CS Introduction to Data Structures Week 1 Thursday

Transcription

1 CS Introduction to Data Structures Week 1 Thursday We assume that you are proficient at object-oriented programming in Java. Please enroll in CS300 if you do not know Java and have not written object-oriented programs. Instructor Lec 1 & 2 Deb Deppeler, 5376 CS, deppeler@cs.wisc.edu Course Website: If Canvas is down, we post updates on: See modules each week for online readings and lecture outlines (no textbook) Waitlisted? Prep 1 and 2 available, complete as soon as possible Read: Module 1 Introduction, Course Info pages, Lists This Week (TODAY) in Lecture: What makes software good? Collections and Data Structures General container intro Abstract Data Types and data structures (ds) Designing an ADT Java interfaces Using the ADT review of autoboxing Generalizing the ADT Java Object Implementing a general ADT casting when using Object Making a generic type using Java Generics ListADT (the list operations you need to know) designing the List ADT coding the List ADT as a Java generic interface using lists via the List ADT implementing the List ADT using an array (class SimpleArrayList) Next Week Java API Lists Iterators concept iterators and the Java API using iterators Copyright Jim Skrentny, 2017 Deb Deppeler CS 367 (F17): W1R - 1

2 It works! Easy to use and modify. Reasonably efficient. What makes software good? What makes code reusable? 1 Documentation 2 Modular components a. functions, methods, procs b. classes c. module, packages d. frameworks 3 Generality a. a collection can store any type of data! What is data? information, items, objects, numbers, How do we store data? in structures: collections, groups, objects, our closets, desks, backbacks, notebooks, computers Q: Why structures or collections? A: It facilitates retrieval of our data. Copyright Jim Skrentny, 2017 Deb Deppeler CS 367 (F17): W1R - 2

3 Collections What is a collection? Definition: A set of rules that defines items in the collection. A group of similar or related items in a shared space. Related terms: items the individual members of the collection container the structure used to store items What operations can you do on a collection? sort *add *remove search, find, get, lookup, retrieve swap replace *is empty is full Place a star by the most fundamental operations. Copyright Jim Skrentny, 2017 Deb Deppeler CS 367 (F17): W1R - 3

4 Collections as Data Structures Data is all around us: paper, notebook, files, physical objects have: mass, volume, weight, location some objects contain other objects (collections): bags, buildings, offices, file cabinets It is useful to categorize data structures by the type of operation one can perform with that structure. - Group data structures by their operations - Give that group a name (that s an ADT) - Use that ADT to solve problems at a higher level. Q: Which data structures share set of operations? Copyright Jim Skrentny, 2017 Deb Deppeler CS 367 (F17): W1R - 4

5 ADTs - Abstract Data Types Why have Abstract Data Type and not just lots of Data Structures? 1. ADTs allow us to use a single name to identify the set of operations that we will need. Thus, the developer team can discuss the problem at a high level without worrying about the details of the final implementation. (Common interview questions are about ADTs) 2. Also, once we abstract (name and describe the operations), multiple implementation variations can be developed. Multiple implementations are then compared for their suitability for our application. 3. If the ADT is designed well and there are multiple implementations, then many apps can reuse. (MODULARITY) Such modularity improves the quality as problems are found (many eyes) and fixed (useful for many users) Abstract Data Type (ADT) ADTs vs. Data Structures ADT specifies what you can do (operations) In Java, an interface is used to code (define) the ADT Data Structure (DS) A construct within a class that implements an ADT (Java interface) that is used to store the collection of items. CODE the ADT as an interface IMPLEMENT the coded interface with a class This allows Java to enforce our ADT design choices. Copyright Jim Skrentny, 2017 Deb Deppeler CS 367 (F17): W1R - 5

6 Concept GENERAL CONTAINER can store any type of item can store different types of items can store duplicate items unordered no explicit internal structure add is fast lookup is slow remove (any) is fast no search remove (specific) is slow must search Example: BagADT Operations (ONLY FUNDAMENTAL OPS) add an item remove any item isempty Problems What problems might occur when doing these operations? add when bag is full remove when bag is empty OOP in Java It is difficult to use a data structure (or ADT) when you do not know what type of object is being returned. Objects must always be cast to the expected type before you can use that object s methods. Let s simplify. Our ADTs will be used to store objects of the same type. Copyright Jim Skrentny, 2017 Deb Deppeler CS 367 (F17): W1R - 6

7 Designing an IntegerBag ADT Conceptual Description General collection of integers that allows duplicate items. Code the Public Interface of the IntegerBagADT import java.util.nosuchelementexception; (add later) public interface IntegerBagADT { void add( Integer item); Integer remove() ; // add throws NoSuchElement boolean isempty() ; Coding Issues add when full, expand bag remove when empty, throw NoSuchElementException Example 1: Using an ADT Write a code fragment to put the numbers 0 through 99 into an IntegerBagADT named mystuff. IntegerBagADT mystuff = new...; //assume is instantiated here We do not need to know how it is implemented to use it in our application for (int i = 0; i < 100 ; i++ ) bag.add( i ) ; // int i is auto-boxed Copyright Jim Skrentny, 2017 Deb Deppeler CS 367 (F17): W1R - 7

8 Java Autoboxing Review Example 2: Using the IntegerBagADT Complete the printitems method so that it prints the contents of the parameter collection. Challenge: Implement your printitems method so that it does not change the contents of the collection. public static void printitems( IntegerBagADT collection ) { Copyright Jim Skrentny, 2017 Deb Deppeler CS 367 (F17): W1R - 8

9 Making the ADT work for items of any type What class in Java can be used for a variable that may reference any Java object? (Object) What modifications are needed to generalize our ADT for any type? import java.util.*; public interface IntegerBagADT { void add(integer item); Object remove() throws NoSuchElementException; boolean isempty(); A general BagADT implementation using an array as the internal data structure import java.util.nosuchelementexception; public class ArrayBag implements BagADT { private Object[] items; private int numitems; public ArrayBag() { items = new Object[10]; numitems = 0; public boolean isempty(){ return numitems == 0; public Object remove() throws NoSuchElementException { if (isempty()) throw new NoSuchElementException(); return items[--numitems]; public void add(object item) { // if items is full, expand array // add the item at the end of array // increment numitems Copyright Jim Skrentny, 2017 Deb Deppeler CS 367 (F17): W1R - 9

10 Use - ADT and Casting Write a statement that makes a BagADT container named bag. BagADT bag = new ArrayBag(); Assume Die is a class representing dice and has a zero parameter constructor. Write a code fragment that adds 6 dice to dicecup. for (int i = 0; i < 6 ; i++ ) bag.add( new Die() ); Assume the bag has had items added to it. Why doesn t the following code compile? while (! bag.isempty() ) { Die mydie = bag.remove(); mydie.roll(); Copyright Jim Skrentny, 2017 Deb Deppeler CS 367 (F17): W1R - 10

11 import java.util.*; public interface BagADT<E> { Java Generics: A better way! void add(e item); E remove() throws NoSuchElementException(); boolean isempty(); public class ArrayBag<E> implements BagADT<E> { private E[] items; private int numitems; public ArrayBag() { items = (E[]) new Object[10]; numitems = 0; public boolean isempty() { return numitems == 0; public void add(e item) { public remove() throws NoSuchElementException{ 1. Java generics specify one or more type parameters list the type after the interface name <E> put the type parameters in between brackets <E,T> 2. Replace the specific types with the type parameter. Let s continue with our first official ADT Copyright Jim Skrentny, 2017 Deb Deppeler CS 367 (F17): W1R - 11

12 Design The Official CS367 List ADT Concept Issues Null item detect then signal with IllegalArgumentException Bad position detect then signal with IndexOutOfBoundsException Empty list handle as a bad position Copyright Jim Skrentny, 2017 Deb Deppeler CS 367 (F17): W1R - 12

13 Interface - Generic ListADT /** * A List is a general container storing a contiguous collection * of items, that is position-oriented using zero-based indexing * and where duplicates are allowed. */ public interface ListADT <E> { /** * Add item to the end of the List. * item the item to add IllegalArgumentException if item is null */ void add(e item); /** * Add item at position pos in the List, moving the items * originally in positions pos through size()- 1 one place * to the right to make room. * pos the position at which to add the item item the item to add IllegalArgumentException if item is null IndexOutOfBoundsException if pos is less than 0 * or greater than size() */ void add(int pos, E item); /** * Return true iff item is in the List (i.e., there is an * item x in the List such that x.equals(item)) * item the item to check true if item is in the List, false otherwise */ boolean contains(e item); Copyright Jim Skrentny, 2017 Deb Deppeler CS 367 (F17): W1R - 13

14 Interface - Generic ListADT (cont.) /** * Return the number of items in the List. * the number of items in the List */ int size(); /** * Return true iff the List is empty. * true if the List is empty, false otherwise */ boolean isempty(); /** * Return the item at position pos in the List. * pos the position of the item to return the item at position pos IndexOutOfBoundsException if pos is less than 0 * or greater than or equal to size() */ E get(int pos); /** * Remove and return the item at position pos in the List, * moving the items originally in positions pos+1 through * size() one place to the left to fill in the gap. * pos the position at which to remove the item the item at position pos IndexOutOfBoundsException if pos is less than 0 * or greater than or equal to size() */ E remove(int pos); Copyright Jim Skrentny, 2017 Deb Deppeler CS 367 (F17): W1R - 14

15 Implementation Java Generic ListADT What changes are needed to make the implementation below a generic type? public class ArrayList implements ListADT { private Object[] items; private int numitems; public ArrayList() { items = new Object[100]; numitems = 0; boolean isempty(){ return numitems == 0; void add(object item) {... Object remove(int i) throws IndexOutOfBoundsException {... Copyright Jim Skrentny, 2017 Deb Deppeler CS 367 (F17): W1R - 15

16 Use - Generic ADT How do we use a generic interface and its generic implementation class to make a container? Write a code fragment to make one generic List ADT container storing String objects and another one storing Die objects. Write a statement to add cs367 into the appropriate container. What happens with? Can we make a single generic List ADT container that can store both String and Die objects at the same time? Copyright Jim Skrentny, 2017 Deb Deppeler CS 367 (F17): W1R - 16