Generics and the ArrayList Class

1. Generics and the ArrayList Class

2. Generics • Added to Java v5.0 • Generics = class and method definitions may include parameters for types • Generic program – type parameter allows one to write code that applies to any class • Ex. list of items of type T • when T=some-object-type, it’s a list of some-object-type, etc.

3. ArrayList • Advantage: • Can grow or shrink (arrays can’t) • import java.util.ArrayList; • See http://java.sun.com/j2se/1.5.0/docs/api/java/util/ArrayList.html

4. ArrayList • Disadvantage: • Less efficient (i.e., slower) than arrays • Lacks familiar [] syntax (Java limitation only; supported by C++) • Base type must be an object (not primitive) type • Can have ArrayList of Integer but can’t have an ArrayList of int. • Less of a problem w/ automatic boxing and unboxing • Java limitation only; supported by C++

5. ArrayList class • Define an ArrayList of Strings: • ArrayList<String> list = new ArrayList<String>(); • Define an ArrayList of Employees: • ArrayList<Employee> empList = new ArrayList<Employee>( 20 ); • Initial capacity is specified as 20.

6. ArrayList class • Set an element: • list.set( 12, “Hi, Mom.” ); • Get an element: • System.out.println( list.get( 12 ) ); • String s = list.get( 5 ); //note: no cast necessary

7. ArrayList class • Add an element: add( value ) form • list.add( “One” ); • list.add( “Two” ); • list.add( “Three” ); • “One” is at position 0, “Two” is at position 1, and “Three” is at position 2.

8. ArrayList class • Add an element: add( i, value ) form • i must be a used position or the first unused position • list.add( “One” ); • list.add( “Two” ); • list.add( “Three” ); • list.add( 1, “Fred” ); • “One” is at position 0, “Fred is at position 1, “Two” is at position 2, and “Three” is at position 3.

9. ArrayList class • size() method for (int i=0; i<list.size(); i++) { System.out.println( list.get(i) ); }

10. ArrayList class methods

11. ArrayList class methods • Ctors • public ArrayList<Base_Type>( int initialCapacity ) • public ArrayList<Base_Type>() • initial capacity is 10

12. ArrayList class methods • Array-like methods • public Base_Type set ( int index, Base_Type newElement ) • where 0<=index<size() • (or IndexOutOfBoundsException) • public Base_Type get ( int index ) • where 0<=index<size() • (or IndexOutOfBoundsException)

13. ArrayList class methods • Methods to add elements • public boolean add ( Base_Type newElement ) • adds the new element to the end • size() increases by 1 • capacity increases if necessary

14. ArrayList class methods • Methods to add elements • public void add ( int index, Base_Type newElement ) • 0<=index<=size() • when index==size(), inserts at end • size() increases by 1 • capacity increases if necessary • when 0<=index<size(), current elements at index move to index+1, at index+1 move to index+2, …, at size()-1 move to size() • otherwise, IndexOutOfBoundsException

15. ArrayList class methods • Methods to remove elements • public Base_Type remove ( int index ) • 0<=index<size() (or exception) • Removes element at index; shifts to the left remaining elements at index+1 … size()-1. • protected void removeRange ( int fromIndex, int toIndex ) • Removes elements with index i s.t. fromIndex<=i<toIndex; shifts to the left remaining elements at toIndex … size()-1

16. ArrayList class methods • Methods to remove elements • public boolean remove ( Object theElement ) • If found then removes the first occurrence of theElement; shifts the remaining elements to the left; size() becomes size()-1; returns true. • If not found then returns false. • public void clear ( ) • Removes all elements • size() becomes 0

17. ArrayList class methods • Search methods • public boolean contains ( Object target ) • Uses equals() method of target • True if ArrayList contains target; false otherwise. • public int indexOf ( Object target ) • Uses equals() method of target • Returns index of first occurrence of target in ArrayList; -1 otherwise. • public int lastIndexOf ( Object target ) • Same as above except index of last occurrence is returned.

18. ArrayList class methods • Memory management (size & capacity) • public boolean isEmpty ( ) • True if empty; false otherwise. • public int size ( ) • Returns number of elements in ArrayList • public void ensureCapacity ( int newCapacity ) • Increases the capacity (if necessary) • public void trimToSize ( ) • Reduces capacity to current size

19. ArrayList class methods • Make a copy • public Object[] toArray ( ) • Returns an array of list elements (in order). • public<Type> Type[] toArray ( Type[] a ) • Returns an array of list elements (in order). • If list will fit in array a, it will be used; remaining elements of a will be null. • If list won’t fit in array a, a new array will be created and used. • public Object clone ( ) • Returns a shallow copy of the ArrayList.

20. ArrayList class methods • Equality • public boolean equals ( Object other ) • True only when both are of the same size, and both have the same elements (in the same order).

21. New syntax! The for-each loop

22. for-each loop for (Array_Base_Type var : Collection_Object) Statement; • Let myList be an ArrayList of Strings. for (String element : myList) System.out.println( element );

23. for-each loop • The for-each loop also works with arrays: // Returns the sum of the elements of a int sum ( int[] a ) { int result = 0; for (int i : a) result += i; return result; }

24. Generics

25. Generics • Classes and methods can have a type parameter (actually, more than one). • The type parameter can be any reference (class) type. • (started with Java version 5.0; in C++ as well but much restricted in Java) • Generic class = parameterized class = class with a parameter for a type

26. Generic methods public class Utility { … public static<T> T getMidpoint ( T[] a ) { return a[ a.length/2 ]; } public static<T> T getFirst ( T[] a ) { return a[0]; } … }

27. Generic methods public class Utility { public static<T> T getMidpoint ( T[] a ) { return a[ a.length/2 ]; } public static<T> T getFirst ( T[] a ) { return a[0]; } } //usage: String midString = Utility.<String>getMidpoint( b ); double firstNumber = Utility.<Double>getFirst( c ); must be Double[] automatic unboxing

28. Generic (parameterized) classes public class Sample<T> { private T data; public void setData ( T newData ) { data = newData; } public T getData ( ) { return data; } }

29. Generic (parameterized) classes public class Sample<T> { private T data; public void setData ( T newData ) { data = newData; } public T getData ( ) { return data; } } //usage: Sample<String> object = new Sample<String>(); object.setData( “Hello” );

30. Generic class for ordered pairs Pair<String> secretPair = new Pair<String>( “Happy”, “Day” ); Pair<Integer> rollOfDice = new Pair<Integer>( new Integer(2), new Integer(3) ); Pet male = new Pet(); Pet female = new Pet(); Pair<Pet> breedingPair = new Pair<Pet>( male, female );

31. Defining the ordered pair class public class Pair<T> { private T first; private T second; public Pair ( ) { first = null; second = null; } public Pair ( T f, T s ) { first = f; second = s; } … }

32. Defining the ordered pair class public class Pair<T> { private T first; private T second; … //These are suggested by our author. //Why are they bad? public T getFirst ( ) { return first; } public T getSecond ( ) { return second; } … }

33. Defining the ordered pair class public class Pair<T> { private T first; private T second; … //These are suggested by our author. //Why are they bad? //Because of potential privacy leaks! public T getFirst ( ) { return first; } public T getSecond ( ) { return second; } … }

34. Defining the ordered pair class public class Pair<T> { private T first; private T second; … public boolean equals ( Object other ) { if (other==null) return false; if (getClass() != other.getClass()) return false; Pair<T> o = (Pair<T>) other; return first.equals( o.first ) && second.equals( o.second ); } … }

35. More then one type parameter can be specified Pair<String,Integer> p = new Pair<String,Integer>( “Kyle Jones”, new Integer(123456789) );

36. Defining the ordered pair class public class Pair<T1,T2> { private T1 first; private T2 second; public Pair ( ) { first = null; second = null; } public Pair ( T1 f, T2 s ) { first = f; second = s; } … }

37. Defining the ordered pair class public class Pair<T1,T2> { private T1 first; private T2 second; … public T1 getFirst ( ) { return first; } public T2 getSecond ( ) { return second; } … }

38. Defining the ordered pair class public class Pair<T1,T2> { private T1 first; private T2 second; … public boolean equals ( Object other ) { if (other==null) return false; if (getClass() != other.getClass()) return false; Pair<T1,T2> o = (Pair<T1,T2>)other; return first.equals( o.first ) && second.equals( o.second ); } … }

39. Bounds for type parameters

40. Bounds for type parameters • Say I wish to restrict my type parameter T to only those things that implement the Comparable interface. • Can I do this? • What happens if I use this new parameterized class using a class (as a parameter) that doesn’t implement the Comparable interface?

41. Bounds for type parameters • Say I wish to restrict my type parameter T to only those things that implement the Comparable interface. • Can I do this? • Yes. • What happens if I use this new parameterized class using a class (as a parameter) that doesn’t implement the Comparable interface? • A compiler error occurs.

42. Bounds for type parameters • Example of stating this restriction: public class Pair<T extends Comparable> { … } The keyword extends is used rather than implements because T must be a class (not an interface). Hmmmm.

43. Multiple bounds • Multiple bounds may also be specified. • For example, public class AnotherGeneric< T1 extends Employee & Comparable, T2 extends Comparable> { … } • But only 1 may be a class! Hmmmm! The rest must be interfaces. Why?

44. Problem 1 • In the sport of diving, seven judges award a score between [0.0,10.0]. • The highest and lowest scores are discarded. • The remaining scores are added together. (Hint: Use Collections.sort().) • The sum is then multiplied by the degree of difficulty between [1.2,3.8] for that dive. • The result is then multiplied by 0.6 to determine the diver’s score. • Write a program that inputs the degree of difficulty and the 7 judges’ score, and outputs the diver’s score. • You are required to use an ArrayList of Double for the judges’ scores.

45. Problem 2 • Write a program that uses an ArrayList of parameter type Contact to store a database of contracts. • The Contract class should store the contact’s first and last name, phone number, and email address. Add appropriate accessor and mutator methods. • Your program should present a (textual – not Swing) menu that allows the user to add a contact, display all contacts, search for a specific contact and displays it, or search for a specific contact and give the user the option to delete it. The searches should find any contact where any member variable contains a target search string. For example, if “elmore” is the search target then any contact where the first name, last name, phone number, or email address contains “elmore” should be returned for display or deletion. • Use the “for-each” loop to iterate through the ArrayList.

46. Problem 3 • Many GPS’ can record waypoints. The waypoint marks the coordinates of a location on a map along with a timestamp. Our GPS stores waypoints in terms of an (X,Y) coordinate on a map together with a timestamp t that records the number of seconds that have elapsed since the unit was turned on. • Write a program that allows the user to enter as many waypoints as desired, storing each in an ArrayList of a class that you design. • As waypoints are entered, calculate the average speed of the trip so far. (The distance between (0,0) to (1,0) is 0.1 miles.)

47. Problem 4 • Write a generic class, MyMathClass, with a type parameter T where T is a numeric object type (e.g., Integer, Double, or any class that extends java.lang.Number). • Add a method named standardDeviation that takes and ArrayList of type T and returns as a double the standard deviation of the values in the ArrayList. • Use the doubleValue() method in the Number class to retrieve the value of each number as a double. • Your program should generate a compile-time error if your standard deviation method is invoked on an ArrayList that is defined for non-numeric elements (e.g., String). • Additionally, use for-each in the standardDeviation method.

48. Problem 5 • Create a generic class with a type parameter that simulates drawing an item at random out of a box. This class could be used for simulating a random drawing. • For example the box might contain Strings representing names written on a slip of paper, or the box might contain Integers representing a random drawing for a lottery based on numeric lottery picks. • Create an add method that allows the user of the class to add an object of the specified type along with an isEmpty method that determines whether or not the box is empty. • Finally, your class should have a drawItem method that randomly selects an object from the box and returns it. If the user attempts to draw an item out of an empty box, return null. • Write a main method that tests your class. To generate a random number x, where 0<=x<=1, use x = Math.random();.