Fundamentals in Java

There are the following major fundamentals in Java:-

Character Set in Java

• Java uses the Unicode character set. Unicode defines a fully international character set that can represent all of the characters found in all human languages and writing systems around the world such as English, Arabic, Chinese, Hindi, Regional languages etc.
• Since there are a large number of languages in the world, therefore a large character set is required and 8 bits, are not sufficient. Therefore, this 16-bit character set fulfills the need. Thus, in Java char is a 16-bit type.

Data Types in Java

• In most of the other programming languages, the format and size of primitive data types depend on the platform/operating system or processor on which a code is running. However, in Java programming language, the size and format of its primitive data types are specified. So we need not worry about the system dependencies while using Java primitive data types. This makes Java programs easily portable on any machine architecture.

Primitive Data Types in Java

• Characters
  • A char is a single character that includes a letter, a digit, a punctuation mark, a tab, a space, or
    something like that. A char literal is a single character enclosed in single quotes such as char ch = ‘a’ ; char ch= ‘ 5 ’ ; char ch= ‘ * ’ ; etc.
• Booleans
  • In Java, Booleans are logical variables, which can contain the value either true or false. Any other value cannot be assigned to a Boolean variable.

Java Keywords/Reserve Words

• A keyword or reserved word in Java has a special & specific meaning and cannot be used as a user-defined identifier/variables/function name in any Java program, because they are used by the compiler for a specific purpose.
• Each keyword has a specific and well-defined purpose.
• When we use keywords as variable names in our Java program the compiler will generate errors.
• All the Java keywords are represented in lowercase.
• A list of Java keywords are – 

• The words true, false, and null are used as reserved words but not in the list.
• Keywords ‘const’ and ‘goto’ are currently not in use in the program.
• In the list of Keywords, certain fresh Keywords have also been added, for example, Java 1.2 adds the strictfp keyword, Java 1.4 adds the assert keyword, etc.

Identifiers

• Identifiers are nothing but the names of variables, methods, classes, packages, and interfaces.
• In Java application programs, String, args, main, and System.out.println all are identifiers.
• Identifiers must be composed of letters, numbers, the underscore ‘_’, and the dollar sign $. But identifiers should begin with a letter, the underscore, or a dollar sign.

Variables

• It is a basic unit of storage in a program.
• Variables represent memory locations in which values can be stored which are used in the program.

Types of Variables in Java

Java shows three kinds of variables –
• Instance variables –

• • Instance variables are used to define attributes or the state of a particular object and are used to store information needed by multiple methods in the objects.

• Class/Global/Universal Variables –

• • Class Variables are similar to instance variables, except their values apply to all the instances of a class (and to the class itself) rather than having different values for each object.
  • Class variables are global to a class and to all the instances of the class. They are useful for communicating between different objects of the same class or keeping track of global states.

• Local/Private Variables –

• • Local variables are declared and used inside blocks as counters or in methods as temporary variables to store information needed by a single method.

Variable Declaration :

• Before using any variable in the program, we must first declare it. Normally after the declaration, we can initialize (assign certain required values) them, if necessary.
• We can also declare and assign a value to a variable at the same time.
• Variable declarations can be put anywhere in the program code but just above the first use of the variable in the program code. However, it is common practice to place the variable declarations at the top of each code block.
• Variable names in Java can only start with a letter, an underscore ( _), or a dollar sign ($). They cannot start with a number. Thus, a variable name can include letters or numbers or a combination of both but must start with a letter.
• It is useful to name the program variables intelligently or according to their role in the program.
• A variable name cannot contain any white space. To represent a variable with a gap, we can use the underscore symbol in between.
• There is no limit to the length of a Java variable name.
• A variable declaration specifies the datatype, the variable name, and, optionally, the default value for the variable.
• The general syntax of the variable declaration is –
         Syntax = datatype variable-name ;
         Examples = byte x; short age; boolean attd; int x; 
• We can also declare multiple variables of similar datatype of one type in one expression such as –

Variable Initialization :

• Once we have declared the data type of a variable, we can initialize it with some related data type’s value.
• Variable Initialization may be of two types – (i) Static Variable Initialization and (ii) Dynamic Variable Initialization.

(i) Static Variable Initialization :

• • • When we assign a certain value to the declared variable before run time, i.e. normally at the time of declaration or just after declaration in a single/double statement is called static initialization.
    • Syntax :-  variable name = some value;
      Example :- int rollno;
      rollno = 2801; int x=10;

(ii) Dynamic Variable Initialization :

• • • When we assign a certain manipulated value to the declared variable during run time in a single statement is called dynamic initialization.
    • Syntax :-  variable name = some value;

Literals

• Literals are nothing but pieces of Java code that indicate/contain explicit values.
• For example “Hello IGNOU!” is a String literal. The double quote marks indicate to the compiler that this is a string literal. The quotes indicate the start and the end of the string, Here, the quote marks themselves are not a part of the string.
• Similarly, Character Literals are enclosed in single quotes and they must have exactly one character.
• TRUE and FALSE are boolean literals that mean true and false.
• Number, double, long, and float literals also exist there.

Constants

• Constants are fixed values that are assigned directly to compatible variables.
• The value of constants never changes during the program execution.
• To declare constants in Java, the final keyword is used.
• For example – The following statement defines an integer constant x containing a value of 100.
  final int x =100;
• float area=3.141*r*r;