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if-thenand if-then-elseconditional statements let a Java program make simple decisions

For example, when making a plan with a friend, you could say "If Mike gets home before 5:00 PM, then we'll go out for an early dinner." When 5:00 PM arrives, the condition (i.e., Mike is home), which determines whether everyone goes out for an early dinner, will either be true or false. It works exactly the same in Java.

The if-then Statement 

Let's say part of a program we're writing needs to calculate if the purchaser of a ticket is eligible for a child's discount. Anyone under the age of 16 gets a 10% discount on the ticket price.

We can let our program make this decision by using an

if-then if (age < 16) isChild = true;

In our program, an integer variable called

ageholds the age of the ticket purchaser. The condition (i.e., is the ticket purchaser under 16) is placed inside the brackets. If this condition is true, then the statement beneath the if statement is executed -- in this case a booleanvariable isChildis set to true

The syntax follows the same pattern every time. The

if if (condition is true) execute this statement

The key thing to remember is the condition must equate to a

boolean

Often, a Java program needs to execute more than one statement if a condition is true. This is achieved by using a block (i.e., enclosing the statements in curly brackets):

if (age < 16)​{ isChild = true; discount = 10;}

This form of the

if-then

The if-then-else Statement

The

if-thenstatement can be extended to have statements that are executed when the condition is false. The if-then-else if (condition) { execute statement(s) if condition is true}else{ execute statement(s) if condition is false}

In the ticket program, let's say we need to make sure the discount is equal to 0 if the ticket purchaser is not a child:

if (age < 16){ isChild = true; discount = 10;}else{ discount = 0;}

The

if-then-elsestatement also allows the nesting of if-then if (age < 16){ isChild = true; discount = 10;}else if (age > 65){ isPensioner = true; discount = 15;}else if (isStudent == true){ discount = 5;}

As you can see, the

if-then-elsestatement pattern just repeats itself. If at any time the condition is true , then the relevant statements are executed and any conditions beneath are not tested to see whether they are trueor false

For example, if the age of the ticket purchaser is 67, then the highlighted statements are executed and the

(isStudent == true)

There is something worth noting about the

(isStudent == true)condition. The condition is written to make it clear that we're testing whether isStudenthas a value of true, but because it is a boolean else if (isStudent){ discount = 5;}

If this is confusing, the way to think about it is like this -- we know a condition is tested to be true or false. For integer variables like

age, we have to write an expression that can be evaluated to true or false (e.g., age == 12, age > 35

However, boolean variables already evaluate to be true or false. We don't need to write an expression to prove it because

if (isStudent)is already saying "if isStudent is true..". If you want to test that a boolean variable is false, just use the unary operator !. It inverts a boolean value, therefore if (!isStudent)