short-circuiting behavior of conditional OR operator(||)

Question

Both conditional operators && and || are short-circuited as per
http://docs.oracle.com/javase/tutorial/java/nutsandbolts/op2.html, which means the second operand

Answer 1

if (true || comparison)

comparison will never be evaluated, since its result is irrelevant (because the first argument is already true).

Answer 2

if(true || (0 > 1))

The first statement is true, and so it is not necessary to evaluate the second.

Answer 3

The or operator is short-circuited when the first operand is true. So,

String foo = null;
if (true || foo.equals("")) {
    // ...
}

doesn't throw a NullPointerException.

As @prajeesh rightly points out in the comments, on way that short-circuiting is used in real code is to prevent a NullPointerException whenever you are dealing with an API that might return null. So, for instance, if there is a readStringFromConsole method that returns either the currently available string or null if the user doesn't type anything, we could write

String username = readStringFromConsole();
while (username == null || username.length() == 0) {
    // No NullPointerException on the while clause because the length() call
    // will only be made if username is not null

    System.out.println("Please enter a non-blank name");
    username = readStringFromConsole();
}

// Now do something useful with username, which is non-null and of nonzero length

As a side note, an API that returns user input should return the empty string whenever the user doesn't type anything, and shouldn't return null. Returning null is a way of saying "there is nothing available," while returning the empty string is a way of saying "the user didn't type anything" and so is preferred.