Java Trying to do reflection(class instance and invoke method at runtime)

Question

Here's my code.

String name = "expevaluator." + super.left.getClass().getSimpleName() + super.right.getClass().getSimpleName() + "Addition";
        try {
            Object instance;
            instance = Class.forName(name).newInstance();
        } catch (ClassNotFoundException | InstantiationException | IllegalAccessException ex) {
            Logger.getLogger(Addition.class.getName()).log(Level.SEVERE, null, ex);
        }

I'm trying to call the evaluate function of the variable "name" class.

Here's the evaluate function.

public Number evaluate(int left, int right) {
    return left + right;
}

That's the evaluate method from IntegerIntegerAddition Class

and I have also the IntegerDoubleAddition Class evaluate method

public Number evaluate(int left, double right) {
    return left + right;
}

So the variable name could have "IntegerIntegerAddition" or "IntegerDoubleAddition" and I want with both of them call to the Evaluate method with the 2 parameters.

Okay, so you need to get the relevant method by reflection and invoke it - what have you tried for that part? — Jon Skeet
– Jon Skeet, Commented Apr 2, 2014 at 15:27
I'm not sure exactly what you're trying to achieve. I feel like theres a bit of missing context here with what you're trying to do. — Julian
– Julian, Commented Apr 2, 2014 at 15:28

Julian · Accepted Answer · 2019-05-08 21:23:18Z

2

You'll want to create a Method object using the name and parameter types that your real method will require. Then you want to use a reference to the object that has that method, and invoke your Method reference on it.

//These are all the types that my example references will use.

Object obj;  //The instance that my method is going to be invoked on.
Object[] params; //The parameters for my method
String nameOfMethod; //The name of my method.
Class<?>[] paramTypes; //The types, in order, that my method accepts as parameters

Method m = obj.getClass().getMethod(nameOfMethod, paramTypes); //Create my Method object

m.invoke(obj, params); //Invoke that method!

getMethod() accepts a vararg for the array paramTypes. So you can just list your types directly in there one by one as seperate parameters. Same thing with Method.invoke() for it's params parameter.

m.invoke(obj, param1, param2, param3) //etc

edited May 8, 2019 at 21:23

answered Apr 2, 2014 at 15:38

Julian

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Peter Flower · Accepted Answer · 2014-04-02 15:39:37Z

0

You could do something like this:

Method evaluate = instance.getClass().getDeclaredMethod( "evaluate", int.class, int.class );
evaluate.invoke( instance, 3, 1 );

This should find the exact method by parameter type ( alternatively use "int.class, long.class" for your second method ) & invoke it, here using example arguments 3 & 1. Remember to catch the possible exceptions.

answered Apr 2, 2014 at 15:39

Peter Flower

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2 Comments

Subiendo Over a year ago

but I dont know the parameters primitive type it could be int int, int double, double int or double double

Julian Over a year ago

If you have overloaded methods that accept different parameters, logic will need to be put somewhere to determine which method is going to be chosen. This might mean combining your methods to accept more generic parameter types and do this type check internally, or checking the types of your values before instantiating the Method object, and having a case to create it using the correct types for the current situation.

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