searches for any number between 100 and 999 in a given string. str.matches(“ “); A. [0-9][0-9][1-9] B. [0-9][0-9][0-9] C. [1-9][0-9][0-9]