Java logical operator short-circuiting

Which set is short-circuiting, and what exactly does it mean that the complex conditional expression is short-circuiting?

public static void main(String[] args) {
  int x, y, z;

  x = 10;
  y = 20;
  z = 30;

  // T T
  // T F
  // F T
  // F F

  //SET A
  boolean a = (x < z) && (x == x);
  boolean b = (x < z) && (x == z);
  boolean c = (x == z) && (x < z);
  boolean d = (x == z) && (x > z);
  //SET B    
  boolean aa = (x < z) & (x == x);
  boolean bb = (x < z) & (x == z);
  boolean cc = (x == z) & (x < z);
  boolean dd = (x == z) & (x > z);

}

Solution 1:

The && and || operators "short-circuit", meaning they don't evaluate the right-hand side if it isn't necessary.

The & and | operators, when used as logical operators, always evaluate both sides.

There is only one case of short-circuiting for each operator, and they are:

• false && ... - it is not necessary to know what the right-hand side is because the result can only be false regardless of the value there
• true || ... - it is not necessary to know what the right-hand side is because the result can only be true regardless of the value there

Let's compare the behaviour in a simple example:

public boolean longerThan(String input, int length) {
    return input != null && input.length() > length;
}

public boolean longerThan(String input, int length) {
    return input != null & input.length() > length;
}

The 2nd version uses the non-short-circuiting operator & and will throw a NullPointerException if input is null, but the 1st version will return false without an exception.

Solution 2:

SET A uses short-circuiting boolean operators.

What 'short-circuiting' means in the context of boolean operators is that for a set of booleans b1, b2, ..., bn, the short circuit versions will cease evaluation as soon as the first of these booleans is true (||) or false (&&).

For example:

// 2 == 2 will never get evaluated because it is already clear from evaluating
// 1 != 1 that the result will be false.
(1 != 1) && (2 == 2)

// 2 != 2 will never get evaluated because it is already clear from evaluating
// 1 == 1 that the result will be true.
(1 == 1) || (2 != 2)

Solution 3:

Short circuiting means the second operator will not be checked if the first operator decides the final outcome.

E.g. Expression is: True || False

In case of ||, all we need is one of the side to be True. So if the left hand side is true, there is no point in checking the right hand side, and hence that will not be checked at all.

Similarly, False && True

In case of &&, we need both sides to be True. So if the left hand side is False, there is no point in checking the right hand side, the answer has to be False. And hence that will not be checked at all.

Solution 4:

boolean a = (x < z) && (x == x);

This kind will short-circuit, meaning if (x < z) evaluates to false then the latter is not evaluated, a will be false, otherwise && will also evaluate (x == x).

& is a bitwise operator, but also a boolean AND operator which does not short-circuit.

You can test them by something as follows (see how many times the method is called in each case):

public static boolean getFalse() {
    System.out.println("Method");
    return false;
}

public static void main(String[] args) {
    if(getFalse() && getFalse()) { }        
    System.out.println("=============================");        
    if(getFalse() & getFalse()) { }
}

Solution 5:

In plain terms, short-circuiting means stopping evaluation once you know that the answer can no longer change. For example, if you are evaluating a chain of logical ANDs and you discover a FALSE in the middle of that chain, you know the result is going to be false, no matter what are the values of the rest of the expressions in the chain. Same goes for a chain of ORs: once you discover a TRUE, you know the answer right away, and so you can skip evaluating the rest of the expressions.

You indicate to Java that you want short-circuiting by using && instead of & and || instead of |. The first set in your post is short-circuiting.

Note that this is more than an attempt at saving a few CPU cycles: in expressions like this

if (mystring != null && mystring.indexOf('+') > 0) {
    ...
}

short-circuiting means a difference between correct operation and a crash (in the case where mystring is null).