Operations on Strings

8/8/2005 Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved L: String Manipulation Slide 2 Strings In Java, strings are objects and String is a “built- in” class with special treatment String s = new String( “Hello” ); is the same as String s = “Hello”; “Hello” is an example of a String literal representing a String object The + operator can be used for String concatenation The String class has certain methods length() substring(…) equals(…) equalsIgnoreCase(…) others

8/8/2005 Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved L: String Manipulation Slide 3 String Assignment Since String is a class (just like BankAccount), a String variable stores a reference to a String object String object: sequence of characters delimited by quotes; e.g., “Hello” Can be viewed as equivalent to new String( “Hello” ) Assignment Points the String reference (left-hand side) to the String instance (right-hand side) Similar to object declaration and assignment Uses the “=” operator String s = “Hello”;

8/8/2005 Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved L: String Manipulation Slide 4 String Concatenation Strings have special treatment in Java in that the + operator works on string objects Concatenation Appends a variable or object with a String Returns the linked String as the result Uses the “+” operator String a = s + “ World!”; // a = “Hello World!” String b = s + 5; // b = “Hello5” “+=” can also be used on Strings b += “ World!”; // b = “Hello5 World!”

8/8/2005 Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved L: String Manipulation Slide 5 String Methods The length() method returns an int representing the number of characters in the string String s = “Hello there”; System.out.println( s.length() ); // prints 11 The substring() method requires two ints as parameters and returns a part of the string bounded by those integers String s = “Hello there”; String t = s.substring(2,7); // assigns “llo t” Other string methods for equality and comparison

8/8/2005 Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved L: String Manipulation Slide 6 String Equality Equality Requires a String parameter and returns a boolean Value equality: equals(), equalsIgnoreCase() method Checks if the value of two Strings are equal String a = “myString”; String b = “this is myString again”.substring(8,16); boolean ansA = a.equals( b ); // true boolean ansB = a == b; // false Pointer equality: “==” Checks if the two Strings are pointing to the same object Not used to check if Strings contain the same value

8/8/2005 Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved L: String Manipulation Slide 7 String Comparison Natural comparison of Strings A more flexible comparison (not just checking for equality) Used by calling the compareTo() method Returns a number Example String a = “application”; int result = a.compareTo( “applet” ); Returns a negative number when a is less than b Returns 0 when a is equal to b Returns a positive number when a is greater than b What if we change the first line to String a = “Application” ?

8/8/2005 Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved L: String Manipulation Slide 8 String Formatting There are string methods that can help with Text alignment Formatting of numeric values Java provides the a String formatting method String.format() The desired effect is also available through System.out.printf()

8/8/2005 Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved L: String Manipulation Slide 9 String Formatting Methods which produce formatted outputs requires a format string and an argument list Example double pi = ; String s = String.format( “Pi: %.2f”, pi ); // s = “Pi: 3.14” Format string Contains fixed text and format specifiers that process the arguments

8/8/2005 Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved L: String Manipulation Slide 10 Format Specifiers Indicates how the arguments should be processed and where they should be inserted Syntax % - start of the specifier arg_index + '$' (optional) position of argument in the argument list flags (optional) provides special formatting (e.g. left align, padding with zeroes)

8/8/2005 Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved L: String Manipulation Slide 11 Format Specifiers Syntax (continued) width (optional) minimum number of spaces to print argument puts blanks in unused spaces '.' + precision (optional) number of digits after decimal ConversionFormatted into: dintegers ffloating-point number sString ccharacter conversion – how the argument should be formatted

8/8/2005 Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved L: String Manipulation Slide 12 Example Print a price list String header = String.format("%s branch sales for %s“, "Katipunan“, “July” ); int num = 1; double price1 = 3.75; double price2 = 100;... double price10 = 12.5; System.out.println( header ); System.out.printf( "%s %s" "Item #", "Price" ); System.out.printf( "%-6d P%5.2f \n", num++, price1 ); System.out.printf( "%-6d P%5.2f \n", num++, price2 );... System.out.printf( "%-6d P%5.2f \n", num++, price10 ); Output: Katipunan branch sales for July Item # Price 1 P P P 12.50

8/8/2005 Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved L: String Manipulation Slide 13 Summary String is a class; there are String objects with which String methods() can be carried out Strings also have a special treatment in Java String literals; e.g., “hello” The concatenation operators ( +, += )