TeachJava! 2003.ppt

1、TeachJava! 2003,Corky Cartwright Dung Nguyen Stephen Wong Charlie Reis, James Hsia, Neal Hororwitz, Peter Centgraf,From C+ To Java,C+ and Java have similar syntax. Dont be misled! Profoundly different semantics: high-level objects vs. bytes in memory Profoundly different programming models: Java is

2、object-oriented (OO) C+ is object-based (OB) in normal usage,Encouraging Note,Writing OO programs in Java is remarkably easyprovided we recognize that it is fundamentally different than writing OB programs in C+. There is little common conceptual ground, other than syntactic conventions, beween OO p

3、rograming (OOP) in Java and OB programming in C+,Guiding Vision,Program design in Java is data-directed. Design the data abstractions first; they will determine the structure of the code. In OOP circles, this data design process is often called object-modeling. Common data abstractions are codified

4、as design patterns.,Secondary Theme,DrJava, our lightweight, reactive environment for Java, facilitates active learning; with DrJava learning Java is a form of exploration. DrJava is not a toy; DrJava is developed using DrJava. It includes everything that we believe is important and nothing more.,Wh

5、at Is an Object?,Collection of fields representing the properties of a conceptual or physical object. Collection of operations called methods for observing and changing the fields of the object. These fields and methods often called the members of the object.,Example: Phone Directory,Task: maintain

6、a directory containing the office address and phone number for each person in the Rice Computer Science Dept.Each entry in such a directory has a natural representation as an object with three fields containing a persons name address phone number represented as character strings.,Summary of Entry fo

7、rmat,Fields:String nameString addressString phoneMethods: String getName() String getAddress() String getPhone(),Java Method Invocation,A Java method m is executed by sending a method call o.m()to an object o, called the receiver. The method m must be a member of o. The code defining the method m ca

8、n refer to the receiver using the keyword this.,Finger Exercise,In the DrJava programming environment, open the program file Entry.java, compile it, and type the following statements in the Interactions pane:Entry e = new Entry(“Corky“,“DH 3104“,“x 6042“); e.getName()e.getPhone(),Java Expressions,Ja

9、va supports essentially the same expressions over primitive types (int, float, double, boolean) as C+. Notable differences: boolean is a distinct type from int no unsigned version of integer types explicit long type,Finger Exercise,Evaluate the following: -5 + 3 -(5 + 3) 5 % 3 5./3. 5 / 0 5./0. 5 6.

10、,Finger Exercise cont.,72. - 32. * 1.8 (72. - 32.) * 1.8 72. - 30. - 12. 72. - (30. - 12.),Java Statements,Essentially the same form as in C+: assignment, if, while, for, return, But well-written Java programs consist primarily of assignment, if, and return statements (with smattering of for). Focus

11、 on assignment and if for most of the week.,Assignment,Restricted form of assignment: variable definitiontype var = exp; Exampleint x = 5;,Finger Exercise,int x = 5; x*x double d = .000001; double dd = d*d; dd dd*dd 1. + dd 1. + dd*dd,Finger Exercise cont.,Evaluate: int x = 7; if (x = 5) y = 0; else

12、 y = 10; yDid you get the behavior that you expected? Repeat the exercise with corrected syntax. Evaluate: boolean switch1 = (x = 7); switch1Repeat the exercise with corrected syntax.,Classes: Object Templates,A Java program is a collection of classes. A class is an object template consisting of a c

13、ollection of membersfields and methods plus a constructor, which is a special method used to initialize objects when created.,Example: Class Entry,class Entry /* fields */String name; String address;String phone;/* constructor */Entry(String n, String a, String p) this.name = n;this.address = a;this

14、.phone = p; /* getters */String getName() return this.name; String getAddress() return this.address; String getPhone() return this.phone; ,Finger Exercise,In the Definitions pane of DrJava, enter the Java program defining the Entry class. In the Interactions pane, evaluate the following program text

15、: Entry e = new Entry(“Corky“, “DH 3104“, “x 6042“); e.getName() e.getAddress() e.getPhone() Save your program for future use in a file named Entry.java.,Terminology,The methods that return the fields of a class are called getters. The special method with the same name as the class is called the con

16、structor. The constructor is executed when a new instance (object) of the class is created using new.,Java Data Types,Two fundamental categories Primitive types: int, boolean, double, float, char, long, short, byte (first three are most common) Object types: all class instances (objects) belong to o

17、bject types, which are disjoint from the primitive types Values of primitive type (e.g., true, 0) are not objects,Object Types,Organized in a strict hierarchy with the universal type Object at the top. Every class C except Object has an immediate superclass, which is the parent of C in the hierarchy

18、. In a class definition (like our Entry example), the default superclass is Object.A descendant in the class hierarchy is called a subclass. B is a subclass of A iff A is a superclass of B. Entry is a subclass of Object; Object is a superclass of Entry.,null,There is a special value null of object t

19、ype on which method invocation utterly fails. null is a reference to nothing; the method invocationnull.m()always generates a NullPointerException aborting execution.,Example: the String class,The String class is built-in to Java, just like Object. Finger Exercise: evaluate String s = “Corky“; Objec

20、t o = s; o o = s String t = “Cork“ + “y”; t = s s.length() o = null; o.length() Morals: multiple copies of the same String may exist; do not use = to test String equality. Do not use null to represent legitimate data values.,Object Types cont.,Each subclass C inherits (includes) all of the members o

21、f its superclass. The declared members of C augment the inherited members with one exception: if C declares a method m with exactly the same name and types as an inherited method, then the new definition of m overrides (replaces) the inherited definition.,Inheritance from Object,The Object class has

22、 several members that its children inherit. They include the methods public String toString() which gives a String representation for the object. public boolean equals(Object o) which compares this to o,Example of Overriding,class Entry /* fields */ String name; String address;String phone;/* constr

23、uctor */Entry(String n, String a, String p) this.name = n;this.address = a;this.phone = p; /* accessors */String getName() return this.name; String getAddress() return this.address; String getPhone() return this.phone; /* overridden methods */public String toString() return “ + name + address + phon

24、e + “; ,Finger Exercise,Open your Entry class into the DrJava Interactions pane. Compile your program and evaluate:Entry e = new Entry(“Corky“, “DH 3104“, “x 6042“); e Add the definition of toString() from the previous slide to your Entry class. Compile your program and evaluate:Entry e = new Entry(

25、“Corky“, “DH 3104“, “x 6042“); e,Adding a Method Definition,class Entry /* return this if its name matches keyName; otherwise return null indicating failure */Entry match(String keyName) if (keyName.equals(this.name) return this; else return null Finger Exercise: Add this method definition to your E

26、ntry class, compile it, and evaluate: Entry e = new Entry(“Corky“, “DH 3104“, “x 6042“); e.match(“Corky“)e.match(“Matthias“),The Wrapper Classes,How do we treat primitive values as objects? Java includes a built-in “wrapper” class for each primitive type. Examples: Integer is the wrapper class for i

27、nt Boolean is the wrapper class for boolean Double is the wrapper class for double,Finger Exercise,In the DrJava Interactions evaluate the following:Integer i = new Integer(5); i Integer j = i; i = j Integer k = new Integer(5); i = k,Constructors,Given a class definition, Java provides a mechanism c

28、alled new for creating new instances of the class. To exploit the new, the class must provide a special method called a constructor that specifies how the fields of the created object are initialized. A constructor method has the same name as the class and does not contain the return type in the hea

29、ding. Example: Entry(String n, String a, String p) this.name = n;this.address = a;this.phone = p; If a class does not include a constructor, Java provides a default constructor (of no arguments) that does nothing.,Finger Exercise,Add a second constructor Entry(String n) to the Entry class where n sp

30、ecifies the name of the entry. Use any reasonable default value, e.g., “unknown“ for the address and phone fields.Recompile your program and evaluate the expressionnew Entry(“Matthias“),Union Pattern,The union pattern is used to represent different forms of related data with some common behavior. Th

31、e pattern consists of an abstract class A together with a collection of variant subclasses B1, ., BN extending A. An abstract class cannot be instantiated using new. The collection of classes A, B1, ., BN is called a union hierarchy and A classes is called the root class of the hierarchy. The common

32、 behavior is codified by a set of methods in A, which may be abstract Each such method m has an associated contract that that the implementation in each variant class must obey.,Class Diagram of Union Pattern,Defining a Method on a Union,abstract m();, m() , m() ,City Directory Example,Assume that w

33、e want to design the data for an online city phone book. In contrast to our DeptDirectory example, such a directory will contain several different kinds of listings: businesses, residences, and government agencies. The entry data for such a directory is represented by using the union pattern to iden

34、tify the common behavior among the various kinds of listings. In this case, the common behavior is the existence of a String name for the listing and the String text of the listing (given by toString().,Definition of CityEntry,A CityEntry is either: a ResidentialEntry(name,address,phone) a BusinessE

35、ntry(name,address,phone,city,state) a GovernmentEntry(name,address,phone,city, state, government) Examples: ResidentialEntry(“Corky Cartwright“,“3310 Underwood“, “713-660-6967“) BusinessEntry(“ToysRUs“,“2101 Old Spanish Trail“, “713-664-1234“,“Houston“, “TX“) GovernmentEntry(“Federal Drug Administra

36、tion“, “800-666-9000“, “Washington“, “DC“, “Federal“),Class Diagram of CityEntry Union,Defining Methods on Unions,Assume that we want to define a method on a union. The method will typically require a separate implementation for each variant (subclass) of the union. But each implementation will sati

37、sfy the same “contract“ (description of behavior). In Java, the method must not only be defined in each variant of the union, it must be declared as abstract in the root class of the union hierarchy. Otherwise, Java will not allow the method to be invoked on objects of the union type.,Methods for Ci

38、tyEntry,To process entries in a city directory, we need two methods:/* Returns the name of this CityEntry */String getName();/* Returns all information in this CityEntry */String toString()The toString() method is already declared in Object (the superclass of CityEntry) but the provided default beha

39、vior does not satisfy the contract given above, so it must be overridden in each variant. The getName must be declared as abstract in CityEntry.,CityEntry,abstract class CityEntry /* Returns the name for the entry */abstract String getName();/* Returns all of the information in the listing in String

40、 form*/* String toString() */ class BusinessEntry extends CityEntry String name, address, phone, city, state; BusinessEntry(String n, String a, String p, String c, String s) this.name = n;this.address = a;this.phone = p;this.city = c;this.state = s;String getName() return this.name; public String to

41、String() return “Business“ + this.name + “,“ + this.address + “,“ +this.phone + “,“ + this.city + “,“ + this.state + “; ,CityEntry cont.,class GovernmentEntry extends CityEntry String name, address, phone, city, state, government; GovernmentEntry(String n, String a, String p, String c, String s, Str

42、ing g) this.name = n;this.address = a;this.phone = p;this.city = c;this.state = s;this.government = g;String getName() return this.name; public String toString() return “Government“ + this.name + “,“ + this.address + “,“ + this.phone + “,“ + this.city + “,“ + this.state + “,“ + this.government + “;

43、class Residential Entry extends CityEntry String name, address, phone, city; ResidentialEntry(String n, String a, String p) this.name = n;this.address = a;this.phone = p;String getName() return this.name; public String toString() return “Residential“ + this.name + “,“ + this.address + “,“ + this.pho

44、ne + “; ,Member Hoisting,In a union hierarchy, the same code may be repeated in every variant. A cardinal rule of software engineering is never duplicate code. We can eliminate code duplication in a union hierarchy by hoisting duplicated code (code that is invariant within the union) into the abstra

45、ct class at the route of the hierarchy.,CityEntry Example,We can hoist the fields name, address, and phone and the getters getAddress and getPhone. To perform this transformation we must introduce a constructor in the abstract class CityEntry to initialize the fields declared in the class. This cons

46、tructor can be called from subclass constructors by using the special method name super. This hoisting process does not affect the form of the class diagram.,CityEntry II,abstract class CityEntry String name, address, phone;CityEntry(String n, String a, String p) name = n;address = a;phone = p;Strin

47、g getName() return name; String getAddress() return address; String getPhone() return phone; /* Returns all of the information in the listing */* String toString() */ class BusinessEntry extends CityEntry String city, state; BusinessEntry(String n, String a, String p, String c, String s) super(n,a,p

48、);this.city = c;this.state = s;String getCity() return this.city; String getState() return this.state; public String toString() return “Business“ + this.getName() + “,“ + this.getAddress() + “,“ +this.getPhone() + “,“ + this.city + “,“ + this.state + “; ,CityEntry II cont.,class GovernmentEntry exte

49、nds CityEntry String city, state, government; GovernmentEntry(String n, String a, String p, String c, String s, String g) super(n,a,p);this.city = c;this.state = s;this.government = g;String getCity() return this.city; String getState() return this.state; String getGovernment() return this.governmen

50、t; public String toString() return “Government“ + this.getName() + “,“ + this.getAddress() + “,“ + this.getPhone() + “,“ + this.city + “,“ + this.state + “,“ + this.government + “; class ResidentialEntry extends CityEntry ResidentialEntry(String n, String a, String p) super(n,a,p); public String toString() return “Residential“ + this.name + “,“ + this.address + “,“ + this.phone +“; ,