Java类的接口 [英] Java Casting Interface to Class

问题描述

public class InterfaceCasting {

    private static class A{}

    public static void main(String[] args) {
        A a = new A();
        Serializable serializable = new Serializable(){};
        a = (A)serializable;
    }

}

编译成功，但运行时异常

Compilation succeed but Runtime exception

Exception in thread "main" java.lang.ClassCastException: InterfaceCasting$1 cannot be cast to InterfaceCasting$A

为什么要编译成功？编译器必须知道serialiazable不是A？

WHY COMPILATION SUCCEED? Compiler must known that serialiazable is not A?

推荐答案

正如你所指出的，

interface MyInterface {}

class A {}

public class InterfaceCasting {
    public static void main(String[] args) {
        MyInterface myObject = new MyInterface() {};
        A a = (A) myObject;
    }
}

编译：编译：

This however, will not compile:

interface MyInterface {}

class A {}

public class InterfaceCasting {
    public static void main(String[] args) {
        A a = (A) new MyInterface() {}; // javac says: "inconvertible types!"
    }
}

有什么区别？

好吧，由于 MyInterface 只是一个接口， / em>非常好地通过扩展A 的类实现，在这种情况下，从 MyInterface 到 A 将是合法的。

Well, since MyInterface is simply an interface, it could very well be implemented by a class that extends A, in which case the cast from MyInterface to A would be legal.

这段代码将在所有执行的50％ / strong>，并说明编译器需要解决可能的不可判定的问题，以便在编译时总是检测非法的转换。

This code for instance, will succeed in 50% of all executions, and illustrates that the compiler would need to solve possibly undecidable problems in order to always "detect" illegal casts at compile time.

interface MyInterface {}

class A {}

class B extends A implements MyInterface {}

public class InterfaceCasting {
    public static void main(String[] args) {
        MyInterface myObject = new MyInterface() {};
        if (java.lang.Math.random() > 0.5)
            myObject = new B();
        A a = (A) myObject;
    }
}

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