Java Programming Language

Java programming language was developed by the Sun Microsystems and was launched in 1995. The language has had many versions, the latest is Java SE 21.0.2.

Keywords are predefined code which has a specific meaning and that meaning cannot be changed. They are total 53 in number and cannot be used elsewhere.

For example, these keywords may not be used as variables, methods, classes, etc.


List of Keywords:

Data Type Access Modifiers Control Statement
byte
short
int
long
float
double
char
Boolean
private
protected
public
default
Else
switch
case
default
for
while
Do
Continue
Break
goto [reserved]

Object & Classes Modifiers Exceptions
class
interface
extends
implements
this
super
new
static
abstract
synchronized
volatile
native
transient
strictfp
final
try
catch
finally
throw
throws

Miscellaneous Packages
return
const(*)
instance of
void
assert
package
import
JAVA 2 -> 49 -> 52
Keywords
JAVA 2 -> 50 -> 53
  • Digits (0-9)

  • Alphabets (A-Z, a-z)

They will be used as names to variables, methods, classes, etc. When you are defining user defined words, you need to remember the following points/rules:

  • User defined words can contain all the digits, all the alphabets and only two special symbols, which are, ‘_’(underscore) and '$'(dollar)

  • The first character must be an alphabet, underscore or dollar

  • Keywords can be used as user defined words


Which of the following are valid identifiers or user defined words?

1) abc 123 Invalid (because of the space between characters)
2) 123abc Invalid (because it starts with a number)
3) -123$98 Invalid (because it starts with a dash ‘-‘)
4) Int Valid
5) hello Valid
6) a Valid

There are two categories of data types:

  • Primitive data type
  • User defined data type

Primitive data type:

Primitive datatype is a predefined type of data, and is supported by java.
They are eight in number and are listed below:

Data Type Size (byte) Default Value Range
byte 1 0 Minimum value is-128(2^7)
Maximum value is 127 (inclusive)(2^7-1)
short 2 0 Minimum value is -32,768(-2^15)
Maximum value is 32,767 (inclusive)(2^15-1)
int 4 0 Minimum value is -2,147,483,648(-2^31)
Maximum value is 2,147,483,647(2^31-1)
long 8 0L Minimum value is - 9,223,372,036,854,775,808(-2^63)
Maximum value is 9,223,372,036,854,775,807(2^63-1)
float 4 0.0f Example : float f1 = 234.5f
double 8 0.0d Example : double d1 = 123.4
char 2 Blank Minimum value is 'u0000'(or 0)
Maximum value is 'uffff'(or 65,535 inclusive)
boolean 1 false Example : boolean one = true
Example : boolean one = false

User defined data type

User defined datatype is a data type which is obtained from an already present data type.
Listed below are the types of user defined types :

  • Class type.
  • Interface type.
  • Two more types called enum and annotation.
  • Class objects, and various type of array variables come under reference variables.
  • Default value of any reference variable is null.
  • A reference variable can be used to refer to any object of the declared or compatible type.
    Example: Animal animal = new Animal("giraffe");

A variable in java is a section of memory that contains or may contain a data value. Therefore, it can be said that is a name allotted to the location of memory.

    Syntax :
      Data type var_name = value;
      e.g.
        int a;
        int b = 20;
  • These two variables a and b are called as primitive variables because they are declared with primitive data type. Here, we can say that b is type of int whose value is 20.

      Hello h =new Hello();

  • The above given variable, h called reference variables because they are declared with user defined data type. Here we can say h is the type of Hello with Hello(); as the new value.


Difference between Primitive and Reference variables:

Primitive Variable Reference Variable
Primitive data type is called primitive variable. Variable declared with user defined datatype is called reference variable.
Memory allocation for Primitive variables will depend on the primitive Data type used. Always allocate eight bytes of memory for reference variable.
The default value for the primitive variables will depend on the primitive data type used. Null will always be assigned.
Primitive variables contain value as address or literal. Reference variable contains the addresses of an object.

Constants may be defined as variables whose values cannot be changed.

  • Constants are also called final variables

  • Value assigned for the final variable can not be modified

    • Syntax:
        final Data type var_name = value;
        For ex.
          final int a = 0;
          //a = a + 1;
          //Showing error, it is not possible to do any operation on a final variable like this.

Literals are a value which you can assign to the variable or a constant.
There are five types of literals:

  • Integer Literal

  • Floating Point Literal

  • Character Literal

  • Boolean Literal

  • String Literal

Learn in Detail

Operators are special symbols that perform operations.
Listed below are the types of operators:

  • Arithmetic Operator (+, -, *, /,%)

  • Relational Operator (>, >=, <, <=)

  • Logical Operator (&&, ||, \)

  • Assign Operator (=, +=, -=, *=, /=, &=)

  • Increment / Decrement Operator (++, --)

  • Ternary Operator (?:)

  • Bitwise Operator (&, \, ^)

  • Equality Operator (==, !=)

  • Unary Operator (\, ++, --, ~)

  • Shift Operator (>>, <<)

  • Instance of Operator (instanceof)

  • New Operator (new)

Learn in Detail

A Control statement in Java works as a determiner for deciding the next task of the other statements whether to execute or not.

Types of control statements:

  • Conditional control statement

  • Looping control statement

  • Unconditional control statement

Learn in Detail

Please refer Arrays Chapter.