Conditional Branches

Conditional branches let a program choose different code paths for different situations. Java uses if, else if, and else for branching.

if

An if statement checks a condition. If the condition is true, Java runs the indented block under it. If the condition is false, Java skips that block.

int age = 17;
if (age > 0 && age < 18) {
    System.out.println("Minor");
}

One possible run:

Enter your age: 17
Minor

In Java, indentation is not decoration. It is part of the syntax. Statements that belong to the same if block must use the same indentation level.

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.

int year = 2020;
boolean leap = (year % 4 == 0 && year % 100 != 0);
        || year % 400 == 0;
if (leap) {
    System.out.println("Leap year");
} else {
    System.out.println("Common year");
}

One possible run:

Enter a year: 2020
Leap year

Saving the complex condition into is_leap_year 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.

char c = 'T';
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:

Enter a character: T
Uppercase

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