FaQs
Q:
ServerSocket or DatagramSocket? What is better to use in my applications?
Answer:
Both of them are good. It depends on task you do. DatagramSocket is designed for UDP and
ServerSocket for TCP. TCP is more
reliable in terms that it ensures delivery, UDP not.
But UDP packets are much smaller and have no such big
headers like TCP packets have.
If it is smaller then there is more probability that it
arrives faster. UDP used mostly in areas where you do no mind about retransmission.
You just do not need this, because information become obsolete. Sound, for
example. It is better to make small pause or repeat last packet than to
play some piece that was send first but received
second. It is up to you to take
decision. If you do not mind that can lose small piece of data - use UDP.
Otherwise use TCP.
Q:
I read this statement: "The main thread must be the last thread to finish
execution. When the main thread stops, the program terminates." Is it
true?
Answer:
Absolutely wrong! The correct one: When
you start a program, JVM creates one thread to run your program. The JVM
creates one user thread for running a program.
This thread is called main thread.
The main method of the class is called from the main thread. If program
spawns new threads from the main thread it does stop until last thread id
died. Even if the main thread dies
program keep running.
Q:
What is asynchronous architecture?
Answer:
In asynchronous architecture a program does not wait for return immediately.
The program continues to do another staff and does not care when it will
happen. Simple example: modeless dialog.
You can do something else and return later to that dialog, change something and
press "Apply" button.
Synchronous example: modal dialog call, the procedure call it will
not continue until the model dialog returns. For example, when you try to delete files you
will be asked: "Are you sure that you want to delete file(s)?" You
can not continue until you press
"Yes", "No" or "Cancel".
Q:
Why C++ is not platform independent?
Answer:
C++ compiles to binary code (.obj, .exe, .dll, a.out etc.). Binary code
(Machine code, 0's and 1's) are machine dependent. Java compiles to byte code which is
independent of any machine. It need to
be interpreted to binary code by JVM, and executed by the machine.
Q:
I have heard that String concatenation operator "+" affects
performance of program if it used much. Is it true?
Answer:
Yes, it affects your program performance if you do a lot of "+"
operations with strings:
A new StringBuffer must be created, then two arguments are
added to it with append(), and the final result must be converted back with a
toString(). Your time and space is
wasted...
In case if you are appending more than one String, try to
use a StringBuffer directly.
Code
example: I want to show you one funny thing! The code is shown below
is simplest that you can imagine and does very unusual thing!
It is slightly bigger
than "Hello World!" program but does much more. It lists all files in
the current directory if you run it like this:
java test * (of
course after compilation) in DOS/CMD prompt on Windows or in any shell in UNIX.
The program shows all files both in Unix and Windows. If you
do: java test. * On UNIX it also shows all hidden files.
class test{
public static void main(String args[]){
for (int i = 0;i < args.length; i++) {
System.out.println("File " + i +
":" + args[i]);
}
if (args.length<=0) {
System.out.println("No files!");
}
}
}
You can ask how can we get this list
without any file handling functionality in the code? Indeed looks mysterious...
But in reality everything is very simple. When you type
"*" (wildcard) OS (DOS, Windows, UNIX), not Java (!!!) sends the list
of files in the current directory to your program as a list of parameters. And
you see this list...
Q:
When can I use System.exit() in servlets?
Answer:
Never! Depends on server you run on...
Security exceptions on good server or full shut down for
simpler one
Q:
I was writing and testing some servlet program. Now I decided to remove it from
our web server. But it is still there causing confusion to another developers I
am working with. How can I unload my servlet explicitly?
Answer:
It depends on server you use. Often you can unregister your servlet somehow.
Easier way to make your servlet empty (remove all code), compile it and copy
class file to server. Many servers reload automatically a new class if they see
it was changed. You will still have your servlet on the server but it will do
nothing and nobody will complain...
Q:
By default an application has no security manager and Java runtime environment
does not create automatically a security manager for my program. How then
applet where I am not creating any security manager already prevented from many
operations?
Answer:
It is true - Java by default let you do whatever you want and then it is your
responsibility to restrict something. In case with applets a little bit
different story - applet viewers and browser have THEIR OWN security manager,
not from JRE. That's why even if you did not define any security manager in the
code of your applet, during the start up time browser/viewers will use their
own security manager. This manager is built in into their application (browser,
viewer), not your Java applet.
Q:
I the see method getClass() in java.lang.Object. Do we need it? I know all my
classes.
Answer:
Exactly. If you know - you do not need it. But if you do not know then it helps
you. For example if you get some object and would like to instantiate it:
Object createNewInstOf(Object obj) { return obj.getClass().newInstance(); }
Q:
I know that Java file should have the same name as public class in the
file.. But what are rules for files
that have no public classes?
Answer:
No rules are defined for such case! It is up to you how to
name it. Please check my example:
// ******
ABCDEFG.java file *******************
class G {
public
static void main(String[] args) {
System.out.println("This is class G!");
}
}
class Z {
public
static void main(String[] args) {
System.out.println("This is another class Z!");
}
}
class AB {}
class CD {}
class EF {}
After compilation you will get AB, CD, EF, G and Z
classes. You can run G and Z (they have
main method), but not AB, CD, EF.
Try it: java G
or java Z
Q:
I am currently working on a program for generating a landscape from a
bitmap. I use lots of 4-Point-QuadArrays
next to each other and set the vertexes z-positions according to values from a
grayscale bitmap. It looks pretty OK but
is very memory intensive (at 150X150 QuadQrrays i get a memory exc.).
Answer:
You might want to try the utilities that I've written and are part of the
j3d.org code repository. Instead of a
QuadArray per pixel, I just generate one large array for the entire image.
You control the size of the quads by the size of the image
:)
It also sounds like you are doing some automatic terrain
generation, so you might want to have a look at the fractal terrain generator
that is also part of the package. Here's the pages:
and Javadoc
Q:
I am planning on making a MRU(Most Recently Used) list and allowing the user to
configure some primitive directory settings, like where to look for images,
other data, etc. I have a class fairly
well worked out in my brain to accomplish these goals. It's not rocket science. It does, however require that a file be
written to store the information, and the application needs to be able to
discern from whence the information should be read when started. In C++, I have stripped the directory from
which the application was run (from argv[0]) and saved my
"ApplicationSettings.cfg" file in that directory.
Any thoughts as to how to accomplish this?
Answer:
Check the API documentation for JDK1.4 beta:
===========================
public abstract class Preferences extends Object
A node in a hierarchical collection of preference data. This
class allows applications to store and retrieve user and system preference and
configuration data. This data is stored persistently in an
implementation-dependent backing store.
Typical implementations include flat files, OS-specific
registries, directory servers and SQL databases. The user of this class needn't be concerned
with details of the backing store.
============================
Note the bit about 'needn't be concerned with details'. The ugly problem of where to put such
preference data has been abstracted away from the developer, which makes life
much easier. The Preferences API is part
of JDK 1.4, which is available in beta form now, with a production release in
the near future.
Q:
I am writing a Java Applet program which pop up a Window, However, there is a
Warning:Java Applet..." message appears at the bottom of the windows. Do you know how can I remove this, or Can I
rewrite the wording, for example, write company info instead of the
"Waring:Java Applet ....."?
Answer:
The rule for writing secure application is to use encapsulation. Always make variables PRIVATE. One of Sun's programmers, whose name I do not
mention here, did not follow that
idea.
He wrote the class Window like this:
public class Window extends Container{
String
warningString;
......
private
FocusManager focusMgr;
private static
int nameCounter = 0;
.....
}
What you do is like this:
Write:
package java.awt;
public class SetWarning {
public static
void setWarning(Window w){
w.warningString = "";
/*disable warning string*/
}
}
Compile this file and then put the class SetWarning.class
inside the subdirectory java\awt somewhere onto the classpath, and you suddenly
have gain access to the internal of the java.awt package. Include whatever need be in your final
product.
Q:
Can I access a private variable of one Object from another Object?
Answer:
Yes, if this object of the same class. You can access private field from static
method of the class through an instance of the same class.
Check one example below:
public class Example {
private int
privateVar;
Example(int
a) {
privateVar
= a;
System.out.println("private privateVar = " + privateVar);
}
public static
void main(String[] args) {
Example ex
= new Example(5);
System.out.println(ex.privateVar);
}
}
Q:
I understand multi-threading as "parallel instances of execution" and
I have a query in this regard. The following is a code and its output. I would
like to how will you describe "what exactly is the join() method
doing?".
Answer: join() is a left over from UNIX days of doing
process synchronization (via the infamous fork/spawn mechanism). join() makes
the current thread (the one where you call T.join()) wait
until the thread that it is called on (namely T) dies (or is
interrupted). This is useful in the case
where, for example, your main thread had to do additional work AFTER the other
threads were finished doing theirs. join guarantees that the only way that the call
to join will return is if the target thread is dead.
There's a good book on java multithreading that you might
want to read "Java Threads" by Scott Oaks & Henry Wong.
Q:
I run some code in my program like this:
Runtime rt = Runtime.getRuntime();
rt.exec("C:/StartMyApp.bat");
I get very strange behavior: sometime it starts from first
time, sometime I need to call it a few times and often it does not start at all after I run it
once..
Answer:
You need to modify your code since you execute your bat file in the MSDOS shell
(Win95/98) or command line shell"cmd.exe" (NT). You should use exec(String[] cmdarray) not
exec(String cmd) method...
Q:
I have a question between C and Java Language. In C, I want to call a Java
Library, as Java calls C library in Java.
In Java, I can call a C function and execute an application program by
using JNI Mechanism. But can I call a
library of Java Class in C ?
Answer:
Not easily. But you can execute a Java program from C. If you really need to call Java from C, write
a Java program that does what you want and "call" it.
Q:
Why Servlets are better than CGI scripts?
Answer: Generally, every new client's request
to CGI script starting a new process. With servlet web server starts just a new
thread that is much cheaper in terms of time for starting up, memory and CPU
consumption. JVM uses just one
process. This answer is right for
"standard" CGI. Now appear new modules for Perl that uses the same
approach as servlet does. Anyway, writing large programs on Java is much easier
than on Perl, atleast for me. Java has
also very rich API.
Q:
I can't find the API documentation on any classes in the sun.* packages. Where
is it?
Answer: The short answer is that SUN provides
documentation only for the public classes in java.*. SUN does not provide
documentation for sun.* because those are the Sun-specific
implementation, and specifically not
part of the Java technology API standard, and are therefore subject to change
without notice.
In
general, SUN doesn't provide javadoc documentation for sun.* classes in order to
discourage developers from writing programs that use them. For further
explanation, see the next question.
However, if you must have it, the documentation for sun.* is available
separately, here: http://java.sun.com/communitysource/index.html For example,
the doc comments for sun.net are in the source files located at: /src/share/sun/sun/net/*.java This source code release does not include
javadoc-generated documentation. You would have to generate those docs yourself
using javadoc. source:
Q:
My friend claim that garbage collectors do not collect int value since they are
not created with new() method.. Is he
right?
Answer: Programmers know about the importance
of initialization, but often forget the importance of cleanup. After all, who
needs to clean up an int? But with libraries, simply "letting go" of
an object once you're done with it is not always safe. Of course, Java has the
garbage collector to reclaim the memory of objects that are no longer used. Now
consider a very unusual case. Suppose
your object allocates "special" memory without using new. The garbage collector knows only how to
release memory allocated with new, so it won't know how to release the bject's
"special" memory. To handle this case, Java provides a method called
finalize( ) that you can define for your class. Here's how it's supposed to
work. When the garbage collector is ready to release the storage used for your
object, it will first call finalize( ), and only on the next garbage-collection
pass will it reclaim the object's memory. So if you choose to use finalize( ),
it gives
you the ability to perform some
important cleanup at the time of garbage collection.
Q: In my program where I use finalizers
to help GC to free the memory faster... But it seems that it is difficult to
know when it happens. I tried to find it and see that even on the same machine
my program runs differently...
Answer: You are right! Garbage collection
happens differently each time because it based not on definite schedule but
quite complicate alghorithms that take into consideration many factors
such as CPU load, number of variables,
memory size and so on. Even developers
of JVMs just guess when some process can start but not exactly. Since we have
many JVMs, Java programmers, therefore, should avoid writing code for which
program correctness depends upon the timely finalization of objects. For
example, if a finalizer of an unreferenced object releases a resource that is
needed again later by the program, the resource will not be made available
until after the garbage collector has run the object finalizer. If the program
needs the resource before the garbage collector has gotten around to finalizing
the unreferenced object, the program is out of luck.
Q: I do not understand the difference
between clone() and new(). Why shouldn't
I just call new()?
Can apply new() to cloned object?
Answer: The difference is that clone() creates
the object that possibly has changed fields (after start up) and this object is
willing to be cloned by implementation Cloneable interface. New() method can be applied without objects "permission" and create a
new instance with fields just initialized and not changed. They will be changed later during runtime.
Clone() "mirroring" this modified object. That's why you NEVER should call new() on
cloned object. You can destroy the clone...
Q: We need to authenticate a user on the
local Linux system before he/she/it can log on to the application and begin
taking over the world. Since the Linux system uses shadow passwords, we must
have a method to retrieve the password and authenticate the user in the user
shadow database. Once authenticated, this user can work with the
application. How it can be done in Java?
Answer: It can be done, it is already
done! Here if you are interested in
interfacing C with Java in Linux (the JNI Solution):
Advice: If you are C/C++ programmer be careful
with shortcut assignment operator "+=" in Java when you work with
Strings!
Why: The shortcut assignment operator +=
when used with Strings may confuse C and C++ programmers at first. Recall that
a += b is equivalent to a = a + b. Let's look at two code samples written in
C++ and the Java programming language:
//C++ code
string*
s1 = new string("hello");
string*
s2 = s1;
(*s1) += " world";
cout<<*s1<<endl<<*s2<<endl;
return
0;
//s1 = s2 = "hello world"
//Java programming language code
String s1 = "hello";
String s2 = s1;
s1 += " world";
System.out.println(s1 + "\n"
+ s2);
//s1 = "hello world" and s2 = "hello"
In the C++ example, the strings s1 and
s2 print the same result because they both point to the same address. In the
Java programming language, Strings can't be modified, so the + operator
must create a new String when
"world" is appended to s1.
Q: What is difference between:
String
My = "Test";
and
String
My = new String ("My"); ?
Answer: Because the compiler automatically
creates a new String object for every literal string it encounters, you can use
a literal string to initialize a String.
String s = "Test";
The above construct is equivalent to,
but more efficient than, this one, which ends up creating two Strings instead
of one:
String s = new
String("Test");
The compiler creates the first string
when it encounters the literal string "Test", and the second one when
it encounters new String.
Q: Why the String does not have a
capacity method?
Answer:
The String class doesn't have a capacity method, because a string cannot
be changed.
Q: Why can not I compile this simple
example?
public class Test{
int a;
final static int fin;
final int finint;
}
Answer: You can not compile it because you
have to initialize final variables. It must be done explicitly.
Q: Can someone point me to some resources
that explain how JSP and Servlets work together?
I am trying to develop a small project
(to learn JSP and Servlets) that is similar to a banking transaction where I
want to use JSP and Servlets.
I would ideally like to do something
like this.
1. Get some input from the user in a
HTML form.
2. Send this data to the database.
3. Get the reply from the database for
user queries.
4. Print the results to the user.
It was suggested that I use JSP to
display data and Servlets to interact with the browser. But I was not able to
find a concrete example. Any help regarding this would be greatly appreciated.
Answer: Here is how this stuff works,
generally:
1) The form (regular html) sends the
request to a servlet that does all of the processing and stores the results in
standard javabeans (not ejb) in the context that you want (either request
context or session context).
The things that the JSP is going to use
need to be properties of the javabeans that are accessible with the normal getters
and setters (i.e. street needs to be accessible by getStreet method
and city needs to be accessible with
getCity method). You can create as many of these beans as you need to
accomplish your purpose and store them in the context that makes sense for your
app.
2)
Get a request dispatcher from the ServletRequest using
getRequestDispatcher("/") and call its forward method to transfer
control to a JSP. If you have been
diligent in designing the beans, you will not need any java code inside the JSP
to write your page back to the browser.
If the JSP has form stuff in it, you can send that form data off to
another servlet and start the cycle over again.
This whole scheme has been enhanced for
internationalization and other stuff with the Struts framework (http://jakarta.apache.com). You will
be able to accomplish your modest
goals with the information above.
Q: Do I need to call Garbage Collector
gc() explicitly? If not why then does exist this method?
Answer: Do not afraid, if you do not call gc()
it will run anyway! You can be sure that GC-ing happens anyway... Why does SUN
provide us such method? I see two reasons at least for having it:
1.
Time critical applications. If you know that in some moment it is safe to run
GC call it. Probably it will run immediately. Not always. Anyway it can provide better distribution of
GC-ing in time increasing GC activity in "safe" time
2.
Test applications. It can help you to be sure that your objects will be
collected faster than usually.
Q: Does Garbage Collection hang my
program for a while?
Answer: Well, of course it somehow
"hungs" if you run your program on one CPU. Not in terms that it
hungs until some GC-ing is over. Usually
well written GC runs in own thread, alongside to your Java program and not more
time than any other thread. Your program
will not wait until some point is reached in GC but rather wait some amount of
time which the GC-ing thread allowed to take.
Q: I write my first applet and it become
very huge! It is an applet but looks like huge Java Application. Could you point me what is most important for
having a small applet?
Answer:
1. Use compiler optimization: javac -O
But check it the size anyway. Sometime it makes the code bigger.
2. Use jar files instead of class
files.
3. Try to use inheritance as much as
possible: than more code you can reuse than less new lines you have to add.
4. Try to use standard APIs. Often they
are better optimized in size than some private exotic packages. Of course often
they have better methods and so on but try to use efficiently what we have
already!
5. Use short names.
6. Do not initialize big arrays
because. They will be initialized and put directly into bytecode. You can do it
later on the fly.
Q: Main disadvantage of Java GUI is that
it is often slow. But I have seen also very fast working GUIs. Unfortunately
the code is hidden and I do not know the technique for writing of fast Java
GUIs.
Answer: I can describe one of main technique
you can use. It does not give you full solution, but will certainly speed up
your GUI. The main idea is to use
"lazy" initialization. Instead of creating and initializing of all
GUI components in constructors during start up time postpone it to later time
until you really need. Let say, you have a lot of tab panels with many elements
on each tab panel. Your constructors
should be "quite" empty and do not create and initialize those small
elements until your tab panel is chosen.
You should have very small constructor for tab panel itself and
additional lazy constructor for the rest. It should be called when user clicks
on that particular tab. This does not
decrease the full time of initialization, but spreads it up. The user will not
feel big delays during start up of your application. You should use these lazy
constructors just before you are going to call any paint method.
Q: Could you give me simplest example how
to do print in Java? I will work out it
myself :-)
Answer: Please compile and run it! It will
draw empty rectangle (you see I save your inks!)
import java.awt.*;
public class print {
public static void main(String args[])
{
Frame frm = new Frame("JavaFAQ_test");
frm.pack();
PrintJob printJob =
frm.getToolkit().getPrintJob(frm, "print", null);
if (printJob != null) {
Graphics grphcs = printJob.getGraphics();
grphcs.drawRect(50, 50, 150, 100);
grphcs.dispose();
printJob.end();
}
System.exit(0);
}
}
Question: Please describe shortly J2ME and what
is different comparing to J2SE and J2EE?
Answer: It will not be shortly :-) ...
J2ME is SUN's Java version for machines
with limited amount of memory (RAM 128 KB)and
low performance (comparing to desktop versions such as PIII)
processors. The main difference from
J2EE and J2SE is that it has a set of different profiles.
J2ME consist of three parts:
1. Java virtual machines* that fit inside the range of consumer devices.
2. APIs that are specialized for each type of device.
3. A profile, that is, a specification of the minimum set of APIs useful
for a particular kind of
consumer device (set-top, screenphone, wireless, car, and digital
assistant) and a
specification of the Java virtual machine functions required to support
those APIs. Each
profile is designed for specific hardware type - mobile phone, PDA,
microwave oven and so
on. Profile contains minimum libraries that are enough to support the
particular device. The
program is designed with profile for mobile phone does not contain many
classes that must
be used by oven oven, for example. Each profile uses JVM that uses some
subset of JVM
from J2SE and J2EE. Usually the program that you write for J2SE or J2EE
will not run on
J2ME JVM.
SUN already released two profiles:
1.
The Foundation Profile is a set of Java
APIs which, together with the Connected Device
Configuration
(CDC), provides a J2ME application runtime environment targeted at
next-generation, consumer electronic and embedded devices.
2.
The Mobile Information Device Profile
(MIDP) is a set of Java[tm] APIs which, together with the Connected, Limited
Device Configuration (CLDC), provides a complete J2ME application runtime
environment targeted at mobile information devices, such as cellular phones and
two-way pagers.
To be able to write a program with J2ME
you must use profile's implementaion - configuration. Profiles define only
specification.
SUN has two profiles now:
1.
Connected Device Configuration (CDC)
and C virtual machine CDC is a Java Community
Process effort that has standardized a portable,
full-featured Java[tm] 2 virtual machine
building block for
next-generation, consumer electronic and embedded devices. CDC runs on top of
the C Virtual Machine (CVM) that is provided as part of this release.
2.
Connected Limited Device Configuration
(CLDC) and K virtual machine. CLDC Java
Community
Process effort that has standardized a portable, minimum-footprint Java
building block for small,
resource-constrained devices. CLDC runs on top of Sun's K Virtual Machine (KVM)
that is provided as part of this
release.
Also SUN provides developers with J2ME
Wireless Toolkit The J2ME Wireless Toolkit is a set of tools that provides
developers with the emulation environment, documentation and examples needed to
develop CLDC/MIDP compliant applications.
Question: Can anybody tell me what an API is...
Answer: It stands for "Application
Programming Interface" and is used to mean the collection of packages,
classes, and methods that expose a proprietary implementation to an external
user.
That's a pretty good definition, but I
would qualify it a little bit.
"Proprietary" usually, in the
vernacular, means "tied to a particular vendor's tools".
A lot of APIs are not tied to any
particular vendor. One example is the C standard library. In that library,
there is a common definition and syntax for a function to print to a file:
int printf(const char *format, ...);
and that function is considered to be
part of the standard C API
and not tied to anything proprietary.
As well, any vendor can write a library
and sell it, and it'll have its API, and that will be considered
"proprietary."
An API can be either proprietary or
non-proprietary.
Now, this is a Java group, and the
standard Java API could be considered to be a proprietary Sun library. I'm
aware that Sun considered and rejected the idea of giving ownership to all
things
Java to the international standards
committees.
But for our intents and purposes, we
can call the standard Java library "non-proprietary", if only by
virtue of the fact that we can get implementations of it from IBM and other
sources.
Some might disagree with this, but
that's okay, I don't feel strongly about it, and I'm not a software contract
lawyer, thank goodness.
Question: I am quite new to JavaBeans and having
some difficulty with making threads work with it. I wrote a thread class and I want to use it
in a visual builder (specifically VisualAge). I want to drop the class as a
bean on the composition screen and then connect its output to other beans
and so on. Only problem is I don't know how to start
this thread
Answer: One way would be subclass the bean
into a runnable subclass that also inherits from Runnable, like so:
MyBean <--extends-- RunnableMyBean
---implements--> Runnable
You could then call the run() method in
the RunnableMyBean class by invoking RunnableMyBean.start().
Question: What are Thread Groups useful for? I
do not see big reason to have them in Java API...
Answer: From the beginning Thread Groups were
added for security reasons. Assumed, that not trusted code from third parties
will run in dedicated Thread Group. Of course it can not affect the
rest of program. Those threads from
external companies were not allowed to start, stop and suspend your threads.
Then this model is changed and we have no such protection now. Now the Thread Groups will let you a little
bit easier manage your threads. You can manage many threads simultaneously
rather than each separately. For example you can set maximum priority level,
call suspend(), interrupt() on all threads in your group. Also giving good
names will help you to easily track and debug your program.
Question: Could you give me an example how to
generate TCP/IP packets? I need to write my first network program.
Answer: There is no need to write such
program. Java hides this implementation from you. Just use java.net.Socket or
java.net.ServerSocket class and use appropriate constructor. Easy and fast! If you want even faster, use the method
setTcpNoDelay(true) :-)
Question: What is difference between Java
Compiler and Java Interpreter?
Answer: Java compiler is a program that
translates Java language code (you write program x.java) into the bytecode
(x.class) for the Java Virtual Machine.
Another program that actually implements the Java Virtual Machine
specification and runs bytecode is an interpreter program...
Question: Well, you say (see tip above) that
compiler is a program that translates source code (x.java) into bytecode
(x.class). What is JIT compiler? How can we have two compilers and where the
JIT compiler takes my source code (if I already compiled it)?
Answer: The name "JIT compiler" is
good enough to confuse people! Indeed,
what does it compile, if we have done the compilation after a program was
checked and compiled by javac, for example?
JIT is a part of the JVM and what it does is an optimization of java
bytecode to be able to run it much more efficiently on a current OS. Main
difference from javac compiler is that JIT does it on the fly and taking into
consideration much more things than javac.
Javac just compiles the code into bytecode and actual optimization
happens during a run time in JVM with help of JIT.
Question: I read it on
http://java.sun.com/docs/books/tutorial/java/javaOO/override.html: "Also, a subclass cannot override
methods that are declared static in the superclass. In other words, a subclass
cannot override a class method. A subclass can HIDE a static method in the superclass
by declaring a static method in the subclass with the same signature as the
static method in the superclass. "
My question: It looks like play with
words: override, hide.. Why is it written HIDE?
If I can write a static method in subclass with the same signature it
means that I override that method in superclass, I really changed that method
here in subclass. Am I wrong?
Answer: Let's start from the very beginning -
definition for overriding. It includes three main points that overridden method
must:
* have the same name
* have the same signature
* have the same data type.
In your case the static method in
subclass can have another data type.
Example:
************ Alex.java *************
package alex;
public class Alex {
static String al(String name){
return name;
}
}
******************** EOF ***********
************ John.java *************
package alex;
public class John extends Alex {
static int al(int name){
return name;
}
}
******************** EOF ***********
It compiles! You see that in Alex class
the method "al" returns String and in John class - int. It is hiding, not overriding. Advice: try to avoid such situation when you
hide methods in super class. Use different names instead and you will save a
lot of time.
Question: Constructors are similar to methods...
I know that they do not return any value. Could I say that they have void type
(no return type)?
Answer: Not. Constructors are not methods and
they are different because they:
* always have the same name as the class name
* have no return type, even void!
* are not inherited and that's why (by
the way) you can declare them final.
Question: I know that void method does not
return any value. But I still write the code like this:
void nothing() {};
void nothing2() {return;};
Why can we still use "return"
in the body of method?
Answer: To be able to exit method when it is
necessary. You can exit the method when
you reach the end of method and do not need to use "return". If due to some condition program must exit
the method then you use "return".
Question: Protected members in Java and C++.
What is difference?
Answer: C++ also has 3 levels of protection:
private, protected and public. They act
pretty much the same way. There are no packages in C++ and that’s why protected
members are accessible to subclasses only, not to whole package as in Java.
Question: I know that it is possible to create
threads by two different ways. What is essential difference? What is better and
when?
Answer: that's right! We can create a new
thread by subclassing Thread:
public class MyBestThread extends
Thread {...
Also we can do it by using Runnable
interface. The difference is that by implementing Runnable we change just run()
method keeping the possibility to subclass something more useful...
The difference between these two
methods is really small and often makes no sense.
