Lecture 15 Review of Aliasing Mutability Floating Point Calculations Last time 1 Unit testing and JUnit 2 Constructors revisited 3 equals Today 1 Project 4 assigned 2 Aliasing and Mutability 3 Floating Point calculations 4 Example class development Rational Numbers CMSC 131 Fall 2007 Jan Plane adapted from Bonnie Dorr Project 4 Is Assigned It is due Wednesday 10 31 at 11 00PM The project is closed You must complete the project by yourself Assistance can only be provided by teaching assistants TAs and instructors You must not look at other students code Start now Read entire assignment from beginning to end before starting to code Check out assignment now from CVS Follow the instructions exactly CMSC 131 Fall 2007 Jan Plane adapted from Bonnie Dorr 1 Taking Care of Corner Cases FancyWord example from Friday String of was a corner case that we needed to test for Write new test cases or new asserts in the test cases that already exist to take care of this What about null references as corner cases public void testNullAndEmpty FancyWord a new FancyWord null assertEquals null a toString FancyWord b new FancyWord assertEquals b toString CMSC 131 Fall 2007 Jan Plane adapted from Bonnie Dorr 2 1 What about Strings and Aliasing String objects are immutable fields cannot be changed once created Mutable objects fields values of instance variables can be changed e g Cat Student etc Immutable objects fields values of instance variables cannot be changed See String API http java sun com j2se 1 3 docs api java lang package summary html In this example y is created as an alias for String x b is created as an alias for Cat a but the results are different x Jan Plane y Jan Fluffy has been created Fluffy has been eaten Fluffy has 8 more lives a Fluffy 8 lives b Fluffy 8 lives CMSC 131 Fall 2007 3 Jan Plane adapted from Bonnie Dorr Which picture represents the current status of memory Stack Heap cat a b Stack Heap cat a b Fred Fred CMSC 131 Fall 2007 Jan Plane adapted from Bonnie Dorr 4 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 2007 Jan Plane adapted from Bonnie Dorr 5 2 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 2007 Jan Plane adapted from Bonnie Dorr 6 3
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