All control structures, unless otherwise noted, make use of block statements. These are denoted by curly braces {}.
This differs from normal statements, which do not require curly braces, but also come with a stiff caveat in that only the line immediately following the previous statement would be considered.
Thus, it is perfectly valid to write any of these control structures without curly braces, so long as only one statement follows the beginning, but it is strongly discouraged, as it can lead to buggy implementations, or broken code.
Example:
// valid, but discouraged
Scanner scan = new Scanner(System.in);
int val = scan.nextInt();
if(val % 2 == 0)
System.out.println("Val was even!");
// invalid; will not compile
// note the misleading indentation here
for(int i = 0; i < 10; i++)
System.out.println(i);
System.out.println("i is currently: " + i);