Getting NoSuchMethodException from getMethod using Java reflection

Question

I'm testing with Java reflection and trying to apply overloaded method to parameters according to their type..

However, I have NoSuchMethodException even though the method I tried to get is public. This exception still appears when I used getDeclaredMethod.

Here's the main program

public class Test {

    public static void main(Object... inputs){

        InputManipulation test = new InputManipulation();

        for (Object input: inputs){
            Class ownerClass = test.getClass();
            Class inputClass = input.getClass();
            Method method = ownerClass.getDeclaredMethod("add", inputClass);
            method.invoke(test, "Testing reflection");
        }

    }
}

And here's the self-defined InputManipulation class

public class InputManipulation {

    Integer total;

    public InputManipulation(){this.total = 0;}

    public void add(Integer num){this.total += num;}
    public void add(String str){System.out.println(str);}
}

Thanks in advance!

I now changed the Test class as follows.. but the problem still exists.

public class Test {

    public static void main(String[] args){
        Test testExample = new Test();
        testExample.testMethod("String1", 1, "String2");
    }

    public void testMethod(Object... inputs){
        InputManipulation test = new InputManipulation();
        for (Object input: inputs){
            Class ownerClass = test.getClass();
            Class inputClass = input.getClass();
            Method method = ownerClass.getDeclaredMethod("add", inputClass);
            method.invoke(test, "Testing reflection");
        }

    }
}

I also tried putting the inputClass into an array of Class, as suggested by another post, but it didn't help..

Answer 1

There seems to be a few issues with the initial code you provided and as others have suggested using an IDE would have pointed some of the issues out pretty quickly. However, I have taken your update and fixed the code to call the proper method in the loop you provided of input types.

First change your InputManipulation class like so:

public class InputManipulation {

    Integer total;

    public InputManipulation() {
        this.total = 0;
    }

    public void add(Integer num) {
        this.total += num;
        System.out.println(this.total);
    }

    public void add(String str) {
        System.out.println(str);
    }

}

Now alter your Test class like so:

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

public class Test {

    public static void main(String[] args) {
         Test testExample = new Test();
            testExample.testMethod("String1", 1, "String2");
    }

     public void testMethod(Object... inputs){
            InputManipulation test = new InputManipulation();
            for (Object input: inputs){

                Class<? extends Object> ownerClass = test.getClass();
                Class<? extends Object> inputClass = input.getClass();
                //Method method;    //not needed

                try {
                    ownerClass.getDeclaredMethod("add", inputClass).invoke(test, input);

                } catch (NoSuchMethodException | SecurityException | 
                        IllegalAccessException | IllegalArgumentException |
                        InvocationTargetException e) {

                    e.printStackTrace();
                }

            }

      }
}

I used these readings to help guide my answer, but altered the way I invoked the method:

http://tutorials.jenkov.com/java-reflection/methods.html

Answer 2

public class Test {

public static void main(String[] args) throws NoSuchMethodException, SecurityException, IllegalAccessException, IllegalArgumentException, InvocationTargetException{
    Test testExample = new Test();
    testExample.testMethod("String1", 1, "String2");
}

public void testMethod(Object... inputs) throws NoSuchMethodException, SecurityException, IllegalAccessException, IllegalArgumentException, InvocationTargetException{
    InputManipulation test = new InputManipulation();
    for (Object input: inputs){
        Class ownerClass = test.getClass();
        Class inputClass = input.getClass();
        Method method = ownerClass.getDeclaredMethod("add", inputClass);
        method.invoke(test, input);
    }
}
}

Your problem was caused by this method.invoke(test, "Testing reflection"); You iterate through 2 types of arguments and depends of this argument you invoke method 'add'. When you tried to invoke method with argument Integer you pass to method String parameter that causes error