Questions on HW2-Summer08

Questions on HW2-Summer08

Question: One of the steps in the instructions suggests to change FrameSequence variable 'frames' to static. I changed this at the very top of the file under data storage section to read

static frames;

but when I try to compile, I get this error in line 4:

Error: C:\CS1316\java-source-Fall06\WalkingSausageMan.java:4: <identifier> expected

Any idea what's wrong?

Thanks

I guess that didn't copy with the brackets, but the error says <_identifier_> expected (where the underscores are not present nor are spaces in Dr. Java error message.

It helps to put the code in <code> </code> tags when posting to the coweb. In Java, everything needs a type. I'm guessing you got confused and thought that static was a type. The type in this case is still FrameSequence. static falls in a category that deals with where a variable or method is stored. So okay long story short, you actually want:
```
static FrameSequence frames
```
Dawn Finney

Question: In my renderScene, the first line (after the "public" line) is currently

ColoringPicture pic = new ColoringPicture(name, t+2);

When I try to compile it, I get the error:

File: /Applications/DrJava/WalkingSausageMan.java  [line: 15]
Error: cannot find symbol
symbol  : constructor ColoringPicture(java.lang.String,int)
location: class ColoringPicture

The same thing happens when I take the unmodified PurplePicture and try to run it. Any ideas? The ColoringPicture.java compiles and runs OK.

The constructor for ColoringPicture only takes in one parameter, whereas your call
```
ColoringPicture pic = new ColoringPicture(name, t+2);
```
is attempting to call the constructor with two parameters. Instead, your call should be
```
ColoringPicture pic = new ColoringPicture(name);
```
(where you must define your variable name before the call). Student1086

Question: I am getting a NullPointerException error every time I try to run my code and it forces me to restart Dr. Java. What could be causing this?

A NullPointerExceptionoccurs when you attempt to call a method on an Object (or you can just think of it as a variable) that is null. Any Object has the potential to be null. If you did the following lines, if would also yield a NullPointerException:
```
  String testString = null;
System.out.println(testString.toString()); //NullPointerException occurs here
```
The problem here is that I am calling a method on an Object that is null. null is essentially a placeholder for an actual Object and is not capable of any actions. So when you call a method on testString who is null, it just goes "Nope. Can't do that" and throws the exception.

Remedy. Usually when an NullPointerException occurs, it will give you a line number where it occurs. Go to that line. Any Objects on that line is a likely culprit. If you can't see which one is null right away, try putting printlns for the variables right before the line with the error. So back to my previous example, you would do something like the following:
```
String testString = null;
System.out.println(testString);
System.out.println(testString.toString());
```
Another similar example:
```
int[]array = null;
for(int i = 0; i < array.length; i++) //NullPointerException occurs here
  System.out.println(array[i]);
```
Using my method for testing/debugging, the above code would be:
```
int[]array = null;
System.out.println(array);
for(int i = 0; i < array.length; i++) //NullPointerException occurs here
  System.out.println(array[i]);
```
It is important to note in this case that we are not trying to call a method on the Object but will still yield a NullPointerException

A side note: the newer version of Dr. Java has a nasty habit of crashing Dr. Java after two NullPointerExceptions occurring in sequence. You can either bite the bullet on this one or switch to a different programming environment. Dawn Finney.

P.S. If you want to be picky about CS terminology. Objects cannot be null, but the references to Objects can be pointing to null. So from the examples, the references testString and array are pointing to null. References are almost synonymous with variables and pointers. Ah okay I'm done. I am so not rehashing a Java textbook.

Question: I am trying to use custom colors in my code, but some colors don't seem to work. For instance,

new Color(50,0,50)

turns out black rather than dark purple, and

new Color(250,100,0)

comes out red instead of orange. Is there a predefined color table that Java is automatically snapping my colors to?

The constructor for Color can be found in the Java Color API. To figure out what RGB values will yield try this site and this site is fun as well. I know I didn't really answer the question but I hope those sites help. Dawn Finney.

Links to this Page

Homeworks-Summer 2008 last edited on 22 July 2008 at 4:06 pm by dhcp-207-151-195-107.vonkarman.net
Soap Box Archive-Summer 2008 last edited on 6 August 2008 at 1:49 pm by dhcp-207-151-195-107.vonkarman.net