Java tokens, variables, constants

Java tokens are the atomic elements of Java. Java program are a collection of Java tokens separated by white spaces and comments.

Java is a free form language so it is not must to follow any special indentation rule. The only rule, which needs to be considered, is that the java tokens must be separated by an operator or by a separator or  by at least one white space character. In java, white space is a space, tab, or new line.

More than one white space characters may also appear wherever one white space character is allowed.

Java tokens can be classified into following categories:

• Keywords
• Identifiers
• Literals
• Operators
• Separators

Keywords


There are appox. 50 reserved keywords in java. These keywords can not be used as identifiers. All keywords are in lowercase like C/C++.

The keywords const and goto are reserved but are not being used as of now. They  are reserved for the future use.

A new keyword assert was added by jdk 1.4.

In addition to the keywords, Java reserves the following:

true, false and null

These are the literal/constant values defined by Java, you may not use these keywords as identifiers.


Identifiers

An identifier is a word used in a program to name a local variable, method, class, interface, data members, package name etc. Java identifiers are case sensitive.

Name of the identifiers must follow certain rules:

• A Java identifier must begin with a letter, dollar sign or underscore.
• The subsequent characters may be digits also.
• There is no length restriction on the identifiers i.e. they can be of any length.
• Java allows Unicode characters in identifiers.

It is suggested that following naming convention may be followed while naming java identifiers:

• Name of all the classes and interfaces start with a leading uppercase letter and each subsequent word also starts with a leading uppercase letter. Rest of the letters must be in lower case. Example: Student, Hello, HelloWorld, EmployeeInfo etc.
• Names of all the public data members and methods start with a leading lowercase character. When more than one words are used in a name, the second and subsequent words start with a leading uppercase letter. Example: annualSalary, grossSalary, calculateSalary etc.
• Names of variables that represent constant values use all uppercase letters and underscores between words.

Literals/Constants

A java literal represents a constant value in java. A literal or constant is a value specified in the source as opposed to one determined at run time. Literals may appear on the right side  in an assignment, or may be passed as arguments in method calls. Literals represent constant values so you can not assign a value to any literal i.e. they can not appear on the left side in an assignment.

Literals represent integer (byte, short, int, long) constants, floating point (float, double), constants, Character (char) constants and Boolean (boolean) constants in Java. Java also supports a literal null, which represents a null reference.

Examples:

Integer Literals: 200, -600, 0x1A, 015, 564L

Floating point Literals: 6.89, -3.15, 2.45f, 3.45d, 1.34E-01, 4.56E11

Character Literals: 'x', 'X', '\101', '\n'

Boolean Literals: true, false

String: "Hello", "abc", "xyz"

Java source file structure

All java source files must end with the extension ".java". A source file may contain at the most one top level public class. If a public class is present, the un-extended file name should be same as the class name i.e. if the class name is Hello then the source file name should be Hello.java. If a source file does not contain any public class then name of the source file can be anything.

A source file may contain an unlimited number of non-public class definitions.

There are four top level elements that may appear in a file. None of these elements is must. If they are present, then they must appear in the following order:

1. Package definitions
2. import statements
3. class and/or interface definitions

A source file may have at the most one package statement. If a package statement is present it must be the first statement in the java program. Only comments may appear before the package statements.

A source file may have zero or more import statements. As of now you can think of import statements to be like #include statements in the Java Program. If present, all the import statements must come after the package statement and before the class/interface definitions.

A source file may have any number of class and/or interface definitions but there can be at the most one public class or interface.

main() method

A source file may or may not contain a class having main() method but a standalone java program always starts its execution from main() just like a C/C++ program. So the class from which we want to start the execution  must have the main() defined in it.

The main() method must always have the following signature:

public static void main(String args[])

Note: Method prototype is referred to as signature in Java.

The void keyword preceding main() method indicates that it does not return any value. The keyword public indicates that the main method can be accessed from anywhere. The main() method must be declared public as it is called by the code, which is part of the JVM and is outside the class containing the main() method.

The main() method takes one argument, which is the array of strings. Each element of the array represents one command line argument.

The main() method can also be invoked like any other static method form anywhere in the java program. It can also be overloaded like any other Java method.

It is very common to get an exception indicating that the main() method is not present while running a java program event when the main() method is present. This happens when the signature of the main() method differs from the above mentioned signature.

Comments

A program can be documented by inserting comments wherever required. The comments are simply ignored by the compiler.

Java provides three types of comments for documentation:

1. Single line comment
2. Multi line comment
3. Documentation comment

Single line Comment






A single line comment spans only one line. It can be given on any line followed by the character "//". The syntax is similar to C++.

Example

// This line is a comment and will be ignored by the compiler
x = 4*y // This is also a comment but appears after assignment statement

Multi line Comment

The syntax of multi line comment is same as C/C++. A multi line comment  is enclosed between "/*" and "*/".

Introduction to Java

Java is an object-oriented programming language developed by Sun Microsystems Inc in 1991.

Java has become the widely used programming language for the internet. Although while designing the language, the primary objective was to develop a programming language that is platform independent.