Styjun

I am trying to write a program that will read a webpage. The following code is not valid:

String[] lines;
try 
 lines = loadStrings(url);

catch(IOException e) 
 return;

But this code is:

try 
 String[] lines = loadStrings(url);

catch(IOException e) 
 return;

I cannot use the code in the second example because I need to use the variable "lines" later in the program. Is there some other way to catch this exception (504 error)?

Example:

PFont f;
String webpage;

void setup() 
 size(400, 400); 
 f = createFont("Arial", 16);



void draw() 
 background(255); 
 textFont(f);
 fill(0); 
 text("Press Return to Start", 25, 90);



void keyPressed() 
 if (key == 'n') 
 webpage = "processing.org";
 loadData(webpage);
 



void loadData(String webpage) 

 String url = "www.processing.org";
 try 
 String[] lines = loadStrings(url);
 
 catch(IOException e) 
 return;
 
 saveStrings("Stuff on Webpage.txt", lines);

This is where the program crashes:

java.io.IOException: Server returned HTTP response code: 504 for URL: http://www.processing.org/

In the event of this exception, I would like the program to exit out of the loadData function and continue with keyPressed.

Let's look at this code:

String message;

if(random(1) < .5) 
 message = "hello";


println(message);

This code will generate an error that says message cannot be resolved to a variable. This is because the compiler is smart enough to know that depending on what the if statement does, message might not contain a value. Processing / Java does not assign a default value to local variables.

We could fix this compiler error by assigning a default value ourselves:

String message = null;

if(random(1) < .5) 
 message = "hello";


println(message);

Now message has a default value of null, which matches what happens with sketch / class variables by default.

Another way we could fix this is by putting the println() call inside the if statement:

String message;

if(random(1) < .5) 
 message = "hello";
 println(message);

We could simplify this a bit:

if(random(1) < .5) 
 String message = "hello";
 println(message);

Now, back to your code:

try 
 String[] lines = loadStrings(url);

catch(IOException e) 
 return;

saveStrings("Stuff on Webpage.txt", lines);

This doesn't work because lines is declared inside the try block, so it goes out of scope outside of that block, and you can't use it afterwards.

String[] lines;
try 
 lines = loadStrings(url);

catch(IOException e) 
 return;

saveStrings("Stuff on Webpage.txt", lines);

This doesn't work, because the compiler is not smart enough to see that return statement. It just knows that one branch of the code skips over the lines = loadStrings(url); line, so it knows that lines might be unassigned when you try to use it.

You can fix this by either assigning a default value yourself, or moving everything to be inside the same block, just like we did with our simplified example above.