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Java Coffee Break Newsletter Volume 2 Issue 04
Java Coffee Break Newsletter Volume 2, Issue 4
http://www.davidreilly.com/jcb/
ISSN 1442-3790
=============================================================================
In this issue
* Java in the news
* Q&A : Why can't my applet read or write to files?
* Q&A : How do I convert from a char to an int?
* Q&A : How do I compare two strings?
=============================================================================
In the News
Here are a selection of recent news items that may be of interest to
Java developers.
/*/ Cygnus Solutions releases GNU native compiler for Java
Cygnus has released a compiler that produces native code for
Solaris and Linux. Unlike byte-code interpreters, or just-in-time
compilers which work with a Java runtime, the new compiler produces
completely native code, and is claimed to offer performance increases
of up to twenty (20) times Java interpreters.
For more information, visit http://www.cygnus.com/news/javalib.html
/*/ Sun releases JDK 1.2.1
A small number of changes, including one relating to security, have
been made. For more information, visit http://java.sun.com/
/*/ Microsoft pulls the plug on keynote speaker
Microsoft has cancelled an address by a keynote speaker at this
year's Software Development '99 Conference, where he was to have
announced the ultimate Java killer - a virtual machine capable
of supporting C++, Java and Visual Basic. The thought of a
portable Visual Basic, while impressive, would represent
a significant threat to Java, and would challenge its supremecy
over its most important feature - portability.
For more information, visit the commentary at the Java Lobby
site. http://www.javalobby.org/
/*/ Internet Explorer 5 lacks Java Virtual Machine for minimal install
The decision to remove the Java Virtual Machine (JVM) from the minimal
install of Microsoft's Internet Explorer 5 has raised some concern
for developers. However, users that choose a custom install, or use
a typical install, will still have access to Java.
Users are free to download the virtual machine from Microsoft. However,
developers also have the flexibility to avoid Microsoft's JVM entirely,
by choosing to use Sun's Java Plug-in.
For more information on the Java plug-in, visit
http://java.sun.com/products/plugin/index.html
=============================================================================
Q&A : Why can't my applet read or write to files?
Applets execute under the control of a web browser. Netscape and Internet
Explorer impose a security restriction, that prohibits access to the
local filesystem by applets. While this may cause frustration for
developers, this is an important security feature for the end-user.
Without it, applets would be free to modify the contents of a user's
hard-drive, or to read its contents and send this information back over
a network.
Digitally signed applets can request permission to access the local
filesystem, but the easiest way around the problem is to read and write
to remote files located on a network drive. For example, in conjunction
with a CGI script or servlet, you could send HTTP requests to store and
retrieve data.
=============================================================================
Q&A : How do I convert from a char to an int?
If you want to get the ASCII value of a character, or just convert it
into an int (say for writing to an OutputStream), you need to cast from
a char to an int.
What's casting? Casting is when we explicitly convert from one primitve
data type, or a class, to another. Here's a brief example.
public class char_to_int
{
public static void main(String args[])
{
char myChar = 'a';
int i = (int) myChar; // cast from a char to an int
System.out.println ("ASCII value - " + i);
}
}
In this example, we have a character ('a'), and we cast it to an
integer. Printing this integer out will give us the ASCII value of 'a'.
We can covert from a char to an int by casting - but we can also take
it back the other way. An int can be cast back to a char as well.
=============================================================================
Q&A : How do I compare two strings?
A common error that we all make from time to time is incorrect String
comparison. Even once you learn how to compare strings correctly, it's
extremely easy to make a mistake and use the == opearator.
When we compare primitive data types, such as two ints, two chars, two
doubles, etc. we can use the == opeartor. We can also use the == operator
to compare two objects. However, when used with an object, the == operator
will only check to see if they are the same objects, not if they hold the
same contents.
This means that code like the following will not correctly compare to
strings :
if ( string1 == string2 )
{
System.out.println ("Match found");
}
This code will only evaluate to true if string1 and string2 are the
same object, not if they hold the same contents. This is an important
distinction to make. Checking, for example, to see if
aString == "somevalue", will not evaluate to true even if aString holds
the same contents.
To correctly compare two strings, we must use the .equals method().
This method is inherited from java.lang.Object, and can be used to
compare any two strings. Here's an example of how to correctly check
a String's contents :
if ( string1.equals("abcdef") )
{
System.out.println ("Match found");
}
This is a simple, and easy to remember tip that will safe you
considerable time debugging applications. Remember - never use the
== operator if you only want to compare the contents of strings.
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