From: elharo@sunsite.unc.edu Newsgroups: comp.lang.java.programmer,comp.lang.java.api,comp.lang.java.misc,comp.lang.java.tech,alt.www.hotjava Organization: Cafe Au Lait Followup-To: poster Subject: comp.lang.java FAQ Archive-name: computer-lang/java-faq/part1 Posting-Frequency: weekly Last-modified: 1997/03/18 Version: 1.4 URL: http://sunsite.unc.edu/javafaq/javafaq.html
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This document is Copyright 1995-1997 by Elliotte Rusty Harold. For information about reproduction, redistribution and other such rights please see the end of this file.
There are several new questions including:
abstract?
int to an Integer? a float to a Float?
for loops have the same syntax in all four languages);
but it is not based on any of those languages, nor have efforts
been made to make it compatible with them.Java is sometimes referred to as C++ ++ --. The language was originally created because C++ proved inadequate for certain tasks. Since the designers were not burdened with compatibility with existing languages, they were able to learn from the experience and mistakes of previous object-oriented languages. They added a few things C++ doesn't have like garbage collection and multithreading; and they threw away C++ features that had proven to be better in theory than in practice like multiple inheritance and operator overloading. (There's still argument over whether they made the right choices. I tend to think they were correct to throw out operator overloading and probably correct to throw out multiple inheritance. For now let's just say that neither choice is likely to be reviewed soon.)
Even more importantly Java was designed from the ground up to allow for secure execution of code across a network, even when the source of that code was untrusted and possibly malicious. This required the elimination of more features of C and C++. Most notably there are no pointers in Java. Java programs cannot (at least in theory) access arbitrary addresses in memory.
Furthermore Java was designed not only to be cross-platform in source form like C, but also in compiled binary form. Since this is frankly impossible across processor architectures, Java is compiled to an intermediate byte-code which is interpreted on the fly by the Java interpreter. Thus to port Java programs to a new platform all that is needed is a port of the interpreter and a few native code libraries.
Finally Java was designed to make it a lot easier to write bugfree code. Shipping C code has, on average, one bug per 55 lines of code. About half of these bugs are related to memory allocation and deallocation. Thus Java has a number of features to make bugs less common:
In 1989 Joy sold his Sun stock, invested heavily in Microsoft and moved out of mainstream Sun to Aspen, Colorado. By the early 90's Bill was getting tired of huge programs. He decided that he wanted to be able to write a 10,000 line program that made a difference. In late 1990 Bill wrote a paper called Further which outlined his pitch to Sun engineers that they should produce an object environment based on C++. Today Joy freely admits that C++ was too complicated and wasn't up to the job.
Around this time James Gosling (of emacs fame) had been working for several months on an SGML editor called "Imagination" using C++. The Oak language (now Java) grew out of Gosling's frustration with C++ on his "Imagination" project.
Patrick Naughton, then of Sun, now vice-president of technology at StarWave, started the Green Project on December 5th, 1990. Naughton defined the project as an effort to "do fewer things better". That December he recruited Gosling and Mike Sheridan to help start the project. Joy showed them his Further paper, and work began on graphics and user interface issues for several months in C.
In April of 1991 the Green Project (Naughton, Gosling and Sheridan) settled on smart consumer electronics as the delivery platform, and Gosling started working in earnest on Oak. Gosling wrote the original compiler in C; and Naughton, Gosling and Sheridan wrote the runtime-interpreter, also in C. (The Green project never wrote a single line of C++.) Oak was running its first programs in August of 1991. Joy got his first demos of the system that winter, when Gosling and Naugton went skiing at Joy's place in Aspen.
By the fall of 1992 "*7", a cross between a PDA and a remote control, was ready This was demoed to Scott McNealy, Sun's president, in October. He was blown away. Following that the Green Project was set up as First Person Inc., a wholly owned Sun subsidiary.
In early 1993 the Green team heard about a Time-Warner request for proposal for a settop box operating system. First Person quickly shifted focus from smart consumer electronics (which was proving to be more hype than reality) to the set-top box OS market, and placed a bid with Time-Warner.
Fortuitously, Sun lost the bid. The Time-Warner project went nowhere, the same place it probably would have gone if Sun had won the bid. First Person continued work on settop boxes until early 1994, when it concluded that like smart consumer electronics settop boxes were more hype than reality.
Without a market to be seen First Person was rolled back into Sun in 1994. However around this time it was realized that the requirements for smart consumer electronics and settop box software (small, platform independent secure reliable code) were the same requirements for the nascent web.
For a third time the project was redirected, this time at the web. A prototype browser called WebRunner was written by Patrick Naughton in one weekend of inspired hacking. After additional work by Naughton and Jonathan Payne this browser became HotJava. The rest, as they say, is history.
Information in this section is primarily based on the first hand accounts of Bill Joy and Patrick Naughton (which don't always agree). No doubt other people have still different memories of what occurred. If you've got any more first hand information about what went on in the Green project I'd like to hear from you.
At the lowest level the advantage of Java to the web is that it provides a secure, cross-platform way for code to be executed. At a somewhat higher level Java adds several features to existing web sites:
An IRIX port has mostly been completed by Simon Leinen. SGI's own port is available at http://www.sgi.com/Fun/Free_webtools.html
A Linux port is in fairly good shape. See http://java.blackdown.org/java-linux.html.
IBM has ported Java to Windows 3.1, OS/2, and AIX. It is working on ports for MVS and OS/400. See http://ncc.hursley.ibm.com/javainfo/hurindex.html .
The OSF has ported Java to Unixware, the Bull Estrella or other PowerPC running AIX4.1, X86 running DASCOM OSF/1 , the Digital Alpha running Digital UNIX 3.2 , the HP700 series running HPUX 10.x, the NCR Globalyst (Pentium) running UNIX SysV, and Sony NEWS (MIPS) running Sony NEWS 6.1.1. See http://www.osf.org/mall/web/JDK/.
Other ports are underway for Nextstep, SunOS 4.1, the Amiga and possibly other platforms.
In the past new versions of Java have most often been made available first for Solaris. If you have to have the latest version as soon as it's released, or if you're developing "Bet your company" applications with Java, you should probably be running Solaris on a SparcStation. Otherwise, if you just want to learn the language, you can get away with an Intel based Windows 95 or NT machine with a lot of RAM.
Netscape 2.0 and later plays Java applets on Windows NT, Windows 95, Solaris, SunOS 4.1, Linux and most other Unix platforms. Netscape 3.0 for the Mac also supports Java. Netscape 2.0 can even compile Java programs. (Netscape 3.0 cannot).
First download the classes.zip file from the Solaris distribution of the JDK. (You have to download the entire JDK. You can't get just the classes.zip file.) Next set your CLASSPATH environment variable to include the classes.zip file. For example, on Unix, if you put the file in /usr/local/lib/classes.zip, then you would set it like this:
sh: % CLASSPATH=$CLASSPATH:/usr/local/lib/classes.zip csh: % setenv CLASSPATH "$CLASSPATH:/usr/local/lib/classes.zip"You'll probably want to put this line in your .login or .cshrc file. Of course you'll need to adjust this to match where you've installed these files and to add any other classes you use. Finally you compile programs from the command line like this:
% netscape -java sun.tools.javac.Main HelloWorld.javaYou can always alias netscape -java sun.tools.javac.Main to just javac to make this more transparent.
You can also run command-line programs that don't use the AWT in the same way, like this:
% netscape -java HelloWorld.classYou can't use netscape -java to run programs that use the AWT, but you can of course play applets in Netscape..
A little further out in left field, it isn't even necessary to know Java to write Java programs. Intermetrics is beta testing an ADA-95 to Java byte code compiler. Other such cross-compilers are probably possible including ANSI Fortran-77. However the one most people probably want, that is a C/C++ to Java byte code compiler, is probably not possible due to Java's lack of pointers.
Java and JavaScript are about as closely related as the Trump Taj Mahal in Atlantic City is to the Taj Mahal in India. In other words Java and JavaScript both have the word Java in their names. JavaScript is a programming language from Netscape which is incorporated in their browsers. It is superficially similar to Java in the same way C is similar to Java but differs in all important respects.
main() method. An applet is a
Java class which extends java.applet.Applet. A class which extends
java.applet.Applet and also has a main() method is both an application
and an applet.More generally and less technically an application is a stand-alone program, normally launched from the command line, and which has more or less unrestricted access to the host system. An applet is a program which is run in the context of an applet viewer or web browser, and which has strictly limited access to the host system. For instance an applet can normally not read or write files on the host system whereas an application normally can.
The actions of both applets and applications, however, can be controlled by SecurityManager objects. If you can change the SecurityManager that's used you can change what an applet or an application is and is not allowed to do. Thus these are not hard and fast differences, though this is normally how they separate out in practice.
For many more web sites see the Cafe Au Lait links page at http://sunsite.unc.edu/javafaq/links.html.
The second book I recommend is
Java in a Nutshell: A Desktop Quick Reference for Java
Programmers
by
For people with no prior experience in programming I recommend Kris Jamsa's Java Now!, ISBN: 1-884133-30-4, $16.95, Jamsa Press.
Finally The Java Handbook
by
None of these books cover Java 1.1. In fact as of this writing, no book truly covers Java 1.1 though several claim to.
For details about these and many more books see the Cafe Au Lait books page at http://sunsite.unc.edu/javafaq/books.html
int) Java specifies
everything.
Here are the detailed data types:
boolean
true and false only.
true and false are defined constants of the language
and are not the same as True and False,
TRUE and FALSE, zero and nonzero, 1 and 0
or any other numeric value. Booleans may not be cast into any other
type of variable nor may any other variable be cast into a boolean.
byte
short
int
long
float
Like all numeric types floats may be cast into other numeric types (byte,
short, long, int, double).
When lossy casts to integer types are done (e.g.
float to short) the fractional part is truncated and the
conversion is done modulo the length of the smaller type.
double
char
Chars are not the same as bytes, ints, shorts or Strings.
sizeof isn't necessary in Java because all sizes are precisely
defined. i.e. an int is always 4 bytes.
This may not seem to be adequate when dealing with objects that
aren't base data types. However even if you did know the size of
a particular object, you couldn't do anything with it anyway.
You cannot convert an arbitrary object into bytes
and back again.
if, else, for,
while, do while and switch
statements. The syntax is identical to C's.
However all condition tests must return boolean values. Since
non-boolean assignment statements
and arithmetic statements do not return a boolean value,
some of the more obfuscated condition tests in C are prohibited.
argv
has become a string array commonly called args and
args[0] is the first command line argument, not the
name of the program. The other arguments are all shifted one to the
left from where they'd be in C or C++.
/* This is a comment */ comment
from C and the // This is a C++ comment
However comments that begin with a /** are special.
These comments should only be used before
a method or class declaration. They indicate that the comment should be included in
automatically generated documentation for that declaration.
Superclasses of a class are indicated with the extends
keyword rather than with a :.
Methods must be defined inside the class to which they belong. They may not be declared inside the class and defined outside the class as is common in C++.
Features removed that make Java easier to read and understand than
C++ include #define, typedef, operator
overloading, enum, unions and structs.
The main feature removed to make Java safer and more robust than C++ is pointer arithmetic.
Other features removed include global variables, standalone functions (everything is a method), friend functions (Everything in a package is a friend of everything else in the package.) and non-virtual functions.
A number of features have been added to Java to make it safer including true arrays with bounds checking, garbage collection, concurrency, interfaces (from Objective C) and packages. There is no need to explicitly allocate or free memory in Java.
Vector class in
java.util. It can do anything
a linked list can do and a little more and saves you a lot of coding
which, after all, is the point of OOP and the class library. However
it is array based so insertions or deletions from the middle of a
Vector are not as efficient as with a true linked list.Long answer: Object variables in Java are all references. A reference acts like a pointer in most other languages. (Though they're handles, not pointers, in most Java implementations. The notable exception is Microsoft's.) The main difference is that you can't do pointer arithmetic on references. Therefore wherever you'd use a pointer to an object in C++, in Java you should just use the object itself.
On the other hand the primitive data types (int, float,
double, char, byte, short, long and boolean) are
not references. If you want to get a reference to one of these you
need to wrap it in a class first. Java provides ready-made
type-wrapper classes in the java.lang package for Boolean,
Character, Integer, Double,
Float, and Long. Bytes and shorts can be
stored in the Integer class as well. Java 1.1 will add Byte,
Short, and Void classes
too.
scanf(), fscanf()
and sscanf() functions, Pascal's read() and readln() function,
or Fortran's READ* function. In particular there's no one method
that lets you get input from the user as a numeric value.
However, roughly equivalent functionality is scattered across several classes.
You first read an input line into a String
using DataInputStream.readline() or
BufferedReader (in Java 1.1)
Next use the
StringTokenizer
class in java.util to split the String into tokens.
By default StringTokenizer splits on white space (spaces,tabs, carriage returns and newlines),
but this is user definable.
For example,
import java.util.StringTokenizer;
class STTest {
public static void main(String args[]) {
String s = "9 23 45.4 56.7";
StringTokenizer st = new StringTokenizer(s);
while (st.hasMoreTokens()) {
System.out.println(st.nextToken());
}
}
}
9 23 45.4 56.7Finally you convert these tokens into numbers using the type wrapper classes as described in the next question.
Integer, Float,
Double and Long
type wrapper classes as indicated by the following code snippet:
class ConvertTest {
public static void main (String args[]) {
String str;
str = "25";
int i = Integer.valueOf(str).intValue();
System.out.println(i);
long l = Long.valueOf(str).longValue();
System.out.println(l);
str = "25.6";
float f = Float.valueOf(str).floatValue();
System.out.println(f);
double d = Double.valueOf(str).doubleValue();
System.out.println(d);
}
}Short and Byte classes
in Java 1.0. There are in Java 1.1.
For shorts and bytes use the Integer class
but use the byteValue() or shortValue() methods instead.
system() call (or your OS's equivalent) to execute the java
interpreter with appropriate command line arguments.In Java 1.1 the Java Native Method Interface in combination with the Invocation API allows native applications to load and access the Java virtual machine.
In Java 1.1 a serialization interface has been added to the language.
However only objects that explicitly implement the java.io.Serializable
interface can be serialized.
Instead you should declare an interface which declares the function you want to pass; for example
public interface Comparable {
public abstract int compare(Object o1, Object o2);
}
public void sort(Object[] o, Comparable c) {
boolean done = false;
while (!done) {
done = true;
for (int i = 0; i < o.length - 1; i++) {
if (c.compare(o[i], o[i+1]) < 0) {
swap(o[i], o[i+1]);
done = false;
}
}
}
Comparable. For example,
public class StringCompare implements Comparable {
public int compare(Object o1, Object o2) {
String s1 = (String) o1;
String s2 = (String) o2;
return s1.compareTo(s2);
}
}
Object is a superclass for all object types.
An applet can also get references to all other applets on the same
page using the getApplets() method of java.applet.AppletContext. Once
you've got a reference to an applet,
you can communicate with it by
using its public members.
It is conceivable to have applets in different virtual machines that talk to a server somewhere on the Internet and store any data that needs to be serialized there. Then, when another applet needs this data, it could connect to this same server. Implementing this is non-trivial.
A number of organizations have developed special http servers that allow applets or other clients to run Java programs on the server in a secure environment. Most notably Sun's Jeeves implements a servlet interface for this purpose. The W3C's JigSaw implements a similar idea called resource objects.
final class can no longer be subclassed. Mostly this is done for
security reasons with basic classes like String and Integer.
It also allows the compiler to make some optimizations,
and makes thread safety a little easier to achieve.
Methods may be declared final as well. This means they
may not be overridden in a subclass.
Fileds can be declared final, too.
However, this has a completely different meaning.
A final field cannot be changed after it's initialized,
and it must include an initializer statement where it's declared.
For example,
public final double c = 2.998;
It's also possible to make a static
field final to get the effect
of C++'s const statement or
some uses of C's #define, e.g.
public static final double c = 2.998;
Static variables and methods are instantiated only once per class.
In other words they are class variables, not instance
variables. If you change the value of a static variable in a
particular object, the value of that variable changes for all
instances of that class.
Static methods can be referenced with the name of the class rather
than the name of a particular object of the class (though that works
too). That's how library methods like
System.out.println() work.
abstract?abstract keyword like this:
public abstract class Container extends Component {
Abstract classes may contain abstract methods.
A method declared abstract is not actually implemented in the current
class. It exists only to be overridden in subclasses. It has no body.
For example,
public abstract float price();
Abstract methods may only be included in abstract classes. However, an abstract class is not required to have any abstract methods, though most of them do.
Each subclass of an abstract class must override the abstract methods
of its superclasses or itself be declared abstract.
For more details, see section 8.1.2.1 of the Java Language Specification.
For example suppose you're writing an inventory database.
The inventory may include many different items of many different
types and classes. However each item in the warehouse needs to be able to tell
you its price. Normally you would implement this by having each class
extend a common superclass. However that's not always convenient.
Instead you can declare an interface called
Price with a price() method
like this:
public interface Price {
public float price();
}
Price
interface must contain a method with the signature
public float price(). The code of the
price() method is included separately in each
separate class which implements Price,
not in the Price interface itself.
Different classes in your warehouse can each implement
the Price interface like this:
public class Monopoly extends BoardGame implements Price {
// other methods
public float price() {
return 14.95;
}
}
Price with the
instanceof operator. For example,
if (o instanceof Price) System.out.println("Subtotal is " + o.price());
In fact, interfaces can be used to tag objects. The java.rmi.Remote interface declares no
methods. Its sole purpose is to indicate that an object is a remote object. In
general, sub-interfaces of java.rmi.Remote will declare remote methods, however.
For example,
public interface Hello extends java.rmi.Remote {
public String sayHello();
}
public class HelloImpl extends UnicastRemoteServer implements Hello {
public String sayHello() {
return "Hello";
}
}
java.rmi package, see
http://chatsubo.javasoft.com/current/rmi/index.html
or Chapter 14 of my book, Network Programming in Java, from O'Reilly & Associates
(available early 1997).
Extensions are planned for the class library though. In particular Sun is working on extensions for 3D, multimedia, telephony, and improved graphics.
int to an Integer? a float to a Float?int or float to an
object such as an Integer or a Float.
However the proper way to do this isn't very
hard. Instead do
int x = 5;
myInteger = new Integer(x);
print().
Note that some of the classes like DataOutputStream have unoverloaded
methods like writeInt() and writeByte().
Java does have references. A reference is an abstract identifier for an object. It is not a pointer. A reference tags a particular object with a name in the Java virtual machine so that the programmer may refer to it. How exactly the virtual machine implements references at the level of machine code is VM-dependent and completely hidden from the programmer in any case. Most VMs including Sun's use handles, not pointers. A handle is a pointer to a pointer. At the level of machine code in the CPU a reference is an address in memory where the address of the object is stored. This way the objects can be moved around in memory and only the master pointer needs to be updated rather than all references to the object. This is completely hidden from the Java programmer, though. Only the implementer of the virtual machine needs to worry about it. Indeed, this is not the only way references can be implemented. Microsoft's VM actually does use pointers rather than handles. Other schemes are possible.
import java.awt.Point;
class changePoint {
public static void main(String args[]) {
Point p1 = new Point(0, 0);
changePoint(p1);
System.out.println(p1);
}
static void changePoint(Point p) {
p.x = 38;
p.y = 97;
}
}
java.awt.Point[x=38,y=97]Therefore the point has been changed. However the reference, which is what was really passed, has not been changed. To see that consider the following program.
import java.awt.Point;
class dontChangePoint {
public static void main(String args[]) {
Point p1 = new Point(0, 0);
dontChangePoint(p1);
System.out.println(p1);
}
static void dontChangePoint(Point p) {
p = new Point(38, 97);
}
}
java.awt.Point[x=0,y=0]What happened in this example was that a copy of the reference
p1
was passed to the dontChangePoint() method. A new Point object was
then assigned to that copy. However this did not change the old
reference in the main method. In the previous example the reference
p in the changePoint() method and p1 in
the main() method both referred
to the same object. In this example p and p1 refer to different
objects after the new Point is assigned to p.
PERFORM
or BECOME in Java 1.0. These would probably open
security holes. There are workarounds for some things you might want
to do using interfaces.
In Java 1.1 the Core
Reflection API and the java.lang.reflect package provides most of the functionality you need.
However this API mostly unavailable to applets due to security issues.
Components are subclasses of java.awt.Component. Examples of
components include buttons, scrollbars, textfields, frames, windows,
dialogs, panels, canvases, and checkboxes. One subclass of
java.awt.Component is java.awt.Container. A container is a
component which can hold other components. Examples of containers
include Frames, Windows, Dialogs, Panels and Applets. An applet is a
subclass of Panel. Panel is a subclass of
Container. Container is a
subclass of Component. Therefore an applet is both a container and a
component.
When the user clicks the mouse, types on the keyboard, drags and drops, or does any of a few other things, the operating system produces an event. This event is passed to Java, and the Java runtime tries to figure out which component the event was intended for.
The Java runtime then passes that event to the
handleEvent(Event e) method of some Component. The
Component's handleEvent() method contains a big if-else statement to
look at the type of event and respond appropriately. What the
handleEvent() method does depends on the type of component. Generally
some events are ignored and other events are passed to methods that
know how to respond to those events. For instance a MOUSE_DOWN event
is passed to mouseDown(). The table below shows the events the
default handleEvent() can deal with. When one of these events
occurs, handleEvent() passes it and various other information to the
specified method.
Event Method Called -------------------------------------------------------- Event.MOUSE_ENTER: mouseEnter(evt, evt.x, evt.y) Event.MOUSE_EXIT: mouseExit(evt, evt.x, evt.y) Event.MOUSE_MOVE: mouseMove(evt, evt.x, evt.y) Event.MOUSE_DOWN: mouseDown(evt, evt.x, evt.y) Event.MOUSE_DRAG: mouseDrag(evt, evt.x, evt.y) Event.MOUSE_UP: mouseUp(evt, evt.x, evt.y) Event.KEY_PRESS: keyDown(evt, evt.key) Event.KEY_ACTION: keyDown(evt, evt.key) Event.KEY_RELEASE: keyUp(evt, evt.key) Event.KEY_ACTION_RELEASE: keyUp(evt, evt.key) Event.ACTION_EVENT: action(evt, evt.arg) Event.GOT_FOCUS: gotFocus(evt, evt.arg) Event.LOST_FOCUS: lostFocus(evt, evt.arg)By default each of these methods does nothing. Furthermore, by default all other events do nothing. If you want to do something when one of the above events occurs in your component, subclass the component and override the appropriate method. For example, to create a
Canvas that prints the message "Don't Tread on Me!" every
time the user clicks on it, use the following subclass of Button:
public class noTread extends Canvas {
public boolean mouseDown(Event e, int x, int y) {
System.out.println("Don't Tread on Me!");
return true;
}
}Button, Choice, TextField, and Checkbox are special because of how
they're implemented as native widgets. They do not see MOUSE_DOWN,
MOUSE_UP, KEY_PRESS, and other such events. Rather they receive one
event and one event only: Event.ACTION_EVENT. When a Button is
pressed, the user hits return in a TextField, the user double clicks on a List item,
clicks a Checkbox, or
selects from a Choice, an ACTION_EVENT is generated. If you haven't
overridden handleEvent(), this is passed to the action() method of the
component's container. These four components are the only things
which generate ACTION_EVENTs. No other component will generate an
ACTION_EVENT.
Each event handler method, including handleEvent(), returns true if
the event was completely handled and false if it was not. If false
is returned, then the event is passed to the component's container
to see if the container wants to process the event.
If you want to respond to a different set of events, you must
override handleEvent(). You can either completely replace it or just
add the extra events you want to handle and then return
super.handleEvent() for other types of events. For example to print
all the events as they happen in your applet you might use this
handleEvent() method:
public boolean handleEvent(Event e, int x, int y) {
System.out.println(e);
return super.handleEvent(e, x, y);
}
mouseDown(), mouseUp(), keyDown(), and all the other event methods would
still be called by the superclass. If you don't handle an event in
the handleEvent() method, return super.handleEvent(e) to give your
superclass an opportunity to process the event. In this case, we've
handled the event but not completely so we still let the superclass
process it by returning super.handleEvent(e, int x, int y).
The return value of handleEvent() is important. If handleEvent() returns
true, that means the event has been fully digested and no further
processing is needed. On the other hand if handleEvent() returns
false, then the event is passed to the component's container to be
handled.
Note that this all works for any component at all. You'll most commonly override these methods in a subclass of Applet or perhaps Frame. However the same methods and techniques apply to all subclasses of Component.
The API documentation confuses the issue because of the ambigious use of the word parent. In object-oriented circles the word parent usually refers to the superclass. However in the context of the AWT, the word parent refers to the container that contains a component. Thus in the statement:
returnthe word parent means the component's container, not its superclass.trueif the event has been handled and no further action is necessary;falseif the event is to be given to the component's parent
You must distinguish between the container/component
hierarchy and the inheritance hierarchy. By default, the way an event
travels has nothing to do with the inheritance
hieararchy. If you override an event handling method, then events
passed to that method will not be passed to a component's superclass unless
you specifically ask them to be. However, in most cases your handleEvent()
method will return super.handleEvent() to give the superclass a crack at
handling the event rather than simply returning false.
To sum up, when you write a subclass of Component, you can either
respond to specific events by overriding the methods that correspond
to those events, such as mouseDown(); or you can respond to all
events by overriding handleEvent(). Each of your event handling
methods must return a boolean. Return true when an event has been
completely processed, and return false when you want the component's
container to try to handle the event. When overriding handleEvent(),
you also have an option to return super.handleEvent() to ask the
superclass to handle the event and decide whether or not to pass it
along to the component's container.
Instead if you need to do clipping,
create separate offscreen Images
for each clipping region.
Each Image should be the size of the clipping region you desire. Draw into
those offscreen images, and then copy them onto the appropriate
section of the of your applet window using java.awt.Graphics.drawImage().
Some coordinate conversion will
almost certainly be necessary.
If the background image isn't a simple color then you'll first need to
copy the appropriate part of that image to your offscreen clipping
Image. You can do this by drawing your background Image into your
offscreen region with Graphics.drawImage() and a suitable shifting of
coordinates.
This all works for rectangular regions only since all Images are rectangular. More complicated geometries can be faked if all but one section contains only simple colors.
public void paint(Graphics g) {
int x1=5;
int x2=278;
int y1=8;
int y2=93;
// Draw a ten pixel thick line
for (int i = -5; i < 5; i++) {
g.drawLine(x1+i,y1+i,x2+i,y2+i);
}
}
There are other hacks you can use. For example, a thick line is essentially
a filled rectangle. Therefore you can calculate the endpoints of the
rectangle and use fillPolygon() to draw it.
The real solution is going to have to wait for a more complete graphics API for Java, possibly in Java 1.2.
String
array of the names of the available fonts by calling the getFontList() method
from java.awt.Toolkit. For example,
String[] fonts = Toolkit.getDefaultToolkit().getFontList()
FileInputStream(String) constructor and use one of
the various read() methods to read the data into an array of
bytes. For example the following program reads raw data
from a file specified on the command line. It then writes
the same data to the standard output.
import java.io.*;
class ReadRawData {
public static void main (String args[]) {
boolean done = false;
byte b[] = new byte[1024];
int num_bytes = 0;
FileInputStream fin = null;
try {
fin = new FileInputStream(args[0]);
}
catch(ArrayIndexOutOfBoundsException e) {
System.out.println("You have to give me the name of a file to open.");
System.exit(0);
}
catch (FileNotFoundException e) {
System.out.println("Could not open input file " + args[0]);
System.exit(0);
}
catch(IOException e) {
System.out.println("Error while opening input file" + args[0]);
System.exit(0);
}
catch (Exception e) {
System.out.println("Unexpected exception: " + e);
System.exit(0);
}
try {
num_bytes = fin.read(b);
}
catch(IOException e) {
System.out.println("Finished Reading: " + e);
done = true;
}
catch (Exception e) {
System.out.println("Unexpected exception: " + e);
System.exit(0);
}
while(!done) {
System.out.write(b, 0, num_bytes);
try {
num_bytes = fin.read(b);
}
catch(IOException e) {
System.out.println("Finished Reading: " + e);
done = true;
}
catch (Exception e) {
System.out.println("Unexpected exception: " + e);
System.exit(0);
}
if (num_bytes == -1) done = true;
} // end while
} // end main
} // end ReadRawDataDataInputStream which gives you a
readLine() method that returns successive lines of the file
as Java Strings. You can then process each String as you
see fit.
// Implement the Unix cat utility in java
import java.io.*;
class cat {
public static void main (String args[]) {
String thisLine;
//Loop across the arguments
for (int i=0; i < args.length; i++) {
//Open the file for reading
try {
FileInputStream fin = new FileInputStream(args[i]);
// now turn the FileInputStream into a TextFileStream
try {
DataInputStream myInput = new DataInputStream(fin);
try {
while ((thisLine = myInput.readLine()) != null) { // while loop begins here
System.out.println(thisLine);
} // while loop ends here
}
catch (Exception e) {
System.out.println("Error: " + e);
}
} // end try
catch (Exception e) {
System.out.println("Error: " + e);
}
} // end try
catch (Exception e) {
System.out.println("failed to open file " + args[i]);
System.out.println("Error: " + e);
}
} // for ends here
} // main ends here
}
In Java 1.1 DataInputStream.readLine() is deprecated.
You should use a BufferedReader instead.
Within an application, however, file access is straight-forward. There are several ways but here is a simple example using formatted output streams:
import java.io.*;
class PrintToAFile {
public static void main (String args[]) {
//First open the file you want to write into
try {
FileOutputStream fout = new FileOutputStream("test.out");
// now convert the FileOutputStream into a PrintStream
PrintStream myOutput = new PrintStream(fout);
// Now you're able to use println statements just as if you were using System.out.println
// to write to the terminal
myOutput.println("Hello There!");
myOutput.println(1 + " + " + 1 + " = " + (1+1));
}
catch (IOException e) {
System.out.println("Error opening file: " + e);
System.exit(1);
}
} // main ends here
}
You should also learn about the DataOutputStream class and the
write() method when you get a chance.
Da