Java中布尔表达式求值顺序？
假设我有以下表达式，
String myString = getStringFromSomeExternalSource();
if (myString != null && myString.trim().length() != 0) {
...
}
Eclipse 警告我布尔表达式的第二个短语中的 myString 可能为 null。但是，我知道如果第一个条件失败，某些编译器将完全退出布尔表达式。 Java 也是这样吗？或者评估顺序无法保证？
Suppose I have the following expression
String myString = getStringFromSomeExternalSource();
if (myString != null && myString.trim().length() != 0) {
...
}
Eclipse warns me that myString might be null in the second phrase of the boolean expression. However, I know some that some compilers will exit the boolean expression entirely if the first condition fails. Is this true with Java? Or is the order of evaluation not guaranteed?
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评论（4）
是的，这就是所谓的短路评估。类似
&& 的运算符;和||是执行此类操作的运算符。不，保证评估顺序（从左到右）
Yes, that is known as Short-Circuit evaluation.Operators like
&&and||are operators that perform such operations.No,the order of evaluation is guaranteed(from left to right)
Java 应该从左到右评估你的语句。它使用一种称为短路评估的机制来防止第二个、第三个和第n个如果第一个条件为假，则条件不被测试。
因此，如果您的表达式为
myContainer != null && myContainer.Contains(myObject)且myContainer为 null，则不会评估第二个条件myContainer.Contains(myObject)。编辑：正如其他人提到的，Java 特别具有用于布尔条件的短路和非短路运算符。使用
&&将触发短路评估，而&则不会。Java should be evaluating your statements from left to right. It uses a mechanism known as short-circuit evaluation to prevent the second, third, and nth conditions from being tested if the first is false.
So, if your expression is
myContainer != null && myContainer.Contains(myObject)andmyContaineris null, the second condition,myContainer.Contains(myObject)will not be evaluated.Edit: As someone else mentioned, Java in particular does have both short-circuit and non-short-circuit operators for boolean conditions. Using
&&will trigger short-circuit evaluation, and&will not.詹姆斯和艾德是正确的。如果您希望对所有表达式进行求值，而不管之前的失败条件如何，则可以使用非短路布尔运算符
&。James and Ed are correct. If you come across a case in which you would like all expressions to be evaluated regardless of previous failed conditions, you can use the non-short-circuiting boolean operator
&.是的，Java 采用这种方式对 if 语句进行惰性求值。如果 myString==null，则 if 语句的其余部分将不会被计算
Yes, Java practices lazy evaluation of if statements in this way. if myString==null, the rest of the if statement will not be evaluated