Alice in Action with Java Chapter 9 Methods

Alice in Action with Java2 Non- void vs. void Methods Alice messages –Methods: messages sent where a statement occurs –Functions: messages sent where an expression occurs All messages in Java are called methods void method in Java –Corresponds to an Alice method –Example: printVerse() non- void method in Java –Corresponds to an Alice function –Must have a return type

Alice in Action with Java3 Non- void Methods How getVerse() revises the HokeyPokey program –Takes a String argument representing a bodyPart –Returns a verse that includes the bodyPart –Is called multiple times in one println() statement getVerse() method vs. printVerse() method –getVerse() is a value-returning method –getVerse() can be used for more than display Non- void methods use Java’s return statement return statement pattern: return Expression –Any Java type can be used as a return type

Alice in Action with Java4 Non- void Methods (continued)

Alice in Action with Java5 Non- void Methods (continued)

Alice in Action with Java6 Method Design Procedure for developing a method –Write a user story –Identify the nouns (objects) –Identify the verbs (operations) –Organize the objects and operations into an algorithm Designing the getVerse() method –User story: method returns verse including body part –Body part will be a String object passed by the sender –Verse modified by the argument is returned to sender Locals: variables and constants declared in a method

Alice in Action with Java7 Einstein’s Formula e = m x c 2 : energy = mass x speed of light 2 –The formula itself serves as the user story –Method returns an expression for right side of formula Developing the massToEnergy() method –Method’s return type is a double –Parameter list includes a double type called mass –Speed of light is declared as a constant outside method –Computation is performed within return statement Example of a call to massToEnergy() –double energy = massToEnergy(1.0);

Alice in Action with Java8 Einstein’s Formula (continued)

Alice in Action with Java9 Computing Initials Method for computing the first initial of a name –firstInitial() takes String value called name –First char value is accessed and returned to caller How to use firstInitial() –char initial1 = firstInitial("Homer Jay Simpson"); Method for computing the last initial of a name –lastInitial() takes String value called name –The index of the last space is computed and stored –char value after index of last space is returned

Alice in Action with Java10 Computing Initials (continued)

Alice in Action with Java11 Computing Initials (continued)

Alice in Action with Java12 Computing Initials (continued) Method for computing first and last initials of a name –twoInitials() takes String value called name –String called result is used to build the two initials –name is passed to firstInitial() –Value returned by firstInitial() stored in result –name is passed to lastInitial() –Value returned by lastInitial() stored in result –result is returned to caller and stored in a String Concatenation operation: joins String values result value is built using concatenation operation

Alice in Action with Java13 Computing Initials (continued)

Alice in Action with Java14 Sound Level Program Revisited Sound level program review –Task: compute loudness of a sound –Inputs: reference loudness and distance, new distance Revised program wraps formula in newSPL() –The parameter list corresponds to the three inputs –Sound pressure level is computed in the method body –The result is returned and stored in a long variable The revision makes the computation reusable –Example: newSPL() is also invoked in MethodTester In general, methods make a computation reusable

Alice in Action with Java15 Sound Level Program Revisited (continued)

Alice in Action with Java16 Method Libraries Repositories for related methods Example: Math class Section objective: build two method libraries

Alice in Action with Java17 Problem Description: Ballooning a Bedroom Problem context –Your friend who plays practical jokes is away –You want to play a practical joke on your friend –You plan to fill your friend’s room with balloons Question: how many balloons should you purchase The question will be answered by a program

Alice in Action with Java18 Program Design The problem is concerned with volumes –Find out how many balloon volumes fit in a room volume The balloon is approximated by a sphere –volume sphere = 4/3 x PI x radius 3 The room is approximated by a box –volume box = length x width x height Another issue: whether to use large or small balloons –Large balloons take long to inflate, but fewer are needed –Small balloons inflate quickly, but more are needed

Alice in Action with Java19 Program Design (continued) Essentials of the user story –Query the user for the radius of the balloon –Read the radius from the keyboard –Compute the volume of one balloon –Compute the volume of the bedroom Note: dimensions of room are declared as constants –Compute number of balloons needed to fill the bedroom –Display the required number of balloons, with labels Identify nouns and verbs to find objects and operations Organize objects and operations into an algorithm

Alice in Action with Java20 Program Design (continued)

Alice in Action with Java21 Program Design (continued)

Alice in Action with Java22 Program Design (continued)

Alice in Action with Java23 Program Implementation First decision: write methods to compute volumes –Rationale: methods allow computations to be reused Second decision: store methods in separate classes –Rationale: makes the program more modular Three classes will be used to implement the program –BalloonPrank : contains the main() driver method –Sphere : library containing sphere methods –Box : library containing box methods Sphere.volume() : takes one argument (radius) Box.volume() : takes three arguments (l, w, h)

Alice in Action with Java24 Program Implementation (continued)

Alice in Action with Java25 Program Implementation (continued)

Alice in Action with Java26 Program Implementation (continued)

Alice in Action with Java27 Unit Testing The sole purpose of a test class –Ensure that methods in the program or library work How to implement unit testing –Build a test class with test methods One test method for each method in a program or library –Run the test methods Illustration of unit testing: BoxTester.java –Test method is named testVolume() –testVolume() tests the volume() method of Box –Note: test methods use Java’s assert statement

Alice in Action with Java28 Unit Testing (continued)

Alice in Action with Java29 Test-Driven Development Reversing the normal testing process –Build the test (this is the starting point) –Use the test to drive subsequent method development Application to the development of methods –Method call indicates number of arguments needed –Number of arguments indicates number of parameters –Type of value expected indicates the return type Example: an initial test for Box.volume() –double vol = Box.volume(2.0, 3.0, 4.0); assert vol == 24.0;

Alice in Action with Java30 Instance Methods Method libraries do not use full capabilities of a class –Methods are used independently of objects Leveraging object-oriented programming features –Build objects with instance methods and variables –Send messages to objects Section objective –Learn how to define an instance method

Alice in Action with Java31 Box Objects Disadvantage of Box.volume() (a class method) –Box dimensions are passed with each method call Alternative: call method against a Box object –Box initialized once, so values are passed only once Enabling Box class to become an object blueprint –Create instance variables for length, width, height Names of double s: myLength, myWidth, myHeight –Define accessor methods for the instance variables –Create a constructor for a Box object –Add an instance method for computing the volume

Alice in Action with Java32 Box Objects (continued)

Alice in Action with Java33 Box Objects (continued)

Alice in Action with Java34 Box Objects (continued)

Alice in Action with Java35 Box Objects (continued) Characteristics of an instance variable –Defined within a class and outside of a method –Omits the keyword static –Each object has its own copy of the instance variables Characteristics of a class variable –Defined within a class and outside of a method –Includes the keyword static –All objects of a class share a class variable Access specifiers: private, protected, public –Guideline: use private access for instance variables

Alice in Action with Java36 Box Objects (continued) Purpose of a constructor –Initialize instance variables with user-supplied values Constructor features –The constructor name is always the name of its class –A constructor has no return type (not even void ) The new operator precedes a call to a constructor –Ex 1: Box box1 = new Box(1.1, 2.2, 3.3); –Ex 2: Box box2 = new Box(9.9, 8.8, 7.7); box1 and box2 contain references to Box objects

Alice in Action with Java37 Box Objects (continued)

Alice in Action with Java38 Box Objects (continued) Instance method –A message sent to an instance of a class –Not defined with the keyword static –Ex: public double volume() {return myLength * myWidth * myHeight;} Invocation: double box1Vol = box1.volume(); Accessor method (getter) –Instance method that returns value of instance variable –Name usually concatenates “get” with an attribute –Ex: public double getWidth() {return myWidth;}

Alice in Action with Java39 Sphere Objects Objective: enhance Sphere to support objects New members of Sphere –A single instance variable: double called myRadius –Instance method for calculating Sphere volume –An accessor to return the value of myRadius Sending messages to a Sphere object –System.out.println(sphere1.volume()); –System.out.println(sphere2.volume());

Alice in Action with Java40 Sphere Objects (continued)

Alice in Action with Java41 Sphere Objects (continued)

Alice in Action with Java42 Sphere Objects (continued)

Alice in Action with Java43 The BalloonPrank Program Using Objects Program produces same results as the original Difference between original and enhanced versions –Sphere and Box objects model balloon and bedroom Chief benefit of the enhanced version –Sphere and Box classes can be used elsewhere –Ex: Sphere earth = new Sphere( );

Alice in Action with Java44 The BalloonPrank Program Using Objects (continued)

Alice in Action with Java45 The BalloonPrank Program Using Objects (continued)