2.4 Nested if Statements

1. An if inside an if

Nested if statements are if, if-else or if-else-if statements written inside the body of another if, if-else or if-else-if statement 2.4.A.1. Use nesting when a second decision only makes sense after the first one.

A cinema charges by age, but only members get the student price:

int age = 15;
boolean isMember = true;
if (age < 18) {
    if (isMember) {
        System.out.println("Ticket: 5 dollars");
    } else {
        System.out.println("Ticket: 8 dollars");
    }
} else {
    System.out.println("Ticket: 12 dollars");
}

Output:

Ticket: 5 dollars

2. The inner condition waits for the outer one

The Boolean expression of the inner if is evaluated only if the outer Boolean expression is true 2.4.A.2. Trace the cinema code with four inputs:

age	isMember	age < 18	isMember checked?	Output
15	true	true	yes, true	Ticket: 5 dollars
15	false	true	yes, false	Ticket: 8 dollars
30	true	false	no	Ticket: 12 dollars
30	false	false	no	Ticket: 12 dollars

When the outer test is false, Java jumps straight to the outer else. The value of isMember does not matter.

3. Multiway selection: if-else-if

A multiway selection (if-else-if) is used when there is a series of expressions, each with its own segment of code. Java checks the expressions from top to bottom, runs no more than one segment (the one for the first expression that evaluates to true) and skips the rest. If no expression is true and there is a trailing else, the else body runs 2.4.A.3.

int score = 84;
String grade;
if (score >= 90) {
    grade = "A";
} else if (score >= 80) {
    grade = "B";
} else if (score >= 70) {
    grade = "C";
} else {
    grade = "F";
}
System.out.println("Grade: " + grade);

Output:

Grade: B

score >= 70 is also true for 84, but it is never checked, because score >= 80 was true first.

score	First true expression	grade
95	score >= 90	A
84	score >= 80	B
70	score >= 70	C
42	none, trailing else	F

4. Order matters in a chain

Because the first true expression wins, the order of the conditions changes the result. This version checks the widest condition first:

int score = 95;
String grade;
if (score >= 70) {
    grade = "C";
} else if (score >= 80) {
    grade = "B";
} else if (score >= 90) {
    grade = "A";
} else {
    grade = "F";
}
System.out.println("Grade: " + grade);

Output:

Grade: C

A score of 95 gets a C, because score >= 70 is true first. The B and A branches can never run. Rule: with >=, put the largest boundary first; with <=, put the smallest boundary first.

Without a trailing else, a chain may run nothing at all:

int windSpeed = 12;
if (windSpeed > 60) {
    System.out.println("Storm warning");
} else if (windSpeed > 30) {
    System.out.println("Strong wind");
}
System.out.println("Report finished");

Output:

Report finished

5. The dangling else

When there are no braces, an else pairs with the nearest earlier if that does not already have an else, no matter how the code is indented. This is called the dangling else.

int x = 5;
int y = 1;
if (x > 3)
    if (y > 3)
        System.out.println("Both big");
else
    System.out.println("x is small");

Output:

x is small

The else is indented under if (x > 3), but it belongs to if (y > 3). Since x > 3 is true and y > 3 is false, the inner else runs and prints a message that is wrong. With braces, the meaning matches the indentation:

int x = 5;
int y = 1;
if (x > 3) {
    if (y > 3) {
        System.out.println("Both big");
    }
} else {
    System.out.println("x is small");
}
System.out.println("End");

Output:

End