Midterm Exam 1 Review-Spring 2007: Understanding Java Types

Midterm Exam 1 Review-Spring 2007: Understanding Java Types

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Questions? Answers? Comments?

a) The error will be "Datatype mismatched" because the data type "integer" does not accept words.

I have to be picky and say it doesn't accept Strings. The exact error is incompatible types. Dawn Finney.

c) I think the method should return void because this type of method usually just modifies the original version instead of creating a copy. I dont know about the static part however...I don't think it should be static...should it?

I agree with the void part. As for static, you have to think about the times where you would use a static method. You use a static method when you don't need to declare a particular instance of the class in order to use the method (such as FileChooser.getMediaPath()). So is that what you want here? Dawn Finney
Think about it this way. If you declare a method static, you know that you can say Picture.staticMethod(), right? We saw that with SongPhrase this morning (you were in class, right?). What do you think this is if you call Picture.staticMethod(). Does that make sense?

d) I think the class can extend picture and still work but I don't think its necessary to do this because the picture class and its methods are already public.

Yeah I agree that it'll work, but whether it is a good thing to do is a matter opinion. It doesn't hurt, but it isn't something I would do in a million years. Dawn Finney
It has nothing to do with anything being public. It has everything to do with what extends means. Mark Guzdial

Looking at the Midterm Review I have a couple of questions.

from part (b), Problem 7 (Understanding Java Types):
I cannot clearly say why the first expression will not work. Is it because you are not allowed to declare a new variable while calling a void method?

No. If you have a statement variable=someExpression, you assume that someExpression has some value, right? What does newpic=void mean? Mark Guzdial
Eh...a good way to think about this try to get all the items on either side of the equal sign to have the same type.
```
String s = "string";
```
String = String. Both sides of the equation are Strings.
```
int counter = 0.05;
```
Here we have int = double. The two sides are not the same, hence an error. REMOVEDke Mark hinted at above, what does newpic = void mean? Dawn Finney

from part(c), Problem 7 (Understanding Java Types):
I believe that you should declare the method void because there is no data to return. I am not sure however if the method should be static. If the method is static then is does not require any instances? What is an instance exactly?

Whenever you say new, you create an instance. A static method can be called without ever creating a new anything, like SongPhrase this morning. Mark Guzdial

from part(d), Problem 7 (Understanding Java Types):
Does this question deal with inheritance from Picture? I'm not sure what the question is asking. When you ask "and still compile?" are you asking if we will be free of errors if we attempt to compile?

Exactly. If we say public class PictureCollage extends Picture, is that a Java error? Is it a good thing to do? Why? Mark Guzdial

b) because it says void increaseRed will still do its job-to increase red- however, it will not return the picture back (it will still compile and execute though). I do not think that you will get any error. In regards to the other one about size, it will return a picture. Also, I'm kind of confused about the feedback Prof. Guzdial gave above. If you could provide some clarification, I would be extremely grateful.

I disagree. The void one will give you an error. Just remember that no variable can be set to void, because void isn't actually anything. void just means that the function doesn't return anything, or returns nothing. You're basically saying Picture newpic = nothing which is wrong. Dawn Finney

c) I'm kind of confused about the 'staticMethod()' thing Prof. Guzdial was talking about. I don't think it should be static because you do need an instance of the class. For example, you are going to use 'this' sometime in your code, right?

As long as you understand it's not static because you need a particular instance of that class, I'm happy. Dawn Finney

d). If we say PictureCollage extends Picture we are adding to what pictures will know what to do. It creates another method (PictureCollage) that pictures will now know how to do.

No...that's not right. When PictureCollage extends Picture, it inherits all of Picture's methods. PictureCollage will able to do everything Picture can and can have its own methods as well. Now is this a good thing? Dawn Finney

d.) PictureCollage can extend Picture and still compile. The collage would stil work but this is not a good idea because you really don't need your PictureCollage to inherit everything that Picture knows how to do.

You got the basic idea. Of course there is still the question of why you wouldn't need PictureCollage to inherit everything from Picture. Good inheritance is when you have something like Man extends Person and Woman extends Person where Man and Woman are and act Persons (or people), but have unique methods ( or actions) that aren't true to all Persons. Such an unique method would be something akin to giveBirth(). Dawn Finney

d. Its because PictureCollage returns a certain picture collage. Its not a class with different components. Plus, PictureCollage is a public static void main (String[]args) so its kind of running a particular program. You can't use it to make a different collage than the one in the program. Picture is different because you use the methods in Picture on different kinds of pictures.

B. I know the void one will not work because you're saying that newpic=void which is an incorrect statement. The one that is declared Picture still wouldn't work thought because you would be redefining smallpic.

You have to assume that the two calls are done at two different times. The explanation we're looking for doesn't involve any error with redefinition. Dawn Finney

(b.) No, because void functions only change the original picture? They don't return new pictures?

Well you need to understand what a void method is for any class, not just Picture. Dawn Finney

d.) Okay for one, anything in a class file that has a main file needs to be static if it is used or called in "main", I know this because I've gotten errors for not making my variables in the class "TurtleDance.java" static. You can't inherit a static class, I've tried.

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Midterm Exam 1 Review-Spring 2007 last edited on 6 February 2007 at 4:57 pm by unclejesse.cc.gatech.edu