Java: Java Programming For Beginners a simple Start To Java Programming (Written By a software Engineer)

Conditional Programming
While  writing  code,  you  will  faced  with  several  situation  where  you  need  to  execute  a
different  set  of  instructions  if  the  condition  is  true  and  another  set  of  instructions  if  the
same is false.
if-else
In order to implement such scenarios, you can use the if-else construct.
Syntax:
If(condition)
{
//code
}
else
{
//code
}
Consider  a  scenario  in  which  you  ask  the  user  to  enter  his  or  her  age  using  prompt
function.  Now,  you  must  validate  if  the  age  is  a  valid  number,  before  performing  any
computation on the value supplied. This is an ideal scenario of implementing conditional
programming. Sample implementation for this scenario is:
var userAge = prompt(‘Enter your age: ’, ‘’);
if(isNaN(userAge))
{

---

alert(‘Age entered is invalid!’);
}
else
{
//code
}
In this sample code, the if clause checks if the entered value is a number. If the condition
is  true,  that  is  the  object  entered  is  not  a  number,  the  user  is  given  an  alert  message.
However, if the condition is false, the code for else is executed.
It  is  important  to  note  here  that  for  single  statements,  it  is  not  necessary  to  use  curly
braces. The above mentioned code can also be written as:
var userAge = prompt(‘Enter your age: ’, ‘’);
if(isNaN(userAge))
alert(‘Age entered is invalid!’);
else
//code
However,  it  is  a  good  practice  to  use  curly  braces  as  there  is  scope  of  adding  additional
code later on.
if-else if
Another  conditional  programming  construct  is  if-else  if  construct.  This  construct  allows
you to declare multiple conditions and the actions associated with them. The syntax is:
if(condition)

---

{
//code
}
else if(condition)
{
//code
}
else
{
//code
}
Switch
Multiple else ifs can be implemented using this construct. The execution overhead is high
for  this  conditional  programming  construct  as  conditions  are  sequentially  checked  for
validity.  As  an  alternative,  another  keyword,  switch,  is  available,  which  implements
multiple  conditions  in  the  form  of  a  jump  table.  Therefore,  the  execution  overhead  for
switch is lesser than that of if-else if.
Sample implementation:
var userChoice = prompt(‘Choose an alphabet: a, b, c’, ‘e’);
switch (userChoice) {
case ‘a’:
alert(‘a chosen\n’);

---

break;
case ‘b’:
alert(‘b chosen\n’);
break;
case ‘c’:
alert(‘c chosen\n’);
break;
default:
alert(‘None of the alphabets chosen\n’);
break;
};
The switch construct matches that value entered by the user with the values presented in
the cases. If a matching value is found, the case is executed. However, in case, none of the
case  values  match  the  entered  input,  the  default  case  is  executed.  Besides  this,  you  can
also  use  conditions  in  case  values  and  the  case  for  which  the  condition  holds  true  is
executed.
If you do not use the break statement after the code of a case, all the cases following the
matching case will be executed. For example, if the user enters ‘b’ for the above example
and there are no break statements after the case code, then the output will be:
b chosen
c chosen
None of the alphabets chosen

---

Also, it is a good practice to use a break statement in the default case as well.
Note:
If you wish to determine if a keyword has been assigned any value or not, you can use the
following code:
if(c)
{
//code
}
else
{
//code
}
If  the  variable  c  has  been  assigned  a  not-null  value,  then  the  if  condition  is  true  and  the
corresponding code is executed. On the other hand, if the value of variable c is undefined
or null, the code within the else construct is executed.
Note:
The value of the following conditions will always be true:
” == 0
null == undefined
‘123’ == 123
false == 0;
Please note that JavaScript converts the type of the variable concerned for compatibility in

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comparisons.
However, if you want to compare both the value and type of two variables, then JavaScript
provides  another  set  of  operators,  ===  and  !===.    When  the  comparisons  for  the  values
mentioned in the above example are done using this operator, the resultant will always be
false.