Java String class

String Class :  String represent a sequence of character.  The String class belongs to the java.lang package, which does not require an import statement.  Sequence of character is represented in java is by using a character array char charArray[ ] = new char[4] charArray[0]= ‘J’; charArray[1]= ‘a’; charArray[2]= ‘v’; charArray[3]= ‘a’; 1 RAJESHREE KHANDE

String Class X[0] X[1] X[2] X[2][2] X[1][3] X[0][1] Variable Size Array 2RAJESHREE KHANDE

String Class  Character array not good enough to support the range of operations we may like to perform on the string.  In java String are class object & implemented using two classes namely Sting & StringBuffer.  A java string is an instantiated object of String Class.  Java string is not character array & is not NULL terminated.  Strings are Immutable. 3 RAJESHREE KHANDE

String Class String stringName = new String(“string”); e.g. String name = new String(“Rajeshree”); To find the length of String length method of string class is used. int m = name.length(); 4 RAJESHREE KHANDE

String Array  We can also create & use array that contain strings. String itemArray[] = new String[3]; We can assign the element to this array by using for loop. 5 RAJESHREE KHANDE

Literal Strings  String literals are anonymous objects of the String class.  are defined by enclosing text in double quotes. “This is a literal String”  don’t have to be constructed.  can be assigned to String variables.  can be passed to methods and constructors as parameters. 6 RAJESHREE KHANDE

Literal String examples String name = “Robert”; //calling a method on a literal String char firstInitial=“Robert”.charAt(0); //calling a method on a String variable char firstInitial = name.charAt(0); 7 RAJESHREE KHANDE

Empty Strings  An empty String has no characters. It’s length is 0.  Not the same as an uninitialized String. String word1 = “ ”; String word2 = new String(); private String errorMsg; errorMsg is null Empty strings 8 RAJESHREE KHANDE

String Methods : Changing Case  toLowerCase : Convert String to lowercase  toUpperCase : Convert String to uppercase String word2 = word1.toUpperCase(); String word3 = word1.toLowerCase();  returns a new string formed from word1 by converting its characters to upper (lower) case String word1 = “HeLLo”; String word2 = word1.toUpperCase(); //”HELLO” String word3 = word1.toLowerCase(); //”hello” //word1 is still “HeLLo“ 9 RAJESHREE KHANDE

String Methods : Changing Case word1 remains unchanged  Example: to “convert” word1 to upper case, replace the reference with a new reference.  A common bug: word1 = word1.toUpperCase(); word1.toUpperCase(); 10 RAJESHREE KHANDE

String Methods : length, chatAt  ”Problem".length();  ”Window".charAt (2); int length(); char charAt(i);  Returns the number of characters in the string  Returns the char at position i. 7 ’n' Returns: Character positions in strings are numbered starting from 0 – just like arrays. 11 RAJESHREE KHANDE

String Methods : substring television i k “lev" “mutable" "" (empty string) ”television".substring (2,5); “immutable".substring (2); “bob".substring (9); television i  String subs = word.substring (i, k);  returns the substring of chars in positions from i to k-1  String subs = word.substring (i);  returns the substring from the i-th char to the end Returns a new String by copying characters from an existing String. 12 RAJESHREE KHANDE

String Methods : concat String word1 = “re”, word2 = “think”; word3 = “ing”; int num = 2; String result = word1 + word2; //concatenates word1 and word2 “rethink“ String result = word1.concat (word2); //the same as word1 + word2 “rethink“ result += word3; //concatenates word3 to result “rethinking” result += num; //converts num to String //and concatenates it to result “rethinking2” 13 RAJESHREE KHANDE

String Methods : indexOf String name =“President George Washington"; name.indexOf (‘P'); 0 name.indexOf (‘e'); 2 name.indexOf (“George"); 10 name.indexOf (‘e', 3); 6 name.indexOf (“Bob"); -1 name.lastIndexOf (‘e'); 15 0 2 6 10 15 (starts searching at position 3) (not found) Returns: 14 RAJESHREE KHANDE

String Methods : Equality boolean b = word1.equals(word2);  returns true if the string word1 is equal to word2 boolean b = word1.equalsIgnoreCase(word2);  returns true if the string word1 matches word2, case b = “Raiders”.equals(“Raiders”);//true b = “Raiders”.equals(“raiders”);//false b = “Raiders”.equalsIgnoreCase(“raiders”);//true if(“team”.equalsIgnoreCase(“raiders”)) System.out.println(“Go You ” + “team”); 15 RAJESHREE KHANDE

String Methods : Comparisons S1.compareTo(S2) : Return negative if S1<S2, positive if S1>S2, zero if s1 equal to S2 int diff = word1.compareTo(word2); int diff = word1.compareToIgnoreCase(word2); Usually programmers don’t care what the numerical “difference” of word1 - word2 is, just whether the difference is negative (word1 comes before word2), zero (word1 and word2 are equal) or positive (word1 comes after word2). Often used in conditional statements. if(word1.compareTo(word2) > 0){ //word1 comes after word2… } 16 RAJESHREE KHANDE

Comparisons :Eaxmples //negative differences diff = “apple”.compareTo(“berry”); //a before b diff = “Zebra”.compareTo(“apple”); //Z before a diff = “dig”.compareTo(“dug”); //i before u diff = “dig”.compareTo(“digs”); //dig is shorter //zero differences diff = “apple”.compareTo(“apple”); //equal diff = “dig”.compareToIgnoreCase(“DIG”);/ /equal //positive differences diff = “berry”.compareTo(“apple”); //b after a diff = “apple”.compareTo(“Apple”); //a after A diff = “BIT”.compareTo(“BIG”); //T after G diff = “huge”.compareTo(“hug”); //huge is longer 17 RAJESHREE KHANDE

String Methods : trim s2 = s1.trim(); Removes white spaces at the beginning & at the end of string s1  String word2 = word1.trim ();  returns a new string formed from word1 by removing white space at both end does not affect whites space in the middle String word1 = “ Hi Bob ”; String word2 = word1.trim(); //word2 is “Hi Bob” – no spaces on either end //word1 is still “ Hi Bob “ – with spaces 18 RAJESHREE KHANDE

String Methods : replace  String word2 = word1.replace(‘x’, ‘y’); returns a new string formed from word1 by replacing all occurrences of x with y String word1 = “rare“; String word2 = “rare“.replace(‘r’, ‘d’); //word2 is “dade”, but word1 is still “rare“ 19 RAJESHREE KHANDE

Numbers to Strings Three ways to convert a number into a string: 1. String s = "" + num; 2. String s = Integer.toString (i); String s = Double.toString (d); 3. String s = String.valueOf (num); Integer and Double are “wrapper” classes from java.lang that represent numbers as objects. They also provide useful static methods. s = String.valueOf(123) ; //”123” s = “” + 123;//”123” s = Integer.toString(123); //”123” s = Double.toString(3.14); //”3.14” 20 RAJESHREE KHANDE

StringBuffer class  StringBuffer is mutable object.  The StringBuffer class is an alternative to the String class.  StringBuffer is more flexible than String. You can add, insert, or append new contents into a string buffer. 21 RAJESHREE KHANDE

StringBuffer : Methods Method Description s1.setCharAt(n, x) Modifies the nth char to x of string s1 s1.append(s2) Append the string s2 to s1 at the end s1.insert(n,s2) Insert the string s2 at the position n of string s1 s1.setLength(n) Set the length of the string s1 to n If n < s1,length() s1 is truncated. If n > s1.length zeros are added to s1 s1.deleteCharAt(n) Delete nth character from string s1 22RAJESHREE KHANDE

Java String class

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