Conditional statement

1. The if-else statement

Programs wouldn't be very useful if they always did the same thing, regardless of how external circumstances change. A program needs to be able to adapt to different situations and take certain actions in some situations, and to act differently in others.

In Java, this is done with a conditional statement, which uses a special keyword that lets you execute different blocks of commands depending on the truth value of a condition.

A conditional statement consists of three parts: condition, statement 1 and statement 2. If the condition is true, then statement 1 is executed. Otherwise statement 2 is executed. Both commands are never executed. Here's the general appearance of this kind of statement:

if (condition)
   statement 1;
else
   statement 2;

It is quite understandable when written in plain English like this:

If condition is true, then
   execute statement 1;
otherwise
   execute statement 2;

Examples:

int age = 17;
if (age < 18)
   System.out.println("You are still a child");
else
   System.out.println("You are now an adult");

You are still a child

int temperature = 5;
if (temperature < 0)
   System.out.println("It's freezing outside");
else
   System.out.println("It's warm");

It's warm

int age = 18;
if (age == 18)
   System.out.println("You've been drafted for military service");
else
   System.out.println("Report for duty anyway");

You've been drafted for military service

2. Block of statements

If the condition is satisfied (or not) and you want your program to execute several commands, you can combine them into a block.

To combine commands into a block, you "wrap" them in curly braces. Here's how it looks in general:

{
   statement 1;
   statement 2;
   statement 3;
}

You can have as many statements as you want in a block. Or even none.

Examples of an if-else statement combined with a block of statements:

int age = 17;
if (age < 18)
{
   System.out.println("You are still a child");
   System.out.println("Don't talk back to adults");
}
else
{
   System.out.println("You are now an adult");
   System.out.println("And thus ends your youth");
}

You are still a child
Don't talk back to adults

int temperature = 5;
if (temperature < 0)
{
   System.out.println("It's freezing outside");
   System.out.println("Put on a hat");
}
else
   System.out.println("It's warm");

It's warm

int age = 21;
if (age == 18)
   System.out.println("You've been drafted for military service");
else
{
}

3. Abbreviated form of the if statement

Sometimes it you need to execute one or statements if the condition is true but nothing should be done if it is false.

For example, we can specify this command: If Bus No. 62 has arrived, then get aboard, but don't react if the bus isn't here. In Java, this scenario lets us use an abbreviated form: an if statement without an else block.

In other words, if statements(s) needs to be executed only if the condition is true and there are no commands to be executed when the condition is false, then you should use the if statement, which is concise and omits the else block. It looks like this:

if (condition)
   statement 1;

Below are three examples of equivalent code:

int age = 18;
if (age == 18)
{
   System.out.println("You've been drafted for military service");
}
else
{
}

You've been drafted for military service

The program has an else block, but it is empty (there are no statements between the curly braces). You can simply remove it. Nothing will change in the program.

int age = 18;
if (age == 18)
{
   System.out.println("You've been drafted for military service");
}

You've been drafted for military service

int age = 18;
if (age == 18)
   System.out.println("You've been drafted for military service");

You've been drafted for military service