Replace ' ' Spaces in a URL with a '%20' - Coding Challenge
From a short review:


If you are going to write your own replace, then you should create a method for it, so that you can call myReplace( string, target, replacement);
Personally I would have checked if args got a String, and only if nothing is provided would I use the hard coded string, that would make it easier to show and test your design
No reason to do this in 2 lines:

str = URLify(str);
System.out.print(str);


you can just write

System.out.print(URLify(str));

Javadoc commenting looks good (but is not actually true?), otherwise there are zero comments
I am pretty you can merge those 2 loops into one if you used a stringbuilder. On the whole the replace algorithm could use work