Getting Declared Methods - Java

Question

Suppose I have the following three classes

A.java

public class A {
    public static void main (String[] args) {
        new C();
    }
}

B.java

import java.lang.reflect.Method;
import java.util.Arrays;

public class B {
    public B() {
        Method[] m = this.getClass().getDeclaredMethods();
        System.out.println(Arrays.toString(m));
    }

    public void hello() {

    }
}

C.java

public class C extends B{
    public C() {
        super();
    }
}

and I run the main method in class A. Well, it should instantiate class C, which in turn should call class B's constructor, to print out the declared methods. The output is []. This is a surprise to me, as I was expecting the output to be (assuming all classes are in package called test):

[public void test.B.hello()]

So, what's wrong? And how do I get it so that this is the actual output?

Answer 1

getClass returns the class of the instance. In this case, this is C. There is no declared method in class C. You can solve your problem by also using the 'getSuperClass' method of the returned Class object:

Class c = this.getClass();
while (c != null) {
  System.out.println(Arrays.toString(c.getDeclaredMethods()));
  c = c.getSuperClass();
}

Answer 2

The key here is use of the word this That implies the present context, which if C is a concrete subclass of B is C. To get B's methods, you need to traverse until you get to B. That's probably application logic dependent. One way would be the following.

import java.lang.reflect.Method;
import java.util.Arrays;

public class B {
    public B() {
        Class<?> parent = getClass();
        while(parent.getSuperclass() != null) {
            if(parent.getSuperclass() != Object.class) {
                parent = parent.getSuperclass();
            } else {
                break;
            }
        }
        Method[] m = parent.getDeclaredMethods();
        System.out.println(Arrays.toString(m));
    }

    public void hello() {

    }
}

Answer 3

//Actually one can get the number of methods in different classes under the //different packages....
//Go and check the results................

import java.lang.reflect.Method;


    public class TestPackage {
        public static void main(String args[]) throws ClassNotFoundException
        {
            int count=0;
            Class c=Class.forName("java.util.Scanner");
            Method[] m=c.getDeclaredMethods();
            for(Method m1:m)
            {
                count++;
                System.out.println(m1.getName());
            }
            System.out.println("The no. of methods are:"+count);

        }

    }