COMP 202 Lecture Notes - Lecture 25: Local Variable

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amaranthjackal733

15 Jan 2018

School

McGill University

Department

Computer Science (Sci)

Course

COMP 202

Professor

Daniel Pomerantz

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Example: write a class to describe a vector. A print method that displays all of the coordinates. A static add method that takes as input two vector objects and returns a new vector containing the sum. (z=x+y) static. See mycourses for example code vector and testvector . If you want to do v. print() (ie: call it on a particular vector object), you will need a non-static method (public void print(){ ) Public class testvector contains the main method where we use the vector class. When you initialize vector v = new vector[]; you get a nullpointerexception with you do v. print() This is because the array coords hasn"t been assigned a value in the main method. This happens when we write two methods in the same class, with the same name, but different input. You cannot overload a method by just changing the return type/static. You have to change the input in a meaningful way number of inputs, type of input.

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2. In this section you will be shown several classes. Inside the code for each class will be a question. You should provide your answer to the question inside the chart. Many of the questions will ask you whether a variable has changed or not. In cases that the variable being asked about is a primitive type, you should provide your answer based on the data stored in the primitive type. In cases that the variable being asked about is a reference type, you should provide your answer based on the data stored at the Object linked to by the reference type. For example, in the first snippet, the question asks about the variable x defined in the main method. Since x is a primitive type, you should answer YES if you think the variable x has had its value changed after the method methodOne() returns. All your answers should go in the chart below: (9 points) Part Answer to question x changed (circle one)? YES NO y changed (circle one)? a changed (circle one)? b changed (circle one)? d1 a changed (circle one)? b changed (circle one)? YES e words changed (circle one)? YES NO Values in array of array referred to by y: What is printed to the screen? (a) public class ReferenceTesti public static void methodOne (int x){ x++; public static void main(String[args) { int x = 0; methodOne (x); //IS THE DATA STORED OR REFERRED TO BY X THE SAME? (ANSWER IN CHART)

9. Finally, you should write a program MontrealMaps which will suggest a path to go along the metro to get from one place to another. The only required method for MontrealMaps is a main method but of course you are allowed to write others. Your program should do the following. For any library methods that you use, make sure that you are properly importing things and handling any exceptions they may throw. (10 points) • Using an appropriate library method, you should create a Subway System object. If this method fails to initialize, you should print the message Error, not enough funding. • Use a Scanner and print statements to read the current x and y position of the user. • Use a Scanner and print statements to read the x and y position that the user wants to go to. . Find the nearest Metro Station to both the start position of the user and his final destination. • If the 2 metro stations have the same name (i.e. are the same metro station) then print It is fastest to walk there. • Otherwise, call the method findPath from the previous part to find a path from these 2 metro stations. . If the method returns null, then print Sorry, there is no path on the metro, you'll have to walk • Otherwise, print the path using the metro using the method printPath defined in the MapUtilities class in the appendix. SUMMARY OF JAVA STANDARD LIBRARY METHODS POR SELECTED CLASSES • String package java.lang) Methods - public boolean gist and e rs is stingeancoject - puble boolean grease Sering another ca t ering String paint i th Stre C h eshilly before the Stripe with striert w Single weiter water N ENE fusha -publish tohy Casting de . File( p age java.io) Methods r eate - publie Yiling path : C • Scanner (package java.ut 1) Method -ple S ule - double Double Sc - le bon when : S Scanner tal de fethede h able othebean - puble String Best Mus eethe u s tepal -pulle haalean haxtLine: Check the hea ther in left PrintStream( p age java.io) Methods pul vaid pri Pribonaat - puli wid pride d oble vile. - pille vaid print Print vei. -pulle vaid printject o: Prints Object -puli vaid printString : Prints String .. - vaid primi The theme line by writing the li n e tie id print in bool bl. Prise n value the - public void platinabled Prints double ved and then the line - public void printit Prints Ent value and then tomis the - Pull vaid pri l l be t he minime publie wild prin t ing with String in the terminates the line • ArrayLiat package java.util) Methods - public boolean Arraybettype was a new ArrayList

COMP 202 Lecture Notes - Lecture 25: Local Variable

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