Java:编译时Object的隐式转换不起作用,但可以使用反射

Question

给出以下代码:

import java.lang.reflect.Array;
import java.lang.reflect.Method;

public class Test {

  public static void test_function(int[] values) {
    System.out.println("Class of values : " + values.getClass());
    System.out.println(values[0]);
  }

  public static void main(String[] args) {

    Object a = Array.newInstance(int.class, 3);
    Array.set(a, 0, 5);

    Class a_class         = a.getClass();
    Class int_array_class = int[].class;

    System.out.println("Class of int[]  : " + int[].class); 
    System.out.println("Class of a      : " + a.getClass());
    System.out.println("int_array_class : " + int_array_class);
    System.out.println("a_class         : " + a_class);

    // These instructions will provoke a compile-time error
    // test_function(a);
    // test_function(a_class.cast(a));
    // test_function(int_array_class.cast(a));

    // The following call won't cause any problem
    test_function((int[]) a);

    Method testf = null;
    try { 
      testf = Test.class.getMethod("test_function", int[].class);
      testf.invoke(null, a); // Does not provoke exception at run-time either
      testf.invoke(null, a_class.cast(a));
      testf.invoke(null, int_array_class.cast(a));
    }
    catch(Exception e) { 
      e.printStackTrace();
    }

  }

}

我注意到显式地将Object传递给方法不起作用.如果取消注释,三条注释说明将引发以下错误:

Test.java:25: error: method test_function in class Test cannot be applied to given types;
    test_function(a);
    ^
  required: int[]
  found: Object
  reason: actual argument Object cannot be converted to int[] by method invocation conversion
Test.java:26: error: method test_function in class Test cannot be applied to given types;
    test_function(a_class.cast(a));
    ^
  required: int[]
  found: Object
  reason: actual argument Object cannot be converted to int[] by method invocation conversion
Test.java:27: error: method test_function in class Test cannot be applied to given types;
    test_function(int_array_class.cast(a));
    ^
  required: int[]
  found: Object
  reason: actual argument Object cannot be converted to int[] by method invocation conversion
3 errors

但是,通过反射传递对象可以正常工作并产生预期的结果.

Class of int[]  : class [I
Class of a      : class [I
int_array_class : class [I
a_class         : class [I
Class of values : class [I
5
Class of values : class [I
5
Class of values : class [I
5
Class of values : class [I
5

所以问题是:为什么编译器拒绝编译在运行时工作正常的三个命令指令？

Answer 1

反射访问能够在运行时确定对象的实际类型.另一方面,编译器只知道声明的类型.它没有任何其他检查实际类型参数的可能性.

您的变量a声明为Object.因此

test_function(a);

您尝试使用void test_function(Object)根本不存在的签名调用方法.一个简单的演员 - 就​​像你和程序员一样知道正确的类型 - 会在这里有所帮助:

test_function((int[]) a); // works fine (even at runtime)

另外两个方法调用是相同的,但有点不同.你声明了a_class和int_array_class类型Class.这是一个原始类型,因此编译器不知道这些变量可以引用哪种类型.编译器必须推断类型Object.这导致了与以前相同的问题(和解决方案).

但是,它Class是一种通用类型,可以参数化.通过以下更改,最后一个方法调用无需显式转换:

Class<int[]> int_array_class = int[].class;
test_function(int_array_class.cast(a));