1. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Chapter 8 Strings 1

2. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 The String Class Constructing a String: ▫ String message = "Welcome to Java“; ▫ String message = new String("Welcome to Java“); ▫ String s = new String(); Obtaining String length and Retrieving Individual Characters in a string String Concatenation (concat) Substrings (substring(index), substring(start, end)) Comparisons (equals, compareTo) String Conversions Finding a Character or a Substring in a String Conversions between Strings and Arrays Converting Characters and Numeric Values to Strings 2

3. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Constructing Strings String newString = new String(stringLiteral); String message = new String("Welcome to Java"); Shorthand initializer: String message = "Welcome to Java"; 3

4. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Strings Are Immutable A String object is immutable; its contents cannot be changed. Does the following code change the contents of the string? String s = "Java"; s = "HTML"; 4

5. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Trace Code String s = "Java"; s = "HTML"; 5 animation

6. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Trace Code String s = "Java"; s = "HTML"; 6 animation

7. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Interned Strings Strings are immutable, frequently used. To improve efficiency and save memory, the JVM stores string literals that have the same contents once. Such an instance is called interned. For example, the following statements: 7

8. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Examples display s1 == s2 is false s1 == s3 is true 8 A new object is created if you use the new operator. If you use the string initializer, no new object is created if the interned object is already created.

9. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Trace Code 9 animation

10. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Trace Code 10

11. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Trace Code 11

12. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 String Comparisons 12

13. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 String Comparisons equals String s1 = new String("Welcome“); String s2 = "welcome"; if (s1.equals(s2)){ // s1 and s2 have the same contents } if (s1 == s2) { // s1 and s2 have the same reference } 13

14. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 String Comparisons, cont. compareTo(Object object) String s1 = new String("Welcome“); String s2 = "welcome"; if (s1.compareTo(s2) > 0) { // s1 is greater than s2 } else if (s1.compareTo(s2) == 0) { // s1 and s2 have the same contents } else // s1 is less than s2 14

15. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 String Length, Characters, and Combining Strings 15

16. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Finding String Length Finding string length using the length() method: message = "Welcome"; message.length() // returns 7 16

17. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Retrieving Individual Characters in a String Do not use message[0] Use message.charAt(index) Index starts from 0 17

18. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 String Concatenation String s3 = s1.concat(s2); String s3 = s1 + s2; s1 + s2 + s3 + s4 + s5 same as (((s1.concat(s2)).concat(s3)).concat(s4)).concat(s5); 18

19. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Extracting Substrings 19

20. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Extracting Substrings You can extract a single character from a string using the charAt method. You can also extract a substring from a string using the substring method in the String class. String s1 = "Welcome to Java"; String s2 = s1.substring(0, 11) + "HTML"; 20

21. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Converting, Replacing, and Splitting Strings 21

22. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Examples 22 Code New String Returned "Welcome".toLowerCase()welcome "Welcome".toUpperCase()WELCOME " Welcome ".trim()Welcome "Welcome".replace('e', 'A')WAlcomA "Welcome".replaceFirst("e", "AB") WABlcome "Welcome".replace("e", "AB") WABlcomAB "Welcome".replace("el", "AB") WABlcome

23. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Splitting a String String[] tokens = "Java#HTML#Perl".split("#", 0); for (int i = 0; i < tokens.length; i++) System.out.print(tokens[i] + "\n"); 23 Java HTML Perl // output

24. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Matching, Replacing and Splitting by Patterns Specify a pattern to Match Replace Split any string Useful and powerful feature known as regular expression. Refer to Supplement III.F, “Regular Expressions” 24 "Java".matches("Java"); "Java".equals("Java"); "Java is fun".matches("Java.*"); "Java is cool".matches("Java.*");

25. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Regular Expression Syntax 25 Companion Website

26. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Matching, Replacing and Splitting by Patterns replaceAll, replaceFirst, and split methods can be used with regular expression. The following statement returns a new string that replaces $, +, or # in "a+b$#c" by the string NNY. //code String s = "a+b$#c".replaceAll("[$+#]", "NNY "); System.out.println(s); // output: aNNY bNNY NNY c 26

27. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Matching, Replacing and Splitting by Patterns Splits string into array of strings (delimiter: punctuation marks): String[] tokens = "Java,C?C#,C++".split("[.,:;?]"); for (int i = 0; i < tokens.length; i++) System.out.println(tokens[i] ); //output: Java C C# C++ 27

28. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Regex What kind of string does this regular expression describe? [\\d]{3}-[\\d]{2}-[\\d]{4} Can you do the same input validation without using a regular expression? 28

29. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Replacing and Splitting Strings 29 Companion Website

30. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Examples String s = "Java Java Java".replaceAll("v\\w", "wi") ; 30 String s = "Java Java Java".replaceFirst("v\\w", "wi") ; String[] s = "Java1HTML2Perl".split("\\d"); Companion Website

31. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Finding a Character or a Substring in a String 31

32. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Finding a Character or a Substring in a String 32 CodeValue Returned (Int) "Welcome to Java".indexOf('W') 0 "Welcome to Java".indexOf('x') "Welcome to Java".indexOf('o', 5) 9 "Welcome to Java".indexOf("come") 3 "Welcome to Java".indexOf("Java", 5) 11 "Welcome to Java".indexOf("java", 5) "Welcome to Java".indexOf("java", 5) 14

33. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Convert Character and Numbers to Strings overloaded static valueOf methods convert character OR array of characters OR numeric values To strings. different argument types: char, char[], double, long, int, float Example: String.valueOf(5.44). Which overloaded method is called? 33

34. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Problem: Finding Palindromes Objective: Checking whether a string is a palindrome: a string that reads the same forward and backward. 34 CheckPalindrome Run

35. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 The Character Class 35

36. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Examples Character charObject = new Character('b'); charObject.compareTo(new Character('a')) returns 1 charObject.compareTo(new Character('b')) returns 0 charObject.compareTo(new Character('c')) returns -1 charObject.compareTo(new Character('d') returns –2 charObject.equals(new Character('b')) returns true charObject.equals(new Character('d')) returns false 36

37. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Problem: Counting Each Letter in a String This example gives a program that counts the number of occurrence of each letter in a string. Assume the letters are not case- sensitive. 37 CountEachLetter Run

38. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 StringBuilder and StringBuffer  alternative to the String class  StringBuilder/StringBuffer can be used wherever String is used.  More flexible than String class  Can be modified after creation.  Add, insert, or append contents to string buffer  Value of String object is fixed. 38

39. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 StringBuilder Constructors 39

40. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Modifying Strings in the Builder 40

41. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Examples stringBuilder = new StringBuilder(“Welcome to ”); stringBuilder.append("Java"); stringBuilder.insert(11, "HTML and "); stringBuilder.delete(8, 11) changes the builder to Welcome Java. stringBuilder.deleteCharAt(8) changes the builder to Welcome o Java. stringBuilder.reverse() changes the builder to avaJ ot emocleW. stringBuilder.replace(11, 15, "HTML") changes the builder to Welcome to HTML. stringBuilder.setCharAt(0, 'w') sets the builder to welcome to Java. 41

42. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 The toString, capacity, length, setLength, and charAt Methods 42

43. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Problem: Checking Palindromes Ignoring Non-alphanumeric Characters This example gives a program that counts the number of occurrence of each letter in a string. Assume the letters are not case- sensitive. 43 PalindromeIgnoreNonAlphanumeric Run