Everyday objects: Strings and Wrappers

1. Everyday objects: Strings and Wrappers CSC 1051 – Data Structures and Algorithms I Dr. Mary-Angela Papalaskari Department of Computing Sciences Villanova University Course website: http://www.csc.villanova.edu/~map/1051/f13 Some slides in this presentation are adapted from the slides accompanying Java Software Solutions by Lewis & Loftus CSC 1051 M.A. Papalaskari, Villanova University

2. Overview • Review what we know about objects: • classes • methods • object creation • String class • useful methods and examples • Wrapper classes CSC 1051 M.A. Papalaskari, Villanova University

3. Some everyday Objects… • Strings - defined by the Stringclass: "This is a string literal." "123 Main Street" "X” • System.outis also an object - it represents a destination (the monitor screen) to which we can send output CSC 1051 M.A. Papalaskari, Villanova University

4. object method name information provided to the method (parameters) Methods • Objects can have methods associated with them • In Lincoln.java we invoked the println method System.out.println ("Whatever you are, be a good one."); CSC 1051 M.A. Papalaskari, Villanova University

5. Invoking Methods • We use the dot operatorto invoke an object’s methods • String name = scan.nextLine(); nextLine() is one of the methods of Scannerobjects (defined in Scannerclass) • intnumOfCharsInName = name.length(); length() is one of the methods of String objects (defined in String class) 0 1 2 3 4 B e t s y CSC 1051 M.A. Papalaskari, Villanova University

6. More String Methods name • intnumOfCharsInName = name.length(); • char initial = name.charAt(0); • String newName = name.replace('s', 't'); newName • String capsName = name.toUpperCase(); capsName 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 • String nickName = name.substring(1, 4); B e t s y nickName See also textbook example StringMutation.java

7. Palindrome tester • Input a string, determine whether it is a palindrome, i.e.: • first char is the same as last char • 2nd char is the same as 2nd last char • and so on… • How to express this as an algorithm? • How to implement it? str 0 1 2 3 4 R A D A R CSC 1051 M.A. Papalaskari, Villanova University

8. PalindromeTester.java (Example from Chapter 5) System.out.println("Enter a potential palindrome:"); str= scan.nextLine(); left = 0; right = str.length() - 1; while (str.charAt(left) == str.charAt(right) && left < right) { left++; right--; } if (left < right) System.out.println ("NOT a palindrome"); else System.out.println ("palindrome"); CSC 1051 M.A. Papalaskari, Villanova University

9. PalindromeTester.java (Example from Chapter 5) System.out.println("Enter a potential palindrome:"); str= scan.nextLine(); left = 0; right = str.length() - 1; while (str.charAt(left) == str.charAt(right) && left < right) { left++; right--; } if (left < right) System.out.println ("NOT a palindrome"); else System.out.println ("palindrome"); Sample Run Enter a potential palindrome: radar palindrome Test another palindrome (y/n)? y Enter a potential palindrome: able was I ere I saw elba palindrome. Test another palindrome (y/n)? y Enter a potential palindrome: abracadabra NOT a palindrome. Test another palindrome (y/n)? n CSC 1051 M.A. Papalaskari, Villanova University

10. Declaring Variables, revisited • Examples of variable declarations: int count = 0; double mpg; String title; Graphics page; Color aquamarine; Scanner scan; • A class name can be used as a type to declare an object reference variable • The object itself must be created separately CSC 1051 M.A. Papalaskari, Villanova University

11. Creating Objects • We have already seen something like this: Scanner scan = new Scanner (System.in); The new operator calls the Scanner constructor, which is a special method that sets up the object Constructing a new object is called instantiation Variable refers to a Scanner object an instance of the Scanner class CSC 1051 M.A. Papalaskari, Villanova University

12. Creating Objects • Another example: String title = newString ("Java Software Solutions"); The new operator calls the String constructor, which is a special method that sets up the object Constructing a new object is called instantiation Variable refers to a String object an instance of the String class CSC 1051 M.A. Papalaskari, Villanova University

13. The String Class is SPECIAL! • Exception to the use of newoperator: Because strings are so common, we don't have to use the new operator to create a String object • This is special syntax that works only for strings String title = new String ("Java Software Solutions"); String title = "Java Software Solutions"; CSC 1051 M.A. Papalaskari, Villanova University

14. Wrapper Classes • The java.lang package contains wrapper classes that correspond to each primitive type:

15. Wrapper Classes • The following declaration creates an Integer object which represents the integer 40 as an object • Integer age = new Integer(25); age CSC 1051 M.A. Papalaskari, Villanova University

16. Wrapper Class methods and constants • Integer.parseInt(): convert String to int • Double.parseDouble(): convert String to double • Integer.MIN_VALUE: smallest int value • Integer. MAX_VALUE: largest intvalue Examples: • String str = scan.nextLine(); • intnum = Integer.parseInt(str); • intcurrentMin = Integer.MAX_VALUE; CSC 1051 M.A. Papalaskari, Villanova University

17. Autoboxing • Autoboxing is the automatic conversion of a primitive value to a corresponding wrapper object: Integer obj; int num = 42; obj = num; • The assignment creates the appropriate Integer object • The reverse conversion (called unboxing) also occurs automatically as needed CSC 1051 M.A. Papalaskari, Villanova University

18. Quick Check Are the following assignments valid? Double value = 15.75; Character ch = new Character('T'); char myChar = ch; CSC 1051 M.A. Papalaskari, Villanova University

19. 38 num1 "Steve Jobs" name1 References • Note that a primitive variable contains the value itself, but an object variable contains the address of the object • An object reference can be thought of as a pointer to the location of the object • Rather than dealing with arbitrary addresses, we often depict a reference graphically CSC 1051 M.A. Papalaskari, Villanova University

20. 38 num1 Before: 96 num2 38 num1 After: 38 num2 Assignment Revisited • The act of assignment takes a copy of a value and stores it in a variable • For primitive types: int num1 = 38; int num2 = 96; num2 = num1; CSC 1051 M.A. Papalaskari, Villanova University

21. 38 num1 Before: 96 num2 38 num1 After: num2 Reference Assignment • For objects, the same is true, but what is copied is the reference to the object (i.e., its address): • For objects: Integer num1 = 38; Integet num2 = 96; num2 = num1; CSC 1051 M.A. Papalaskari, Villanova University

22. "Steve Jobs" name1 Before: "Steve Wozniak" name2 "Steve Jobs" name1 After: name2 Another example • For object references, assignment copies the address: name2 = name1; CSC 1051 M.A. Papalaskari, Villanova University

23. Aliases • Two or more references that refer to the same object are called aliases of each other • That creates an interesting situation: one object can be accessed using multiple reference variables • Aliases can be useful, but should be managed carefully • Changing an object through one reference changes it for all of its aliases, because there is really only one object CSC 1051 M.A. Papalaskari, Villanova University

24. Garbage Collection • When an object no longer has any valid references to it, it can no longer be accessed by the program • The object is useless, and therefore is called garbage • Java performs automatic garbage collection periodically, returning an object's memory to the system for future use • In other languages, the programmer is responsible for performing garbage collection CSC 1051 M.A. Papalaskari, Villanova University