问题
Possible Duplicate:
Does java evaluate remaining conditions after boolean result is known
Why do we usually use || not |, what is the difference?
I missed my class lecture the other day and I was wondering if anyone could give an explanation what short circuiting is and maybe an example of it being used in a simple Java program. Thanks for your help!
回答1:
Short-circuiting is where an expression is stopped being evaluated as soon as its outcome is determined. So for instance:
if (a == b || c == d || e == f) {
// Do something
}
If a == b is true, then c == d and e == f are never evaluated at all, because the expression's outcome has already been determined. if a == b is false, then c == d is evaluated; if it's true, then e == f is never evaluated. This may not seem to make any difference, but consider:
if (foo() || bar() || baz()) {
// Do something
}
If foo() returns true, then bar and baz are never called, because the expression's outcome has already been determined. So if bar or baz has some other effect than just returning something (a side effect), those effects never occur.
One great example of short-circuiting relates to object references:
if (a != null && a.getFoo() != 42) {
// Do something
}
a.getFoo() would normally throw a NullPointerException if a were null, but because the expression short-circuits, if a != null is false, the a.getFoo() part never happens, so we don't get an exception.
Note that not all expressions are short-circuited. The || and && operators are short-circuited, but | and & are not, nor are * or /; in fact most operators are not.
回答2:
Short-circuit evaluation means that when evaluating boolean expressions (logical AND and OR) you can stop as soon as you find the first condition which satisfies or negates the expression.
For example, suppose you were evaluating a logical OR with several sub-expressions, each of which is very costly to evaluate:
if (costlyTest1() || costlyTest2() || costlyTest3()) { // ...
The JVM can stop evaluating the "costlyTest" functions as soon as it finds one that returns true, since that will satisfy the boolean expression. So if costlyTest1() returns true then the other tests will be skipped. Similarly:
if (costlyTest1() && costlyTest2() && costlyTest3()) { // ...
The JVM can stop evaluating the functions as soon as it finds one that returns false, since that also satisfies the expression; so if costlyTest1() returns false then the other functions will not be called.
These rules pertain with any level of nesting of boolean expressions and can be taken advantage of to avoid unnecessary work, as demonstrated in the examples above.
回答3:
Short Circuit: If the first part is true don't bother evaluating the rest of the expression. Same logic applies for false in the case of && which is also short circuiting
回答4:
Short-circuiting the evaluation of an expression means that only a part of the expression needs to be evaluated before finding its value. For example:
a == null || a.size() == 0
If a is null, the a.size() == 0 subexpression won't be evaluated, because the boolean operator || evaluates to true if one of its operands is true.
Similarly, for this expression:
a != null && a.size() > 0
If a is null, then the a.size() > 0 won't be evaluated, because the boolean operator && evaluates to false if one of its operands is false.
In the above examples the boolean operators && and || are said to be short-circuit, given the fact that the second operand might not be evaluated if the value of the first operand is enough to determine the value of the whole expression. For comparison, the & and | operands are the equivalent non-short-circuit boolean operators.
回答5:
Short circuiting is an alternative way of using the logical AND or OR operators (& or |)
e.g. a non short-circuit OR
if(false | true) {
}
The first condition and second condition are both evaluated even if false is not true (which it always is).
However it is was written as a short-circuit OR:
if(false || true) {
}
The first condition is only evaluated since it's false, true isn't evaluated since it's not required.
来源：https://stackoverflow.com/questions/9344305/what-is-short-circuiting-and-how-is-it-used-when-programming-in-java