Conditional statement

The conditional statement is a construction that allows a program to perform different computation depending on the value of a Boolean expression. If it is true, the program performs one computation; otherwise, it is false, the program performs another computation. Here are some examples of Boolean expressions: a > b, i - j == 1 and so on.

The conditional statement has different forms. We will use all of them.

The single if-case

The simplest form of the conditional statement consists of the keyword if, a Boolean expression and a body enclosed in curly braces.

if (expression) {
    // body: do something
}

If the expression is true, the statements inside the code block are executed; otherwise, the program skips them.

See the following example.

int age = ...; // it has a value
if (age > 100) {
    System.out.println("Very experienced person");
}

In this example, if the age is greater than 100 the code prints "Very experienced person", otherwise, it does nothing.

Sometimes you will see a situation when the expression in a condition is a single boolean type variable. Instead of writing b == true or b == false, use this variable (or its negation with !)as the Boolean expression:

boolean b = ...; // it is true of false
if (b) { // or !b
    // do something
}

A conditional statement can be used in any place in a program where statement is expected. It can be even nested inside another conditional statement to perform multistage checks.

The if-else-cases

The if-case above can be extended with the keyword else and another body to do alternative actions when the expression is false.

if (expression) {    
    // do something
} else {
    // do something else
} 

In this case, if the expression is true, then the first code block is executed; otherwise, the second code block is executed, but not both together.

In the example below, the program outputs different text depending on the value of num (even or odd). Note, the number is even if it can be divided exactly by 2; otherwise it's odd.

int num = ...; // the num is initialized by some value

if (num % 2 == 0) {
    System.out.println("It's an even number");
} else {    
    System.out.println("It's an odd number");
}

Since a number can only be even or odd, only one message will be displayed. If num is 10, the program outputs "It's an even number". If the value is 11, it outputs "It's an odd number".

The if-else-if-cases

The most general form of the conditional statement consists of several conditions and else-branches.

if (expression0) {
    // do something
} else if (expression1) {
    // do something else 1
// ...
} else if (expressionN) {
    // do something else N
}

long dollars = ...; // your budget

if (dollars < 1000) {
    System.out.println("Buy a laptop");
} else if (dollars < 2000) {
    System.out.println("Buy a personal computer");
} else if (dollars < 100_000) {
    System.out.println("Buy a server");
} else {
    System.out.println("Buy a data center or a quantum computer");
}