Java - Programming Fundamentals (Part 2)

In this section we are going to understand how to create a class and add attributes (variables) to it.

Creating class

We start with creating Person class. To start follow the steps given below:-

• Create a Text File.
• Rename the file as Person.java
• Use the text editor or an IDE to write the code
• Use keyword “class” to create a class.

    //creating the class.
    public class Person {
        
    }

Now the Person class is created but still it has no attributes or functionality, so now we will provide a set of attributes to it as variables.

Variables

• Every variable has a type
• Place the type first, followed by the name of the variable
• The semicolon is necessary because a declaration is a complete Java statement.
• A variable name must begin with a letter and must be a sequence of letters or digits.
• The length of a variable name is essentially unlimited.
• You can have multiple declarations on a single line.
• Names are highly case sensitive so variable newAccount is different from newaccount
• Local variables must be declared and initialized before use.

Initializing Variables

After you declare a variable, you must explicitly initialize it by means of an assignment statement—you can never use the values of uninitialized variables.

    public class RunnerClass {
    
        public static void main(String[] args) {
            int IntegerVariable;
            String StringVariable;
            
            System.out.println(IntegerVariable);
            System.out.println(StringVariable);
        }
    
    }

$ javac RunnerClass.java 
RunnerClass.java:7: error: variable IntegerVariable might not have been initialized
        System.out.println(IntegerVariable);
                           ^
RunnerClass.java:8: error: variable StringVariable might not have been initialized
        System.out.println(StringVariable);
                           ^
2 errors

To use a local variable, one must declare the variable before using it.

In Java you can put declarations anywhere in your code. For example, the following is valid code in Java:

• double salary = 65000.0;
• System.out.println(salary);
• int vacationDays = 12; // ok to declare a variable here

In Java, it is considered good style to declare variables as closely as possible to the point where they are first used.

    public class Main {
    
    	public static void main(String[] args) {
    		int IntegerVariable = 1;
    		String StringVariable = "Hello";
    		
    		System.out.println(IntegerVariable);
    		System.out.println(StringVariable);
    	}
    
    }

1
Hello

List of key words which cannot be used as variable names

Keyword	Meaning
abstract	an abstract class or method
assert	used to locate internal program errors
boolean	the Boolean type
break	breaks out of a switch or loop
byte	the 8-bit integer type
case	a case of a switch
catch	the clause of a try block catching an exception
char	the Unicode character type
class	defines a class type
const	const
continue	continues at the end of a loop
default	the default clause of a switch
do	the top of a do/while loop
double	the double-precision floating-number type
else	the else clause of an if statement
enum	an enumerated type
extends	an enumerated type
final	a constant, or a class or method that cannot be overridden
finally	the part of a try block that is always executed
float	the single-precision floating-point type
for	a loop type
goto	not used
if	a conditional statement
implements	defines the interface(s) that a class implements
import	imports a package
instanceof	tests if an object is an instance of a class
int	the 32-bit integer type
interface	an abstract type with methods that a class can implement
long	the 64-bit long integer type
native	a method implemented by the host system
new	allocates a new object or array
null	a null reference
package	a package of classes
private	a feature that is accessible only by methods of this class
protected	a feature that is accessible only by methods of this class,its children, and other classes in the same package
public	a feature that is accessible by methods of all classes
return	returns from a method
short	the 16-bit integer type
static	a feature that is unique to its class, not to objects of its class
strictfp	Use strict rules for floating-point computations
super	the superclass object or constructor
switch	a selection statement
synchronized	a method or code block that is atomic to a thread
this	the implicit argument of a method, or a constructor of this class
throw	throws an exception
throws	the exceptions that a method can throw
transient	marks data that should not be persistent
try	a block of code that traps exceptions
void	denotes a method that returns no value
volatile	ensures that a field is coherently accessed by multiple threads
while	a loop

Let’s return back to our Problem statement, since we have learned how to declare variables we will add variables to our Person class.

    //Adding variables to the class.
    public class Person {
        
        /* The will hold a string data type */
        private String firstName;
        
        /* The will hold a string data type */
        private String lastName;
        
        /* The will hold a string data type */
        private String accountNumber;
        
        /* The will hold a integer data type */
        private double balance;
        
    }

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