在下面的代码中,x的类型是I(虽然x也实现了J但在编译时不知道),为什么(1)处的代码不会导致编译时错误.
因为在编译时只考虑引用的类型.
因为在编译时只考虑引用的类型.
public class MyClass {
public static void main(String[] args) {
I x = new D();
if (x instanceof J) //(1)
System.out.println("J");
}
}
interface I {}
interface J {}
class C implements I {}
class D extends C implements J {}
解决方法
instanceof用于运行时确定对象的类型.您正在尝试确定在程序运行时x是否真的是J类型的对象,因此它会进行编译.
您是否认为它会导致编译时错误,因为您认为编译器不知道x的类型？
编辑
正如Kirk Woll评论的那样(感谢Kirk Woll！),如果你正在检查x是否是具体类的实例,并且编译器可以确定x的类型,那么在编译时会出现错误.
从Java语言规范：
If a cast of the RelationalExpression to the ReferenceType would be rejected as a compile-time error,then the instanceof relational expression likewise produces a compile-time error. In such a situation,the result of the instanceof expression could never be true.
作为一个例子：
import java.io.Serializable;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.io.ObjectInputStream;
class SerializableClass implements Serializable
{
private writeObject(ObjectOutputStream out) {}
private readObject(ObjectInputStream in) {}
}
public class DerivedSerializableClass extends SerializableClass
{
public static void main(String[] args)
{
DerivedSerializableClass dsc = new DerivedSerializableClass();
if (dsc instanceof DerivedSerializableClass) {} // fine
if (dsc instanceof Serializable) {} // fine because check is done at runtime
if (dsc instanceof String) {} // error because compiler knows dsc has no derivation from String in the hierarchy
Object o = (Object)dsc;
if (o instanceof DerivedSerializableClass) {} // fine because you made it Object,so runtime determination is necessary
}
}
