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String and string manipulation x


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String and string manipulation x

  2. 2. CONTENTS1. What Is String?2. Creating and Using Strings • Declaring, Creating, Reading and Printing3. Manipulating Strings • Comparing, Concatenating, Searching, Extracting Substrings, Splitting4. Other String Operations • Replacing Substrings, Deleting Substrings, Changing Character Casing, Trimming
  3. 3. CONTENTS5. Building and Modifying Strings • Using StringBuilder Class6. Formatting Strings
  5. 5. FREQUENTLY ASKED QUESTIONS FAQ• What is string ?• Why string is final?• What are the ways to declare and initialize the String Object?• What is the difference b/w Reference values and literal strings?• What is the difference b/w + opretor and concat() method?• What is the effect when comparing strings with == and equals() ?• What is Difference b/w String class and String Buffer?• What is String pool in Java?• What does intern() method do in Java?• Why String is thread-safe in Java?• What is the difference b/w System.out.println(1+2+” text”) and System.out.println(” text”+1+2) ?
  6. 6. WHAT IS STRING?• String is: • A sequence of characters • Each character is a Unicode character • Represented by the String (java.lang.String) data type in Java • Example: String s = "Hello, Java"; s H e l l o , J a v a
  7. 7. JAVA.LANG.STRING• We use java.lang.String to work with strings in Java• String objects contain an immutable (read-only) sequence of characters• Use Unicode in order to support multiple languages and alphabets• Stores strings in the dynamic memory (managed heap)• java.lang.String is class • It is reference type
  8. 8. JAVA.LANG.STRING• String objects are like arrays of characters (char[]) • Have fixed length (String.length()) • Elements can be accessed by index • Using charAt() method • The index is in the range 0...length()-1 String s = "Hello!"; int len = s.length(); // len = 6 char ch = s.charAt(1); // ch = e„` index = 0 1 2 3 4 5 s.charAt(index) = H e l l o !
  9. 9. STRINGS – FIRST EXAMPLEString s = “Hidaya Institute of Science & Tchnology.";System.out.printf("s = "%s"%n", s);System.out.printf("s.length() = %d%n", s.length());for (int i = 0; i < s.length(); i++) { System.out.printf("s[%d] = %c%n", i, s.charAt(i));}
  10. 10. STRINGS – FIRST EXAMPLE Live Demo
  11. 11. Creating and Using Strings Declaring, Creating, Reading and Printing
  12. 12. DECLARING STRINGS• We use Java String class for declaring string variables: String str;
  13. 13. CREATING STRINGS• Before initializing a string variable is equal to null• Strings can be initialized by: • Assigning a string literal to the string variable • Assigning the value of another string variable • Assigning the result of operation of type string
  14. 14. CREATING STRINGS (2)• Not initialized variables has value of null String s; // s is equal to null• Assigning a string literal String s = "I am string literal!";• Assigning another string variable String s2 = s;• Assigning the result of string operation String s = "Im " + 42 + " years old.";
  15. 15. READING AND PRINTING STRINGS• Reading strings from the console • Use the method input.nextLine() String s = input.nextLine(); • Printng Strings to the console • Use methods print() and println() System.out.print("Please enter your name: "); String name = input.nextLine(); System.out.printf("Hello, %s!%n", name);
  16. 16. MANIPULATING STRINGSComparing, Concatenating, Searching, Extracting Substrings, Splitting
  17. 17. COMPARING STRINGS• There are a number of ways to compare two strings: • Dictionary-based string comparison • Case-insensitive int result = str1.compareToIgnoreCase(str2); // result == 0 if str1 equals str2 // result < 0 if str1 if before str2 // result > 0 if str1 if after str2 • Case-sensitive str1.compareTo(str2);
  18. 18. COMPARING STRINGS (2)• Equality checking by equalsIgnoreCase() • Performs case-insensitive compare • Returns boolean value if (str1.equalsIgnoreCase(str2)){ … }• The case-sensitive equals() method if (str1.equals(str2)){ … }
  19. 19. COMPARING STRINGS (3)• Operators == and != does not check for equality!• These operators returns boolean value, but check if the addresses of the object are equal• Use equals() and equalsIgnoreCase() instead String str1 = new String("Hello"); String str2 = str1; System.out.println((str1==str2)); // true String str1 = "Hello"; String str2 = "Hello"; System.out.println((str1==str2)); // true!!! String str1 = new String("Hello"); String str2 = new String("Hello"); System.out.println((str1==str2)); // This is false!
  20. 20. COMPARING STRINGS – EXAMPLE• Finding the first in a lexicographical order string from a given list of stringsString[] towns = {"Jamshoro", "hyderabad","Qasimabad","Latifabad", "Kotri", "Heerabad"};String firstTown = towns[0];for (int i=1; i<towns.length; i++) { String currentTown = towns[i]; if (currentTown.compareTo(firstTown) < 0) { firstTown = currentTown; }}System.out.println("First town: " + firstTown);
  21. 21. COMPARING STRINGS Live Demo
  22. 22. CONCATENATING STRINGS• There are two ways to combine strings: • Using the concat() method String str = str1.concat(str2); • Using the + or the += operator String str = str1 + str2 + str3; String str += str1;• Any object can be appended to string String name = "Peter"; int age = 22; String s = name + " " + age; //  "Peter 22"
  23. 23. CONCATENATING STRINGS – EXAMPLE String firstName = "Shahjahan"; String lastName = "Samoon"; String fullName = firstName + " " + lastName; int age = 23; String nameAndAge = "Name: " + fullName + "nAge: " + age; System.out.println(nameAndAge); // Name: Shahjahan Samoon // Age: 23
  25. 25. SEARCHING STRINGS• Finding a character or substring within given string • First occurrence indexOf(String str) • First occurrence starting at given position indexOf(String str, int fromIndex) • Last occurrence lastIndexOf(String) • Last occurrence before given position lastIndexOf(String, int fromIndex)
  26. 26. SEARCHING STRINGS – EXAMPLE String str = "Java Programming Course"; int index = str.indexOf("Java"); // index = 0 index = str.indexOf("Course"); // index = 17 index = str.indexOf("COURSE"); // index = -1 // indexOf is case sensetive. -1 means not found index = str.indexOf("ram"); // index = 9 index = str.indexOf("r"); // index = 6 index = str.indexOf("r", 7); // index = 9 index = str.indexOf("r", 10); // index = 20 i = 0 1 2 3 4 5 6 7 8 9 10 11 12 …s.charAt(i) = J a v a P r o g r a m m …
  27. 27. SEARCHING STRINGS Live Demo
  28. 28. EXTRACTING SUBSTRINGS• Extracting substrings • str.substring(int beginIndex, int endIndex) • lastIndex is not included String filename = "C:PicsRila2005.jpg"; String name = filename.substring(8, 16); // name is Rila2005 • str.substring(int beginIndex) String filename = "C:PicsSummer2005.jpg"; String nameAndExtension = filename.substring(8); // nameAndExtension is Rila2005.jpg 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 C : P i c s R i l a 2 0 0 5 . j p g
  30. 30. SPLITTING STRINGS• To split a string by given separator(s) use the following method: String[] split(String regex)• String regex – String with special format• We can list the character which we want to use for separator in square brackets […] String[] parts = "Ivan; Petar,Gosho".split("[;,]"); // this wil separate the stirng into three parts // "Ivan", " Petar" and "Gosho"
  31. 31. SPLITTING STRINGS - EXAMPLE String listOfBeers = "Amstel, Zagorka, Tuborg, Becks."; String[] beers = listOfBeers.split("[ ,.]"); System.out.println("Available beers are:"); for (String beer : beers) { if (!"".equalsIgnoreCase(beer)) { System.out.println(beer); } }
  32. 32. SPLITTING STRINGS Live Demo
  33. 33. OTHER STRING OPERATIONS Replacing Substrings, Changing Character Casing, Trimming
  34. 34. REPLACING SUBSTRINGS• replace(String, String) – replaces all occurrences of given string with another • The result is new string (strings are immutable) String cocktail = "Vodka + Martini + Cherry"; String replaced = cocktail.replace("+", "and"); // Vodka and Martini and Cherry
  35. 35. CHANGING CHARACTER CASING• Using method toLowerCase()String alpha = "aBcDeFg";String lowerAlpha = alpha.toLowerCase(); // abcdefgSystem.out.println(lowerAlpha);• Using method toUpperCase()String alpha = "aBcDeFg";String upperAlpha = alpha.toUpperCase(); // ABCDEFGSystem.out.println(upperAlpha);
  36. 36. TRIMMING WHITE SPACE• Using method trim() String s = " example of white space "; String clean = s.trim(); System.out.println(clean);
  38. 38. BUILDING AND MODIFYING STRINGS Using StringBuilder Class
  39. 39. CONSTRUCTING STRINGS• Strings are immutable • concat(), replace(), trim(), ... return new string, do not modify the old one• Do not use "+" for strings in a loop! • It runs very inefficiently! public static string dupChar(char ch, int count){ String result = ""; for (int i=0; i<count; i++) result += ch; return result; } Bad practice. Avoid this!
  40. 40. CHANGING THE CONTENTS OF A STRING – STRINGBUILDER• Use the java.lang.StringBuilder class for modifiable strings of characters:• Use StringBuilder if you need to keep adding characters to a string public static String reverseIt(String s) { StringBuilder sb = new StringBuilder(); for (int i = s.length()-1; i >= 0; i--) sb.append(s.charAt(i)); return sb.ToString(); }
  41. 41. THE STRINGBUILDER CLASS Capacity StringBuilder: H e l l o , J a v a ! length() = 11 capacity() = 15 used buffer unused (length()) buffer• StringBuilder keeps a buffer memory, allocated in advance • Most operations use the buffer memory and do not allocate new objects
  42. 42. THE STRINGBUILDER CLASS (2)• StringBuilder(int capacity) constructor allocates in advance buffer memory of a given size • By default 16 characters are allocated• capacity() holds the currently allocated space (in characters)• charAt(int index) gives access to the char value at given position• length() hold the length of the string in the buffer
  43. 43. THE STRINGBUILDER CLASS (3)• append(…) appends string or other object after the last character in the buffer• delete(int start, int end) removes the characters in given range• insert(int offset, String str) inserts given string (or object) at given position• replace(int start, int end, String str) replaces all occurrences of a substring with given string• toString() converts the StringBuilder to String object
  44. 44. STRINGBUILDER – EXAMPLE• Extracting all capital letters from a string public static String extractCapitals(String s) { StringBuilder result = new StringBuilder(); for (int i = 0; i < s.length(); i++) { char ch = s.charAt(i); if (Character.isUpperCase(ch)) { result.append(ch); } } return result.toString(); }
  45. 45. HOW THE + OPERATOR DOES STRING CONCATENATIONS?• Consider following string concatenation: String result = str1 + str2;• It is equivalent to this code: StringBuffer sb = new StringBuffer(); sb.append(str1); sb.append(str2); String result = sb.toString();• Actually several new objects are created and leaved to the garbage collector • What happens when using + in a loop?
  47. 47. FORMATTING STRINGSUsing toString() and String.format()
  48. 48. METHOD TOSTRING()• All classes have this public virtual method • Returns a human-readable, culture-sensitive string representing the object • Most Java Platform types have own implementation of toString()
  49. 49. METHOD STRING.FORMAT()• Applies templates for formatting strings • Placeholders are used for dynamic text • Like System.out.printf(…) String template = "If I were %s, I would %s."; String sentence1 = String.format( template, "developer", "know Java"); System.out.println(sentence1); // If I were developer, I would know Java. String sentence2 = String.format( template, "elephant", "weigh 4500 kg"); System.out.println(sentence2); // If I were elephant, I would weigh 4500 kg.
  50. 50. FORMATTING DATES• When we print Dates we use prefix t or T • d, e – day (with/without leading zero) • m – month • y, Y – year (2 or 4 digits) • H, M, S – hour, minute, secondDate now = (new GregorianCalendar()).getTime();System.out.printf("Now is " + "%1$td.%1$tm.%1$tY %1$tH:%1$tM:%1$tS", now);// Now is 23.05.2006 21:09:32
  51. 51. FORMATTING STRINGS Live Demo
  52. 52. EXERCISES Write a program that reads a string, reverses it and prints it on the console. Example: "sample"  "elpmas". Write a program to check if in a given expression the brackets are put correctly. Example of correct expression: ((a+b)/5-d). Example of incorrect expression: )(a+b)).
  53. 53.  Write a program that finds how many times a substring is contained in a given text (perform case insensitive search).Example: The target substring is "in". The text is as follows: We are living in a yellow submarine. We dont have anything else. Inside the submarine is very tight. So we are drinking all the day. We will move out of it in 5 days. The result is: 9.
  54. 54.  You are given a text. Write a program that changes the text in all regions identified by the tags <upcase> and </upcase> to uppercase. The tags cannot be nested. Example: We are living in a <upcase>yellow submarine</upcase>. We dont have <upcase>anything</upcase> else. The expected result: We are living in a YELLOW SUBMARINE. We dont have ANYTHING else.
  55. 55.  Write a program that parses an URL address given in the format: [protocol]://[server]/[resource] and extracts from it the [protocol], [server] and [resource] elements. For example from the URL following information should be extracted: [protocol] = "http", [server] = "", [resource] = "/forum/index.php "
  56. 56.  Write a program that extracts from a given text all the sentences that contain given word. Example: The word is "in". The text is: We are living in a yellow submarine. We dont have anything else. Inside the submarine is very tight. So we are drinking all the day. We will move out of it in 5 days. The expected result is: We are living in a yellow submarine. We will move out of it in 5 days.Consider that the sentences are separated by "." and the words – by non-letter symbols.
  57. 57.  We are given a string containing a list of forbidden words and a text containing some of these words. Write a program that replaces the forbidden words with asterisks. Example: Microsoft announced its next generation Java compiler today. It uses advanced parser and special optimizer for the Microsoft JVM. Words: "Java, JVM, Microsoft" The expected result: ********* announced its next generation **** compiler today. It uses advanced parser and special optimizer for the ********* ***.
  58. 58.  Write a program that reads a string from the console and lists all the different letters in the string along with information how many times each letter is found. Write a program that reads a string from the console and lists all the different words in the string with information how many times each word is found. Write a program that reads a string from the console and replaces all series of consecutive identical letters with a single one. Example: "aaaaabbbbbcdddeeeedssaa" -> "abcdedsa".
  59. 59.  Write a program that reads a list of words, separated by spaces ( ) , and prints the list in an alphabetical order. Write a program that lets the user input a string of maximum 20 characters. If the length of the string is less, the rest of the characters should be filled with *. Print the string into the console.