Per definition braces are superfluous on single statements, but it is a common recommendation that braces should be always used in such cases. With this option, you can specify whether missing braces for single statements should be inserted for the control statements if, for, while and do-while and labeled statements inside switch blocks.

if...else

When enabled, braces are inserted around the body of if...else statements when necessary.

if (true)
    break;
else
    return;

if (true) {
    break;
} else {
    return;
}

for

When enabled, braces are inserted around the body of for statements when necessary.

for (int i = 0; i < count; i++)
    System.out.println(i);

for (int i = 0; i < count; i++) {
    System.out.println(i);
}

while

When enabled, braces are inserted around the body of while statements when necessary.

while (!isDone)
    doSomething();

while (!isDone) {
    doSomething();
}

do...while

When enabled, braces are inserted around the body of do...while statements when necessary.

do
  something();
while (condition);

do {
  something();
} while (condition);

switch

When enabled, braces are inserted around the body of labeled statements inside switch blocks when necessary.

Please note that braces are only inserted if the statement is not empty!

Since 1.4

switch (c) {
    case 'a':
    case 'b':
        System.out.println();
        break;
}

switch (c) {
    case 'a':
    case 'b': {
        System.out.println();
        break;
    }
}

Only insert when statement takes more than one line

When enabled, brace inseration only happens when the block statement takes more than just one line to print.

Since 1.8

Take the following example with two block statements:

if (arg == null)
    for (int i = 0; i < 10; i++)
        System.out.println("arg " + i);

Enabling brace insertion for if and for statements would normally yield:

if (arg == null) {
    for (int i = 0; i < 10; i++) {
        System.out.println("arg " + i);
    }
}

You see braces inserted for both block statements.

But when you enabled the multi-line option, you will get:

if (arg == null) {
    for (int i = 0; i < 10; i++)
        System.out.println("arg " + i);
}

The statement of the if-block happens to be another block statement which is printed in two lines here, therefore the braces are inserted for the if-statement. The for-statement on the other hand does not have any braces inserted, because here the block can be printed in just one line.