Java getMethod with subclass parameter

Question

I'm writing a library that uses reflection to find and call methods dynamically. Given just an object, a method name, and a parameter list, I need to call the given method as though the method call were explicitly written in the code.

I've been using the following approach, which works in most cases:

static void callMethod(Object receiver, String methodName, Object[] params) {
    Class<?>[] paramTypes = new Class<?>[params.length];
    for (int i = 0; i < param.length; i++) {
        paramTypes[i] = params[i].getClass();
    }
    receiver.getClass().getMethod(methodName, paramTypes).invoke(receiver, params);
}

However, when one of the parameters is a subclass of one of the supported types for the method, the reflection API throws a NoSuchMethodException . For example, if the receiver's class has testMethod(Foo) defined, the following fails:

receiver.getClass().getMethod("testMethod", FooSubclass.class).invoke(receiver, new FooSubclass());

even though this works:

receiver.testMethod(new FooSubclass());

How do I resolve this? If the method call is hard-coded there's no issue - the compiler just uses the overloading algorithm to pick the best applicable method to use. It doesn't work with reflection, though, which is what I need.

Thanks in advance!

Answer 1

It's a bit longer than what you started with, but this does what you asked for... and a little more besides - for example, callMethod(receiver, "voidMethod") where voidMethod takes no arguments also works.

static void callMethod(Object receiver,
      String methodName, Object... params) {
  if (receiver == null || methodName == null) {
    return;
  }
  Class<?> cls = receiver.getClass();
  Method[] methods = cls.getMethods();
  Method toInvoke = null;
  methodLoop: for (Method method : methods) {
    if (!methodName.equals(method.getName())) {
      continue;
    }
    Class<?>[] paramTypes = method.getParameterTypes();
    if (params == null && paramTypes == null) {
      toInvoke = method;
      break;
    } else if (params == null || paramTypes == null
        || paramTypes.length != params.length) {
      continue;
    }

    for (int i = 0; i < params.length; ++i) {
      if (!paramTypes[i].isAssignableFrom(params[i].getClass())) {
        continue methodLoop;
      }
    }
    toInvoke = method;
  }
  if (toInvoke != null) {
    try {
      toInvoke.invoke(receiver, params);
    } catch (Exception t) {
      t.printStackTrace();
    }
  }
}

Answer 2

receiver.testMethod(new FooSubclass());
even though this works:

If your testMethod function has parameter of FooSuperClass type:

 public void testMethod(FooSuperClass object){}

then, while you are trying to get a matching method with reflection: getClass().getMethod("testMethod", FooSubclass.class) will result in NoSuchMethodException . Because this getMethod(String name, Class<?>... parameterTypes function returns a Method object which is a public member method with the given name where parameterTypes parameter is an array of Class objects that identify the method's formal parameter types . There is actually no such method is declared with signature testMedthod(FooSubClass object) as the formal parameter type of the function is FooSuperClass . So, the correct invocation is:

receiver.getClass().getMethod("testMethod", FooSuperClass.class)
                        .invoke(receiver, new FooSubclass());

or , passing the super class by calling SubClass.class.getSuperClass() as follows:

receiver.getClass().getMethod("testMethod", FooSubClass.class.getSuperclass())
                            .invoke(receiver, new FooSubclass());

or , changing the method signature to: public void testMethod(FooSubClass object){} and then invoke as you are doing now:

receiver.getClass().getMethod("testMethod", FooSubclass.class)
                         .invoke(receiver, new FooSubclass());