Introduction to Java Programs
There are two types of Java programs - applications and applets. An
application is a stand alone program whereas an applet is a program
embedded in a web page that runs on the client machine.
The Java compiler compiles the source code of an application to the
machine code of the Java Virtual Machine (JVM). The machine code of
the JVM is known as byte-code. The byte code can be interpreted or it
can be compiled by the JIT (just-in-time) compiler to machine code,
or it can be run on computer chips whose machine language is byte-code.
A compiler translates the source code of a program into machine language
of the computer. The compiler produces a file containing the optimized
executable code. But this executable code is not platform independent,
since different processors have different instruction set. Interpreters
on the other hand directly parse and execute line by line the source
code. The interpreter does not optimize the execution nor does it
produce an executable file. Interpretation is platform independent but
A program in Java consists of one or more class definitions, each of
which has been compiled into its own .class file of the JVM
byte code. One of these classes must define a method main()
which is where the program starts running.
If a file contains a public class called ClassName then it
must be saved in a text file called ClassName.java. JAVA IS
A language's syntax is the set of rules that determine whether a
particular statement is correctly formulated. Semantics is the meaning
of each statement.
- C style: /* */
- C++ style: //
- Documentation style: /** */
Other languages, like C or C++, use the ASCII character set. The ASCII
character set represents characters by a byte. Actually 7 bits of the
byte are used. So at most 128 characters can be represented. This is
sufficient to represent the letters of the English alphabet as well
as the decimal digits from 0 to 9, and all the punctuation marks. However,
if you have to represent letters of the alphabet of other languages,
a byte does not offer sufficient storage. Java from the beginning has
adopted the 16-bit Unicode system. The ASCII representation is a subset
of the Unicode system. To represent a character that from another
language one has to use the acutal Unicode. The convention used is
\uxxxx where xxxx are 4 hexadecimal numbers.
- An identifier must start with a letter, an underscore (_), or a
dollar sign ($).
- An identifier cannot be a reserved word.
- An identifier can be any sequence of letters, digits, underscore,
and dollar signs subject to the above constraints.
A variable marks a place in memory to store data. Every variable has
a type which tells the compiler how to interpret the string of bits
in that memory location. A variable has to be declared before using.
In Java there are 8 primitive types. The sizes of these types are fixed
for the Java Virtual Machine.
Octal integer literal begins with 0 and hexadecimal integer
literal begins with 0x or 0X.
- boolean: true or false
- byte: 8 bit
- char: 16 bit
- short: 16 bit
- int: 32 bit
- long: 64 bit
- float: 32 bit
- double: 64 bit
Declaring a Variable
The generic way of declaring a variable is
variable = expression;
int i = 0;
short x = 0xBF;
float area = length * width;
final datatype CONSTANTNAME = VALUE;
final double PI = 3.14159;
final int RATE = 6;
Constants have declared and initialized. You cannot change the
value of a constant.
A literal is a numeric value that is assigned to a variable like
2 or 5.8. Floating point numbers can be represented in the
scientific notation 5.43e+2 which is 543. The numeric operators
+, -, *, / can be applied to all integers and floating
point numbers. Integer division truncates the result. There are
two symbols used to handle overflow in case of floating point
- Addition: +
- Subtraction: -
- Multiplication: *
- Division: /
- Remainder: %
- NaN (not a number) e.g. 0.0 / 0.0
- ∞ (Infinity) e.g. 1.0 / 0.0
| x += 4 || x = x + 4 |
| x -= 4 || x = x - 4 |
| x *= 4 || x = x * 4 |
| x /= 4 || x = x / 4 |
| x %= 4 || x = x % 4 |
Increment and Decrement Operators
Increment and decrement operators can be applied to all
integers and floating point types. They can be used either prefix
(--x, ++x) or postfix (x--, x++) mode.
Prefix Increment Operation
int x = 2;
int y = ++x; // x == 3, y == 3
Postfix Increment Operation
int x = 2;
int y = x++; // x == 3, y == 2