As in any language that supports imperative programming, Java has control flow statements that “break up the flow of execution by employing decision making, looping, and branching, enabling your program to conditionally execute particular blocks of code” [The Java Tutorials, Oracle].

The “if-then-else” statement is the most basic control flow statement that executes one section of code if a condition evaluates to true, and another — if it evaluates to false. The statement has two forms, short:

if( <condition> )
  <statement if true>

and full:

if( <condition> )
  <statement if true>
else
  <statement if false>

For example, this code assigns a client to a salesman only if the salesmen is currently following up less than 10 clients.

if( salesman.clients.size() < 10 )
  salesman.assign( client );

And this code tests if there are tasks in a certain queue and, if yes, assigns the first one to a truck, otherwise sends the truck to the parking position:

if ( tasks.isEmpty() )
  truck.setDestination( truck.parking );
else
  truck.assignTask( tasks.removeFirst() );

In case “then” or “else” code section contains more than one statement, they should be enclosed in braces { … } to become a block, which is treated as a single statement, see the code below. We however recommend to always use braces for “then” and “else” sections to avoid ambiguous-looking code. Braces are specifically important when there are nested “if” statements or when lines of code in the “if” neighborhood are added or deleted during editing or debugging.

if( friends == null ) {
  friends = new ArrayList<Person>();
  friends.add( john );
} else {
  if( ! friends.contains( john ) )
  friends.add( john );
}

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