Control Structures — Decision Making

By default a program runs its statements sequentially, top to bottom. But real problems need the program to choose between actions depending on conditions. Decision-making (selection) statements let a program run different code for different situations.

The if statement

Runs a block only when a condition is true:

if (marks >= 35) {
    System.out.println("Pass");
}

The if...else statement

Chooses between two blocks:

if (marks >= 35) {
    System.out.println("Pass");
} else {
    System.out.println("Fail");
}

The if...else if ladder

Tests several conditions in turn and runs the first one that is true:

if (marks >= 75) {
    System.out.println("Distinction");
} else if (marks >= 60) {
    System.out.println("First Division");
} else if (marks >= 35) {
    System.out.println("Pass");
} else {
    System.out.println("Fail");
}

The conditions are checked from top to bottom; as soon as one is true its block runs and the rest are skipped, so order matters — the strictest condition must come first.

Nested if

An if may be placed inside another if to test a second condition only when the first is already true:

if (age >= 18) {
    if (hasVoterId) {
        System.out.println("Can vote");
    }
}

The switch statement

When one variable is compared against several fixed values, a switch is clearer than a long ladder:

switch (day) {
    case 1: System.out.println("Monday");   break;
    case 2: System.out.println("Tuesday");  break; …