2.2 Conditional Branches

Conditional branches let a program choose different code paths for different situations. Java uses if, else if, and else for branching, and groups the statements that belong to a branch into a block with braces {}.

if

An if statement checks a condition. If the condition is true, Java runs the block inside the braces. If the condition is false, Java skips that block.

Scanner input = new Scanner(System.in);
int age = input.nextInt();

if (age > 0 && age < 18) {
    System.out.println("Minor");
}

One possible run:

17
Minor

In Java, the statements that belong to an if go inside braces {}. Even for a single line, adding braces is recommended to avoid mistakes when the code grows later.

if-else

if-else is used for a two-way choice: run the if block when the condition is true; otherwise run the else block.

Here is a leap-year example. A year is a leap year if it satisfies one of these conditions:

• It is divisible by 4, and not divisible by 100.

• Or it is divisible by 400.

Scanner input = new Scanner(System.in);
int year = input.nextInt();

boolean isLeapYear = (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0);

if (isLeapYear) {
    System.out.println("Leap year");
} else {
    System.out.println("Common year");
}

One possible run:

2020
Leap year

Saving the complex condition into isLeapYear makes the if line easier to read. In real programs, this is often more maintainable than placing a long condition directly inside if.

if-else if-else

When there are more than two cases, use if-else if-else. Java checks conditions from top to bottom. It runs the first branch whose condition is true, then skips the remaining branches.

Scanner input = new Scanner(System.in);
char c = input.next().charAt(0);

if (c >= 'a' && c <= 'z') {
    System.out.println("Lowercase");
} else if (c >= 'A' && c <= 'Z') {
    System.out.println("Uppercase");
} else if (c >= '0' && c <= '9') {
    System.out.println("Digit");
} else {
    System.out.println("Special character");
}

One possible run:

T
Uppercase

The order of else if branches matters. Put more specific or higher-priority conditions earlier, and use else as the fallback case.