Lecture Set 6 Encapsulaton this junit testing and Libraries 1 2 3 4 5 Review of Parameter passing this public vs private Choices junit testing Libraries CMSC 131 Fall 2009 Jan Plane adapted from Bonnie Dorr Tracing Methods and their Parameters Issues to discuss Primitive type parameters Parameters passed to the constructor Parameters where a value not variable is the argument Non primitive type parameters CMSC 131 Fall 2009 Jan Plane adapted from Bonnie Dorr 1 1 Parameters and Constructors Recall that methods constructors can have parameters public Student String newName int IDDesired name newName id IDDesired tokenLevel 3 What is printed by the following String newName Joe Student s new Student newName Schmoe 123456789 System out println s name System out println newName Joe Schmoe Joe CMSC 131 Fall 2009 Jan Plane adapted from Bonnie Dorr 2 Public Declarations public variables methods and classes Keyword public used in declaration Every user of an object can access any public element Sometimes access should be restricted To avoid giving object users unnecessary info keep API small To enforce consistency on instance variables CMSC 131 Fall 2009 Jan Plane adapted from Bonnie Dorr 3 2 Reference type Parameters Recall that methods constructors can have parameters public int Student giveMore Student s if numOfTokens s numOfTokens s numOfTokens 3 else numOfTokens 3 Trace Calling assume there are Student objects stu1 and stu2 Where stu1 has 5 tokens and stu2 has 12 tokens Called with stu1 giveMore stu2 stu2 giveMore stu1 CMSC 131 Fall 2009 Jan Plane adapted from Bonnie Dorr 4 How Does Java Evaluate Method Constructor Calls int newName Joe Student s new Student newName Schmoe 123456789 Arguments are evaluated using stack in effect at call site place where method called 1 newName Schmoe evaluates to Joe Schmoe 123456789 evaluates to 123456789 Stack frame temporary addition to stack created to associate method parameters with values Stack frame put into stack Body of method executed in modified stack Stack frame removed from stack 2 3 4 5 CMSC 131 Fall 2009 Jan Plane adapted from Bonnie Dorr 5 3 this a reference to the current object Only makes sense in a non static method In an instance method this is the object that is assumed easy to refer to members data or methods using the assumed object difficult to refer to the whole object without having a name to call it Only use when needed using it all the time makes the code more difficult to read CMSC 131 Fall 2009 Jan Plane adapted from Bonnie Dorr 6 Private Declarations private variables methods and classes private int tokenLevel 3 Private variables members cannot be accessed outside the class definition Declaring instance variables private means they can only be modified using public methods Now getters accessors and setters mutators are required CMSC 131 Fall 2009 Jan Plane adapted from Bonnie Dorr 7 4 What Should Be Public Private Class interface API public variables methods Only make something public if there is a reason to Why Encapsulation As long as interface is preserved class can change without breaking other code The more limited the interface the less there is to maintain Rule of thumb Make instance variables private Implement set get methods Make auxiliary methods private CMSC 131 Fall 2009 Jan Plane adapted from Bonnie Dorr 8 Separate API and the workings of the class Design so that you can change how the class works without having to change the API the only things in the API are things the user will absolutely need make the interface as simple as possible Demonstrations in Class Significantly Modifying the Student class without changing the API or the driver The Cat class and its drivers with adding a copy constructor Project 3 API described you are using those classes documentation comments needed CMSC 131 Fall 2009 Jan Plane adapted from Bonnie Dorr 9 5 Floating Point Calculations What will this print public class SimpleMath public static void main String args if 3 9 3 8 0 1 System out println I am a very smart computer else System out println I can t do simple arithmetic I can t do simple arithmetic Why Conversion of floating point to binary leads to precision errors What can we do CMSC 131 Fall 2008 Jan Plane adapted from Bonnie Dorr 10 Floating Point Calculations cont Two important rules You can never use to compare floating point values Instead check if two numbers are within a certain tolerance of each other Never use floating point values to represent money e g 3 52 to represent 3 52 Instead use integer 352 to represent 352 pennies CMSC 131 Fall 2008 Jan Plane adapted from Bonnie Dorr 11 6 Documentation Types Three Styles Two Purposes Internal those reading code External those using the class CMSC 131Fall 2009 Jan Plane adapted from Bonnie Dorr 12 Javadoc Documentation Standard When documenting a method list exceptions that method can throw Use exception tag Be sure to include unhandled exceptions that operations in method may throw Example Returns the year part of a date string param d date string in mm dd yyyy format return an integer representing the date exception IndexOutOfBoundsException exception NumberFormatException public static int getYear String d CMSC 131Fall 2009 Jan Plane adapted from Bonnie Dorr 13 7 Libraries in Java Library implementation of useful routines that are shared by different programs Java mechanism for creating libraries packages Package group of related clases Example java util contains Scanner class To use a class from a package you can use a fully qualified name package name class name java util Scanner s new java util Scanner System in You can also import the class in the beginning of the file import java util Scanner To import class in a package import java util Imports Scanner as well as other classes in package CMSC 131 Fall 2009 Jan Plane adapted from Bonnie Dorr 14 Package java lang A special package containing widely used classes String Math etc java lang is automatically imported by every Java program CMSC 131 Fall 2009 Jan Plane adapted from Bonnie Dorr 15 8 Package Management A class can be added to a package by including package name of package in source file usually very first line The variables methods provided by a class package are often called its API Application Programmers Interface APIs should be documented java lang documentation http java sun com j2se 1 3 docs api java lang packagesummary html On the resources page of the class web site javadoc generated
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