反射中方法的参数数量未知

Question

For example I have a class PartE such:

public class PartE implements Part
{
  protected String method = "getIndex" ;

  protected String getIndex ( double dummy,int dummyInt,String dummyString )
  {
    return "E" ;
  }
}

In this code I should invoke a method with unknown number of arguments. In this case it is getIndex(double,int,String) .

It can be in this way:

getIndex()
getIndex(int)
getIndex(boolean,int)
getIndex(int,double,boolean)

So whatever combination of arguments, I have done this but it seems that I am doing error in params[i].newInstance() because it returns not proper thing for instantiation. That's why I am taking InstantiationException . I got stuck what can I do?

Class c1 = null;

try {
    c1 = Class.forName( "PartE" );
} catch (ClassNotFoundException ex) {}

Object obj = null;
obj = c1.newInstance();

Method [] methods = c1.getDeclaredMethods();

for(Method m:methods)
  {
      m.setAccessible(true);

      Class<?>[] params = m.getParameterTypes();
      Object[] paramObjects = new Object[params.length];

      for (int i = 0; i < params.length; i++) 
      {
          paramObjects[i] = params[i].newInstance();
      }

      try {
         System.out.println(m.invoke(obj, paramObjects));  
      } catch (IllegalAccessException | IllegalArgumentException | InvocationTargetException ex) {}

Answer 1

You need to handle the possibility that the class you need doesn't support a no-params constructor, since newInstance requires one but neither Integer nor Double has one. That means you'll need to add branching within your loop over the parameter types, to at least handle Integer and Double . Presumably the assignment tells you what values you should use (or that it doesn't matter what values you use) for those arguments, since you can't pass null (you'll get an unboxing error if you use null since the method expects int and double , not Integer and Double ) and they don't have a no-params constructor.

This answer is intentionally given as prose rather than code because the point of the assignment, presumably, is to learn to write that code. :-)