CS 170, Section /3/2009 CS170, Section 000, Fall

Transcription

1 Lecture 18: Objects CS 170, Section November /3/2009 CS170, Section 000, Fall

2 Lecture Plan Homework 5 : questions, comments? Managing g g Data: objects to make your life easier ArrayList: (more examples) Hashtable: (more examples) Objects t in Java (Chapter 7 cont d) CS170, Section 000, Fall

3 Homework 5: (due Wed Nov 4) Problem: given encrypted text file, break the cipher and print the plain-text (original) message to standard output. Command line arguments: encrypted text, dictionary file (both file names given from command line). Output: original plaintext Simplifying assumptions: No punctuation, numbers in input All words in plaintext are in dictionary, and are in lower case. Dictionary is small (less than 2000 words). 11/3/2009 CS170, Section 000, Fall

4 HW5 (cont d): Approach Overall approach (brute force/dictionary attack) Assume plain text comes from a dictionary Try all possible shifts until each word in a message is found in the dictionary Implementation approach: Read encrypted text from a file, split into words, store into array of words (separate string/entry for each word) Read dictionary, store each word (e.g., in array for now), use individual id string for each word Write a method to search for a given word from the encrypted text in the dictionary can use Hashtable so store/lookup words in dictionary! 11/3/2009 CS170, Section 000, Fall

5 Lecture Plan Homework 5 : questions, comments? Managing g gdata: objects to make your life easier ArrayList: (more examples) Hashtable: (more examples) Objects t in Java (Chapter 7 cont d) CS170, Section 000, Fall

6 What is an ArrayList A dynamically re-sizeable array, so that the total number of elements does not need to be known when created Full documentation: Full documentation: tml

7 ArrayList, cont'd. ArrayList features think of it as an auto-resizing array, that can hold any type of object, with many convenient methods maintains most of the benefits of arrays, such as fast random access can call tostring on an ArrayList to print it remember to import java.util.*; 7

8 ArrayList vs. array import java.util.*; public void main(string [] args){ //construction String[] names = new String[5]; ArrayList nameslist = new ArrayList(); //storage names[0] = "Jennifer"; nameslist.add("jennifer"); //retrieval String name = names[0]; String name = (String) nameslist.get(0); } 8

9 ArrayList vs. array, cont'd. removal (of element #2) for (int i = 2; i < names.length - 1;i++) names[i] = names[i+1]; nameslist.remove(2); search to see if "Marty" is there for (int i = 0; i < names.length; i++) if (names[i].equals( equals("marty")) {... } if (nameslist.contains("marty")) {... } erase all names from the list for (int i = 0; i < names.length; i++) names[i] = null; nameslist.clear(); 9

10 Is an Object in the ArrayList? int indexof(object elem) Searches for the first occurrence equal to the given argument Tests for equality using the equals method Returns position in the list, or -1 if not found boolean contains(object elem) Returns true if this list contains the specified element

11 Important: What s Really Stored ArrayLists (as declared here) store object-references to data of type Object So anything added to an ArrayList must be converted to an Object first Some of this is automatic in Java 5.0 Important: when retrieving an item with get(), you must convert it to the type you orignally stored If you re storing primitive iti types (e.g. int, double) you need to use a wrapper class (e.g. Integer, Double) See your favorite Java book for more on those

12 Adding and Retrieving in Java 5.0 When putting data into an ArrayList: The parameter to add() or set() can be either a primitive type or an object-reference without a problem When retrieving: You must cast the return value to the appropriate class int x = (Integer) myintlist.get(i); String p = (String) mystringlist.get(i); ArrayList list = new ArrayList(); list.add(42); int value = (Integer) list.get(0);

13 Hash Tables Hash Tables 13

14 Constructor Methods Hashtable() Constructs a new, empty hashtable with a default initial capacity (11) and load dfactor, which h is Hashtable(int initialcapacity) Constructs a new, empty hashtable with the specified initial capacity and default load factor, which is Hashtable numbers = new Hashtable(); numbers.put("one", new Integer(1)); numbers.put("two", new Integer(2)); numbers.put("three", new Integer(3)); Integer n = (Integer)numbers.get("two"); if (n!= null) { System.out.println("two = " + n); } 14

15 Methods Object get(object key) Returns the value to which the specified key is mapped in this hashtable. Object put(object key, Object value) Maps the specified key to the specified value in this hashtable. Objec remove(object key) Removes the key (and its corresponding value) from this hashtable void clear() Clears this hashtable so that it contains no keys. boolean contains(object value) Tests if some key maps into the specified value in this hashtable. boolean isempty() Tests if this hashtable maps no keys to values. boolean containskey(object key) Tests if the specified object is a key in this hashtable. 15

16 Objects (Chapter 7) Formal introduction 11/3/2009 CS170, Section 000, Fall

17 Motivation We already know a lot about programming But Java is Object-Oriented dlanguage, so where are all the objects? For developing graphical user interfaces and large scale software systems we need to know OO programming techniques 17

18 Levels of Abstraction: Software Design Old times: computer programs manipulated primitive types such as numbers and characters Manipulating too many of these primitive quantities ii is too much hfor programmers and leads to errors Solution: Encapsulate routine computations to software black boxes

19 Levels of Abstraction: Software Design

20 Objects and Classes Object-oriented programming (OOP) involves programming using objects Object: entity that you can manipulate in your programs Data fields: state of an object Methods: instructions that accesses or modifies the state of an object Class: construct that defines objects of the same type (set of objects with the same behaviour) Definition of data fields: properties of defined objects Definition of methods: behaviours of defined objects Each object belongs to a class

21 Objects and Classes Object: entity that you can manipulate in your programs Class: blueprint for the objects 21

22 Programming with Objects and Defining a class Classes Creating objects using a class and using the objects Using existing classes in Java API

23 Classes A Java class uses variables to define data fields A class uses methods to define behaviors which accesses or modifies the data fields A class provides a special type of methods, known as constructors, which are used to construct and initialize objects from the class 23

24 Circle example Circle Properties: radius Behavior: compute area Circle class Data fields: radius Constructors Methods: getarea()

25 UML design UML Class Diagram Circle Class name radius: double Data fields Circle() Circle(newRadius: double) Constructors and Methods getarea(): double circle1: Circle radius: dus: 10 circle2: Circle radius: 25 circle3: Circle radius: 125 UML notation for objects 25

26 Classes class Circle { /** The radius of this circle */ double radius = 1.0; /** Construct a circle object */ Circle() { } /** Construct a circle object */ Circle(double newradius) { radius = newradius; } Data field Constructors Where is the main method? } /** Return the area of this circle */ double getarea() { return radius * radius * ; } Method 26

27 Syntax: Class Definition accessspecifier class ClassName { constructors methods fields } Example: public class Circle { Circle (double newradius) {... } getarea() {... } double radius;... } Purpose: To define a class

28 Constructors Circle() { } No-args constructor Circle(double newradius) { radius = newradius; Constructor that takes one double parameter } Constructors are a special kind of methods that are invoked to construct objects 28

29 Constructors Constructors must have the same name as the class itself Constructors do not have a return type - not even void A class may be declared without constructors In this case, a no-arg constructor with an empty body is implicitly declared in the class (default constructor) 29

30 Syntax: Constructor Definition accessspecifier ClassName(parameterType parametername,...) { constructor body } Example: public Circle (double radius) {... } Purpose: To define the behavior of a constructor

31 Creating and Using Objects Declaring object reference variables Creating objects Accessing objects

32 Declaring Object Reference Variables To declare a reference variable, use the syntax: ClassName objectrefvar; Example: Circle mycircle; 32

33 Creating Objects Using Constructors Invoking a class constructor using the new operator to create an object new ClassName(); To reference an object, assign the object to a reference Example: variable. Circle mycircle; mycircle = new Circle(5.0); 33

34 Declaring/Creating Objects in a Single Step ClassName objectrefvar = new ClassName(); Assign object reference Create an object Example: Circle mycircle = new Circle(); 34

35 Accessing Objects Referencing the object s data: objectrefvar.data e.g., mycircle.radius Invoking the object s method: objectrefvar.methodname(arguments) eg e.g., mycircle.getarea() 35

36 Example: Circle Class and Tester Class Circle1.java defines a Circle class which can be used to create Circle objects TestCircle1.java a tester class that tests Circle class by creating Circle objects and invoking methods Trace the program of TestCircle1.java 36

37 Primitive data types vs. object data types Variables of primitive data types store the actual value Variables of object types store the reference to the object 37

38 Copying Variables of Primitive Data Types and Object Types Primitive type assignment i = j Before: After: i 1 i 2 j 2 j 2 Object type assignment c1 = c2 c1 Before: c1 After: c2 c2 c1: Circle C2: Circle c1: Circle C2: Circle radius = 5 radius = 9 radius = 5 radius = 9 38

39 The null Value What if we forget to create an object and assign the reference to our object variable? If a data field of a reference type does not reference any object, the data field holds a special value: null Circle c; double r = c.getradius(); Compilation Error c Reference value: null 39

40 The null Value Create an object and initialize the object reference variable before trying to use it Circle c; double r = c.getradius(); Circle c = new Circle(10); double d = c.getradius(); c Reference value 40

41 Reference Data Fields The data fields can be of reference types For example, the following Student class contains a data field name of the String type String is a REFERENCE TYPE as well (it is a class) public class Student { } String name; // name has default value null int age; // age has default value 0 boolean issciencemajor; // issciencemajor has default value false char gender; //gender has default value (char)0 41

42 Default Value for a Data Field The default value of a data fields: null for a reference type 0 for a numeric type false for a boolean type (char)0 for a char type 42

43 Default Value for a Data Field public class Student { String name; // name has default value null int age; // age has default value 0 boolean issciencemajor; // issciencemajor has default value false char gender; //gender has default value (char)0 } public class Test { public static void main(string[] args) { Student student = new Student(); System.out.println("name? " + student.name); System.out.println("age? " + student.age); System.out.println("isScienceMajor? " + student.issciencemajor); System.out.println("gender? " + student.gender); } } 43

44 Warning! Java assigns no default value to a local variable inside a method public class Test { public static void main(string[] args) { int x; // x has no default value String y; // y has no default value System.out.println( println("x is " + x); System.out.println("y is " + y); } } Compilation error: variables not initialized 44

45 Bank account example State (property) of bank account current balance Behaviour of bank account deposit money withdraw money get balance

46 Define and implement BankAccount class Instance fields balance Constructors initialize instance fields Methods may nor may not change the instance fields, and may or may not return an output value withdraw() deposit() getbalance()

47 Test the Class BankAccountTester Create a test class: a class with a main method that contains statements to test another class. 1. Construct one or more objects of the class that is being tested 2. Invoke one or more methods 3. Print out one or more results BankAccount.java BankAccountTester.java

48 Review When you run the BankAccountTester program, how many objects of class BankAccount are constructed? How many objects of type BankAccountTester? nttester?