在调用instanceof之前需要进行空检查(Is null check needed before calling instanceof?)

在调用instanceof之前需要进行空检查(Is null check needed before calling instanceof?)

将null instanceof SomeClass返回false或抛出NullPointerException

Will null instanceof SomeClass return false or throw a NullPointerException?

最满意答案

不，在使用instanceof之前不需要空检查。

如果x为null则表达式x instanceof SomeClass为false 。

从Java语言规范， http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.20.2

“在运行时，如果RelationalExpression的值不为null，并且可以将引用转换为ReferenceType，而不引发ClassCastException，则instanceof运算符的结果为true，否则结果为false。

所以如果操作数为null，结果为false。

No, a null check is not needed before using instanceof.

The expression x instanceof SomeClass is false if x is null.

From the Java Language Specification, section 15.20.2, "Type comparison operator instanceof":

"At run time, the result of the instanceof operator is true if the value of the RelationalExpression is not null and the reference could be cast to the ReferenceType without raising a ClassCastException. Otherwise the result is false."

So if the operand is null, the result is false.