How to Use if Statements in Java

By Doug Lowe from Java For Dummies Quick Reference

In its most basic form, an if statement executes a single statement or a block of statements if a boolean expression evaluates to true. Here’s the syntax:

if (boolean-expression)
    statement

The boolean expression must be enclosed in parentheses. If you use only a single statement, it must end with a semicolon. However, the statement can also be a statement block enclosed by braces. In that case, each statement within the block needs a semicolon, but the block itself doesn’t.

Here’s an example:

double commissionRate = 0.0;
if (salesTotal > 10000.0)
    commissionRate = 0.05;

In this example, a variable named commissionRate is initialized to 0.0 and then set to 0.05 if salesTotal is greater than 10000.0.

Here’s an example that uses a block rather than a single statement:

double commissionRate = 0.0;
if (salesTotal > 10000.0)
{
    commissionRate = 0.05;
    commission = salesTotal * commissionRate;
}

In this example, the two statements within the braces are executed if salesTotal is greater than $10,000. Otherwise, neither statement is executed.

An if statement can include an else clause that executes a statement or block if the boolean expression is not true. Its basic format is

if (boolean-expression)
    statement
else
    statement

Here’s an example:

double commissionRate;
if (salesTotal <= 10000.0)
    commissionRate = 0.02;
else
    commissionRate = 0.05;

In this example, the commission rate is set to 2% if the sales total is less than or equal to $10,000. If the sales total is greater than $10,000, the commission rate is set to 5%.

Here’s an if statement with an else clause that uses a block instead of a single statement:

double commissionRate;
if (salesTotal <= 10000.0)
{
    commissionRate = 0.02;
    level1Count++;
}
else
{
    commissionRate = 0.05;
    level2Count++;
}

The statement that goes in the if or else part of an if-else statement can be any kind of Java statement, including another if or if-else statement. This arrangement is nesting, and an if or if-else statement that includes another if or if-else statement is a nested if statement.

The general form of a nested if statement is this:

if (expression-1)
    if (expression-2)
        statement-1
    else
        statement-2
else
    if (expression-3)
        statement-3
    else
        statement-4

In this example, expression-1 is the first to be evaluated. If it evaluates to true, expression-2 is evaluated. If that expression is true, statement-1 is executed; otherwise, statement-2 is executed. But if expression-1 is false, expression-3 is evaluated. If expression-3 is true, statement-3 is executed; otherwise, statement-4 is executed.

Here’s an example that implements a complicated commission structure based on two variables, named salesClass and salesTotal:

if (salesClass == 1)
    if (salesTotal < 10000.0)
        commissionRate = 0.02;
    else
        commissionRate = 0.04;
else
    if (salesTotal < 10000.0)
        commissionRate = 0.025;
    else
        commissionRate = 0.05;

The trick of using nested if statements is knowing how Java pairs else keywords with if statements. The rule is actually very simple: Each else keyword is matched with the most previous if statement that hasn’t already been paired with an else keyword.