Table of Contents
Elements or Tokens
Java program contains different types of elements like white spaces, comments and tokens. A token is the smallest program element which is recognized by the compiler and which treats them as defined for the compiler. A program is a set of tokens which comprise the following elements:
Identifiers or names: Identifier is the name of variables, methods, classes etc.
Rules for framing Names or Identifiers.
- It should be a single word which contains alphabets a to z or A to Z, digits 0 to 9, underscore (_).
- It should not contain white spaces and special symbols.
- It should not be a keyword of Java.
- It should not be Boolean literal, that is, true or false.
- It should not be null literal.
- It should not start with a digit but it can start with an underscore.
- It can comprise one or more unicode characters which are characters as well as digits.
Conventions for Writing Names
- Names of packages are completely in lower-case letters such as mypackage, java.lang.
- Names of classes and interfaces start with an upper-case letter.
- Names of methods start with a lower-case character.
- Names of variables should start with a lower-case character.
Tokens are the smallest unit of the Java program. The compiler breaks down the lines of code into tokens. Let’s see an example to understand tokens.
// program to understand java tokens
class HelloToken
{
public static void main(String[] args)
{
System.out.println(“Hello Ninja!!”);
}
}
In the above program, “class, HelloToken, {, static, void, main, (, String, args, [, ], ), System, ., out, println” etc. are tokens.
Types of Java Tokens
There are five types of java tokens: keywords, identifiers, literals, operators and separators. The classification is based on their work type; some are used to define names, and others for arithmetic operations.
- Keywords
- Constants
- Identifiers/Variable
- Literals
- Operators
- Seprators
- Comments
Java Keywords
Java reserve words are keywords that are reserved by Java for particular uses and cannot be used as identifiers (e.g., variable names, function names, class names). The list of reserved words is Java is provided below.
| abstract | default | if | package | syhronized |
| assert | do | implements | private | this |
| boolean | double | import | protected | throw |
| break | else | instanceof | public | throws |
| byte | extends | int | return | transient |
| case | false | interface | short | true |
| catch | final | long | static | try |
| char | finally | native | strictfp | void |
| class | float | new | super | volatile |
| const | for | null | switch | while |
| continue | goto |
Java keywords are also known as reserved words. Keywords are particular words which acts as a key to a code. These are predefined words by Java so it cannot be used as a variable or object name.
A list of Java keywords or reserved words are given below:
- abstract : Java abstract keyword is used to declare abstract class. Abstract class can provide the implementation of interface. It can have abstract and non-abstract methods.
- boolean :Java boolean keyword is used to declare a variable as a boolean type. It can hold True and False values only.
- break : Java break keyword is used to break loop or switch statement. It breaks the current flow of the program at specified condition.
- byte : Java byte keyword is used to declare a variable that can hold an 8-bit data values.
- case : Java case keyword is used to with the switch statements to mark blocks of text.
- catch : Java catch keyword is used to catch the exceptions generated by try statements. It must be used after the try block only.
- char : Java char keyword is used to declare a variable that can hold unsigned 16-bit Unicode characters
- class : Java class keyword is used to declare a class.
- continue : Java continue keyword is used to continue the loop. It continues the current flow of the program and skips the remaining code at the specified condition.
- default : Java default keyword is used to specify the default block of code in a switch statement.
- do : Java do keyword is used in control statement to declare a loop. It can iterate a part of the program several times.
- double : Java double keyword is used to declare a variable that can hold a 64-bit floating-point numbers.
- else : Java else keyword is used to indicate the alternative branches in an if statement.
- enum : Java enum keyword is used to define a fixed set of constants. Enum constructors are always private or default.
- extends : Java extends keyword is used to indicate that a class is derived from another class or interface.
- final : Java final keyword is used to indicate that a variable holds a constant value. It is applied with a variable. It is used to restrict the user.
- finally : Java finally keyword indicates a block of code in a try-catch structure. This block is always executed whether exception is handled or not.
- float : Java float keyword is used to declare a variable that can hold a 32-bit floating-point number.
- for : Java for keyword is used to start a for loop. It is used to execute a set of instructions/functions repeatedly when some conditions become true. If the number of iteration is fixed, it is recommended to use for loop.
- if : Java if keyword tests the condition. It executes the if block if condition is true.
- implements : Java implements keyword is used to implement an interface.
- import : Java import keyword makes classes and interfaces available and accessible to the current source code.
- instanceof : Java instanceof keyword is used to test whether the object is an instance of the specified class or implements an interface.
- int : Java int keyword is used to declare a variable that can hold a 32-bit signed integer.
- interface : Java interface keyword is used to declare an interface. It can have only abstract methods.
- long : Java long keyword is used to declare a variable that can hold a 64-bit integer.
- native: Java native keyword is used to specify that a method is implemented in native code using JNI (Java Native Interface).
- new : Java new keyword is used to create new objects.
- null : Java null keyword is used to indicate that a reference does not refer to anything. It removes the garbage value.
- package : Java package keyword is used to declare a Java package that includes the classes.
- private : Java private keyword is an access modifier. It is used to indicate that a method or variable may be accessed only in the class in which it is declared.
- protected : Java protected keyword is an access modifier. It can be accessible within package and outside the package but through inheritance only. It can’t be applied on the class.
- public : Java public keyword is an access modifier. It is used to indicate that an item is accessible anywhere. It has the widest scope among all other modifiers.
- return : Java return keyword is used to return from a method when its execution is complete.
- short : Java short keyword is used to declare a variable that can hold a 16-bit integer.
- static : Java static keyword is used to indicate that a variable or method is a class method. The static keyword in Java is used for memory management mainly.
- strictfp : Java strictfp is used to restrict the floating-point calculations to ensure portability.
- super : Java super keyword is a reference variable that is used to refer parent class object. It can be used to invoke immediate parent class method.
- switch : The Java switch keyword contains a switch statement that executes code based on test value. The switch statement tests the equality of a variable against multiple values.
- synchronized : Java synchronized keyword is used to specify the critical sections or methods in multithreaded code.
- this : Java this keyword can be used to refer the current object in a method or constructor.
- throw : The Java throw keyword is used to explicitly throw an exception. The throw keyword is mainly used to throw custom exception. It is followed by an instance.
- throws : The Java throws keyword is used to declare an exception. Checked exception can be propagated with throws.
- transient : Java transient keyword is used in serialization. If you define any data member as transient, it will not be serialized.
- try : Java try keyword is used to start a block of code that will be tested for exceptions. The try block must be followed by either catch or finally block.
- void : Java void keyword is used to specify that a method does not have a return value.
- volatile : Java volatile keyword is used to indicate that a variable may change asynchronously.
- while : Java while keyword is used to start a while loop. This loop iterates a part of the program several times. If the number of iteration is not fixed, it is recommended to use while loop.
Constants
Constants are declared using the final keyword. The values of the constant can’t be changed once its declared.
public class ConstInJava
{
public static final int constint=5;
public static void main(String[] args)
{
int intvariable;
for (int i = 0;i <= 10;i++)
{
intvariable = i;
System.out.println(“All the values are : \n intvariable = ” + intvariable + “\nconstint = ” + constint);
}
}
}
Identifiers/Variable:-
A method name, class name, variable name, or label is an identifier in Java. The user typically defines these. The identifier names cannot be the same as any reserved keyword. Let’s see an example to understand identifiers:
public class Test
{
public static void main(String[] args)
{
int num = 10;
}
}
Identifiers present in the above program are:
Test: The name of the class.
main: The name of a method.
String: A predefined class name.
args: A variable name.
num: A variable name.
There are rules for naming identifiers.
- The characters allowed are [A-Z], [a-z], [0-9], _ and $.
- Identifiers are case-sensitive. That is, “ninja” is not the same as “NINJA”.
- Identifier names should not start with a digit. For example, “007IamNinja” is an invalid identifier.
- Whitespace is not allowed inside an identifier.
- Keywords can’t be used as an identifier.
Literals
Literals represent fixed values in a source code. These are similar to standard variables with the difference that these are constant. These can be classified as an integer literal, a string literal, a boolean etc. The user defines these mainly to define constants.
Syntax to define literals:
final data_type variable_name;
There are five types of literals in Java:
- Integer
- Floating Point
- Boolean
- Character
- String
| Type | Example |
| int | 22 |
| double | 25.08 |
| boolean | TRUE |
| char | N |
| string | “Coding Ninjas” |
Operators
As the name suggests, operators perform operations between different entities. Whenever the compiler sees an operator, it tokenizes it and proceeds further. Java has other operators based on their functionality. In total, there are eight types of operators in Java.
Let’s see these operators along with their examples.
| Operator | Examples |
| Arithmetic | +.-.*,/,% |
| Assignment | =.+=,-=,*=,/= |
| Unary | ++,–,≠ |
| Logical | &&,|| |
| Relational | ==,≠,≥,≤ |
| Ternary | condition?stat1:stat2 |
| Bitwise | &,|,~,^ |
| Shift | >>,<<,>>> |
Separators
These are the special symbols used to separate java tokens. These are sometimes called punctuators. The separators have special meaning and thus should not be used for anything else.
- Brackets []:These are used to define arrays and represent single and multi-dimension subscripts.
- Braces {}: These mark the start and end of multi-line code blocks.
- Parenthesis (): Used for function calls and parameters.
- Comma (,): These separate statements. For example, separate parameters.
- Semicolon ( : ): It invokes the initialization list.
- Asterisk (*): This is used to generate pointer variables.
- Assignment operator (=): It assigns values to variables. For example, a=10, here we are giving value 10 to variable ‘a’.
Comments
In java there are multiple types of comments available in java :
- Single-line comments:These are the comments that are denoted by // and are used to comment a single line of code.
- Multi-line comment: These comments start with /* and end with */. When a user wants to comment on multiple lines in the code, these comments are used for it.
- Java doc comments: These comments are denoted by “/**” and “*/”. To provide documentation of Java code, these comments are used. Also, the Java doc tool is used to process these comments and generate HTML documents.
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