下面哪个比较好?

a instanceof B

or

B.class.isAssignableFrom(a.getClass())

我所知道的唯一区别是,当'a'为空时,第一个返回false,而第二个抛出异常。除此之外,它们总是给出相同的结果吗?


当前回答

我们在团队中做的一些测试表明,A.class. isassignablefrom (B. getclass())比B instanceof a工作得更快,如果你需要对大量元素进行检查,这是非常有用的。

其他回答

isAssignableFrom(A, B) =

if (A == B) return true
else if (B == java.lang.Object) return false
else return isAssignableFrom(A, getSuperClass(B))

The pseudo code above is a definition of, if references of type/class A is assignable from references of type/class B. It is a recursive definition. To some it may be helpful, for others it may be confusing. I add it in case somebody should find it useful. This is just an attempt to capture my understanding, it is not the official definition. It is used in a certain Java VM implementation and works for many example programs, so while I cannot guarentee that it captures all aspects of isAssignableFrom, it is not completely off.

Instanceof只能用于引用类型,不能用于基本类型。isAssignableFrom()可以用于任何类对象:

a instanceof int  // syntax error
3 instanceof Foo  // syntax error
int.class.isAssignableFrom(int.class)  // true

看到http://java.sun.com/javase/6/docs/api/java/lang/Class.html isAssignableFrom (. lang . class)。

更直接的等价于B的实例是

B.class.isInstance(a)

当a为空时,这也可以工作(返回false)。

在使用instanceof时,您需要在编译时知道B的类。当使用isAssignableFrom()时,它可以是动态的,并在运行时更改。

还有另一个区别:

不管X是什么,null instanceof X都是假的

null.getClass().isAssignableFrom(X)将抛出NullPointerException