int
is a built-in type (some classes are predefined in Java).
struct
you would write in a C program.
class Fruit {
// fields (data members)
int grams;
int cals_per_gram;
//a method (i.e. member function)
int total_calories()
{
return grams * cals_per_gram;
}
}
struct in C).
malloc a pointer to
the struct in C.
Fruit is a class.
Fruit apple;
apple is called an instance of the
class Fruit, or an object.
int i;
i = 0; i++; // all fine
apple above, does not give you an
object of class type Fruit.
apple that can reference
an object of Fruit - after you have filled in the reference so
it points to such an object.
Fruit object for apple or
you can make it point to an existing object.
apple = new Fruit();
Fruit apple = new Fruit();
classname (parameterlist) { ... }
void.
class Fruit {
int grams;
int cals_per_gram;
// constructor
Fruit ()
{
grams = 55;
cals_per_gram = 0;
}
// another constructor
Fruit (int g, int c)
{
grams = g;
cals_per_gram = c;
}
}
...
Fruit melon = new Fruit(4,5), banana = new Fruit();
/opt/jdk, but CUNIX has had softlinks set to go
through /opt/local.
/opt/local/bin.
CLASSPATH=/opt/local/lib/classes.zip:.
.class files it needs.
.class files
in them, and also zip files containing classes.
CLASSPATH=/usr/lib/java/classes.zip:/home/vicky/Java/classes:.
.java
files into .class files.
% javac [options] filename.java
-classpath path
This overrides the CLASSPATH enviornment variable setting with
path
-directory d
Specifies the root directory of the class file hierarchy. In other words,
this is essentially a destination directory for compiled classes. If you leave
this unspecified, the .class file will end up next to the
.java file.
java. To start a stand
alone application with it, you specify the initial class to be loaded (a
.class file).
% java [interpreter options] classname [program arguments]
main method.
.class
at the end of it.
% java HelloWorld
import java.lang.*;
public class HelloWorld {
public static void main (String args[]) {
System.out.println("Hello, World!");
}
}
import - like #include in C. This specifies
which packages and classes this program is using. This line means "include
all classes from the java.lang package." All java.lang classes are already
automatically included, so this line can be omitted.
class - everything is a class in Java, even the simplest
program.
public - this class is visible to all other classes in the
VM, and so is the main method.
void - Java has a void type just like C/C++.
main - a class needs a main() method if it is to
be a runnable program.
String - Java has a built-in, first-class support for
strings.
args - like argc, argv constructs in C; except
we only need an array of Strings as the incoming command-line
arguments. No need for argc because you can easily get any
array's length (in this case args.length).
HelloWorld.java.
% javac HelloWorld.java
HelloWorld.class in your
directory.
% java HelloWorld
.class at the end of
HelloWorld.
public class with
a public static void main (String args[]) method in it. It needs
that main method as a starting point, like a C programs main
function.
main method when you call
java.
args, but it should
be.
args[0] is the first parameter, not the program
name (like in C).
java interpreter.
Type Definition
boolean true or false
char 16-bit Unicode character
byte 8-bit signed integer
short 16-bit signed integer
int 32-bit signed integer
long 64-bit signed integer
float 32-bit IEEE 754 floating-point value
double 64-bit IEEE 754 floating-point value
0,
booleans are false, chars are set to '\0', etc.
public class Circle {
int radius; // automatically initialized to 0
public setRadius (int x)
{
radius = x;
}
}
public int returnOneMoreThan (int x)
{
int y = 0; // must be initialized
y = x + 1;
return y;
}
Just a couple of words about these books. "Java - How to Program" is a comprehensive book on how to program in Java. "Just Java" is only recommended for those of you who are already familiar with C or C++. It is at a level that allows you to pick up Java quickly, but it lacks some in depth details and assumes some prior Object Oriented programming experience.
This is a good book to use as a Java reference (you should always have a reference book at hand to remember syntax and library functions). It is too in depth to be used to learn Java from scratch.