Java Syntax from the Ground Up - Java in a Nutshell - page 51

In this example, the inner if statement forms the single statement allowed by the
syntax of the outer if statement. Unfortunately, it is not clear (except from the hint
given by the indentation) which if the else goes with. And in this example, the
indentation hint is wrong. The rule is that an else clause like this is associated with
the nearest if statement. Properly indented, this code looks like this:

a x

if ( i == j )
if ( j == k )
System . out . println ( "i equals k" );
else
System . out . println ( "i doesn't equal j" ); // WRONG!!
This is legal code, but it is clearly not what the programmer had in mind. When
working with nested if statements, you should use curly braces to make your code
easier to read. Here is a better way to write the code:

if ( i == j ) {
if ( j == k )
System . out . println ( "i equals k" );
}
else {
System . out . println ( "i doesn't equal j" );
}

The else if clause
The if/else statement is useful for testing a condition and choosing between two
statements or blocks of code to execute. But what about when you need to choose
between several blocks of code? This is typically done with an else if clause, which
is not really new syntax, but a common idiomatic usage of the standard if/else
statement. It looks like this:

if ( n == 1 ) {
// Execute code block #1
}
else if ( n == 2 ) {
// Execute code block #2
}
else if ( n == 3 ) {
// Execute code block #3
}
else {
// If all else fails, execute block #4
}
There is nothing special about this code. It is just a series of if statements, where
each if is part of the else clause of the previous statement. Using the else if idiom
is preferable to, and more legible than, writing these statements out in their fully
nested form:

if ( n == 1 ) {
// Execute code block #1