Lists

1. Lists Chapter 4

2. Chapter Contents • Specifications for the ADT List • Redefining the Specifications • Using the ADT List • Using a List Is Like Using a Vending Machine

3. Specifications for the ADT List • A list provides a way to organize data A to-do list, address list, grocery list etc… ADT list must be considered in general, not necessarily a list of strings. It can contain any type of objects.

4. Specifications for the ADT List • Operations on lists • Add new entry – at end, or anywhere • Remove an item • Remove all items • Replace an entry • Look at any entry • Look for an entry of a specific value • Count how many entries • Check if list is empty, full • Display all the entries

5. Specifications for the ADT List • To specify an ADT list • Describe its data( A collection of objects in a specific order and having the same data type, the number of objects in the collection.) • Specify the operations • ADT does not indicate how to store the data or how to implement the operations. • In contrast, a data structure is an implementation of an ADT within a programming language.

6. Example The effect of ADT list operations on an initially empty list. Convenient way of identify a particular entry is by the entry’s position within the list. Position-oriented ADT

7. Potential Problem Operations • add, remove, replace, getEntry work OK when valid position given • remove, replace and getEntry not meaningful on empty lists • A list could become full, what happens to add? • What else operations may cause problems?

8. Possible Solutions • Assume the invalid situations will not occur • Ignore the invalid situations • Make reasonable assumptions, guess at the client’s intention. act in predictable way • Return a signal or Boolean value indicating success or failure of the operation • Throw an exception The documentation for ADT should describe these possible solutions.

9. Redefining Specifications • A first draft of an ADT specifications may ignore potential problems • Simplifies the first draft • Concentrate on details after major portions of specifications written • Makes the specifications complete • After writing specifications, • Write Java interface for its operations • Java interface can be used for class to implement the ADT

10. Generic Types Within an Interface and Class public interface Pairable<S> { public void setPairs(S firstItem, S secondItem) } A class that implements this interface could begin with the statement public class OrderedPair<T> implements Pairable<T> A class to represent pair of objects of the same type. Since each pair can be of any class type, we used a generic type in the definition of the class. The method setPair has parameters of a generic type. Imagine an interface Pairable that declares this method.

11. ListInterface /** An interface for the ADT list. * Entries in the list have positions that bewgin with 1.*/ publicinterface ListInterface<T> { /** Task: Adds a new entry at a specified position within the list. The list’s size is * increased by 1. * @param newPosition an integer that specifies the desired position of the new entry * @param newEntry the object to be added as a new entry. * @return true if the addition is successful, or false if either the list is full, newPosition <1, or newPosition > getLength() +1 */ public boolean add( int newPosition, T newEntry); How to write the comments for method remove? public T remove( int givenPosition); }

12. Using the ADT List A list of numbers that identify runners in the order in which they finish a race

13. Using the ADT List public class ListClient { public static void main(String[] args) { testList(); } // end main public static void testList() { ListInterface<String> runnerList = new AList<String>(); // has only methods // in ListInterface runnerList.add("16"); // winner runnerList.add(" 4"); // second place runnerList.add("33"); // third place runnerList.add("27"); // fourth place runnerList.display(); } // end testList } // end ListClient AList implements interface ListInterface. The data type of runnerList is ListInterface<String>. This declaration obliges runnerList to call only methods in the interface. If the data type was Alist<String>, what methods are available to runnerList?

14. Java Class Library: The Interface List • The standard package java.util contains a list interface – called List • Methods provided public boolean add(Object newEntry) public void add(int index, Object newEntry) public Object remove(int index) public void clear() public Object set(int index, Object anEntry) // like replace public Object get(int index) // like getEntry public boolean contains(Object anEntry) public int size() // like getLength public boolean isEmpty()

15. A List Interface is Like a Vending Machine A vending machine.

16. A List Interface is Like a Vending Machine Observations about vending machines • Can perform only tasks shown on interface • Must understand the tasks • Cannot see inside the machine • Can use the machine even though don’t know what happens inside • If inside of machine replaced with new improved version • Interface remains unchanged • Customer uses machine in same way as before

17. A List Interface is Like a Vending Machine Observations about clients and List ADT • Client can perform only operations from the ADT List • Client must adhere to specifications • Client cannot access data without an ADT operation. Encapsulation hides the data representation within the ADT • Client can use the list – even though unable to access entries directly. Don’t need to know how the data is stored. • If implementation is changed, client still uses list in same way as before

18. Exercise • Suppose that you have a list that is created by the following statement: ListInterface<Double> quizScores = new AList<Double>(); Imagine that someone has added to this list the quiz scores received by a student throughout a course. The professor would like to know the average of these quiz scores, ignoring the lowest score. • a. Write Java statements at the client level that will find and remove the lowest score in the list. • b. Write Java statements at the client level that will compute the average of the scores remaining in the list.