CMPSCI 187: Programming With Data Structures

David Mix Barrington

Fall, 2012

Q&A for Programming Project #3: Maps With Continents and Capitals

Question text in black, answers in blue.

• Question 3.1, posted 19 October: I'm confused about where the constructor for MapGrid should be and what it should do. Since you say that MapGrid should extend Grid, doesn't that mean that the MapGrid constructor should call the Grid constructor? But you said on the assignment that we should rewrite the Grid constructor...

• Yes, you are right. We want MapGrid to extend Grid, and that means that every MapGrid object is also a Grid object and has the fields that a Grid object has. But we need to erase all the arrays that the Grid constructor made, because we want new ones that are made using the constructor that takes a seed (like the one on the lecture notes).

• So the MapGrid constructor doesn't need to create the arrays that the Grid constructor made, just repopulate them with the data made from the new Random object that has a seed. If you create new fields in MapGrid, as you probably want to, the MapGrid constructor will need to initialize and create these.

• Question 3.2, posted 19 October: You say that the labels for continents should start with "a" and then later go on to "A". But don't you have this backward? In ASCII, "A" through "Z" come first, followed by six punctuation characters and then "a" through "z".

• Yes, this was a mistake which I will fix on the assignment.

• So every continent has a number, with the first continent numbered "0", and the label of continent i is now supposed to be the character 'A' + i?

• Yes, that's right. You probably want to create an array of Continent objects, with the information that you need to print out in the continentTable method.

• Question 3.3, posted 19 October: Here's a problem -- DJW wrote markBlob as a private method, so mapGrid can't use it! Should I change the DJW code to make it protected, or write an entirely new method in MapGrid?

• Yeah, DJW weren't thinking that Grid was going to be extended. I think the best thing to do is to write a new markBlob method in MapGrid, particularly if you want to have it access the data fields that you are building in MapGrid. But if you want to change the markBlob method in Grid to be protected, that's ok too -- we will grade your code using the Grid class that is in your edlab directory, not the code straight from DJW.

• <Question 3.4, posted 24 October: I'me getting a strange error -- I created the array visited, initialized it, and then tried to access it and got an NullPointerException. Any idea how that could happen?

  I know that some people have declared visited in MapGrid after it was already declared in DJW's Grid class. That means that there are now two arrays around with the same name -- no good can come of that. It's quite possible that you initialized one of those arrays and then tried to access the other, especially if you used this in one of the two places. Also remember that if you declared the array in the constructor rather than as a field, as so many people did on the exam, you might have initialized an array that ceased to exist when the constructor finished, leaving the other one to cause your exception.

• Question 3.5, posted 24 October: I put in stubs for capitalMap and continentTable like you said, but my MapGrid class still does not work with MapGridTest.

  We've now put up another driver, Project3.java, that doesn't refer to those two methods. But if you said return null in your stubs, they won't work in MapGridTest because that method takes the output String objects from your methods and calls their equals methods. Replacing return null with return "foo" will solve that problem.

• Question 3.6, posted 24 October: I declared a variable char c and tried to increment it by saying c = c + 1;, but this did not compile. Neither did c = (char) c + 1. Oddly, c++; and c += 1; both worked, so I can keep coding, but what was wrong with the other two?

  (Thanks to various students in today's lecture.) It's more or less true that c++; translates to c = c + 1;, but the fomer is smarter about realizing that you want the result to be the same type that you started with. When you add the char and the int, the result is an int, and for some reason it won't do the cast automatically when you use the + sign alone. Using the cast operator as in your second example would solve the problem, except that the cast operator has high precedence and applies only to the first thing it sees. So c = (char) c + 1; is the same as c = ((char) c) + 1; which is just c = c + 1; because c is already a char. But c = (char) (c + 1); does work.