Session 5

1. Session 5 java.lang package Using array java.io package: StringTokenizer, ArrayList, Vector Using Generic

2. java.lang package • The package consists classes and interface which are fundamental to Java programming • All the program automaticlly import java.lang package • It contains

3. Wrapper classes

4. Wrapper classes (cont…)

5. Common methods

6. Math class • Defining methods for basic numeric operations as well as trigonomethic functions • All the methods are static • This is final class • Methods • public static double pow(double x, double y) • public static double exp(double x) • public static double log(double x)

7. Methods of Math class • Trigonometric methods • public static double sin(double x) • public static double cos(double x) • public static double tan(double x) • Arirthmetic methods

8. Array • Stored many values • Elements in array have the same datatype • Elements in array are accessed throught by subscript • Types of array: • Single dimension array (one dimension array) • Multi demension array

9. One Dimensional Array Declarations • Three ways to declare an array are: • datatype identifier [ ]; int a[]; or int [] a; • datatype identifier [ ] = new datatype[size]; char ch[] = new char[10]; • datatype identifier [ ] = {value1,value2,….valueN}; byte b[] = {12, 3, 5};

10. Initialize elements in array Init value for each element a[0] = 12; a[1] = 15; Init when creating array int a[] = {12, 15, 17}; Using loop for(int i = 0 ; i < a.length ; i++) a[i] = …;

11. Example class ArrDemo { public static void main(String [] arg) { double nums[] = {10.1, 11.3, 12.5,13.7, 14.9}; System.out.println(" The value at location 3 is : " + nums[3]); } }

12. Two dimension array Array that has more than one dimention Syntax: int a[][]; int []a[] = new int[2][3]; int [][]a = {{1, 2}, {3, 4}, {-9, 0}};

13. Initialize 2 dimension array Initialize for each elements a[0][0] = ‘c’; a[0][1] = ‘e’; Initialize when declaring array int a[][] = {{1, 2}, {13, -78}}; Using loop for(int i = 0 ; i < n ; i++) for(int j = 0 ; j < m ; j++) a[i][j] = …;

14. String class • In Java, a string literal is an object of type String class. • Hence manipulation of strings will be done through the use of the methods provided by the String class. • String class is in java.lang package • Every time we need an altered version of the String, a new String object is created with the modifications in it.

15. Compare two String • String length(): This method determines the length of a string. • The == operator and equals(), compareTo() method can be used for strings comparison. • The == operator checks if the two operands being used are one and the same object. • The equals() method checks if the contents of the two operands are the same. • compareTo() return value = 0 if s1 = s2, return value >0 if s1 > s2, return value < 0 if s1 < s2

16. Methods of String • trim () • substring (int start, int end) • equals (Object s) • equalsIgnoreCase (String s) • charAt(inti) • endsWith (String s) • startsWith (String s) • indexOf (String s) • lastIndexOf (String s) • toLowerCase () • toUpperCase ()

17. Example class StringTest { public static void main(String[] args) { String name = args[0]; if(name.startsWith("M")) System.out.println("Hey my name also starts with an M! "); int length = name.length(); System.out.println("Your name has "+length+" characters"); String name_in_caps = name.toUpperCase(); System.out.println(name_in_caps); } }

18. Convert String into number String s = “12”; String to int int a = Interger.parseInt(s); String to float float f = Float.parseFloat(s);

19. Array of Strings String s[]; s = new String[3]; Or String s[] = new String[3]; Or String []s = {“a”, “b”, “c”};

20. Immutability of String • Strings in Java once created cannot be changed directly. • This is known as immutability in Strings.

21. Example class Testing { public static void main(String[] args) { String str = "Hello"; str.concat("And Goodbye"); System.out.println(str); } }

22. java.util package • Contains useful classes providing a broard range of functionality • Collection classes are useful for working with groups of objects • Contains classes that provides date and time, calendar, dictionary facilities

23. StringTokenizer • Placed at java.util • Seperated String into token • Methods • countTokens() • hasMoreElements() • hasMoreTokens() • nextElement() • nextToken() • Example: s = “22+3-5/3” • Token: 22 3 5 3 • Separator: + - /

24. ArrayList • An ArrayList object is a variable length array of object references. • Used to create dynamic arrays • ExtendsAbstractList and implementsListinterface. • ArrayLists are created with an initial size. • As elements are added, size increases and the array expands.

25. ArrayList (cont…) • Constructors • Methods • size() • add(int index, E element) • get(int index) • indexOf(Object elem) • lastIndexOf(Object elem) • remove(int index) • remove(Object o) • set(int index, E element)

26. Vector class • Similar to ArrayList class, allows to implement dynamic array • Storing an array of objects that size can increase or decrease • At any given point of time, an instance of type Vector has the capacity to hold a certain number of elements. • When it becomes full, its capacity is incremented by an amount specific to that Vector object. • Diffirence between Vector and ArrayList is that methods of Vector are synchronized and are thread-safe

27. Vector class (cont…)

28. Generic • Earlier Collection treated elements as a collection of objects. • To retrieve an element from a Collection required an explicit cast • Thus, there was always a risk of runtime exception, ClassCastException • Generic allows the programmer to communicate the type of a collection to the compiler so that it can be checked. • The compiler consistently checks for the element type of the collection. • And inserts the correct cast on elements being taken out of the collection.

29. Example

30. Example of Generic Code required explicit cast Type of element No required explicit cast