Java: NoSuchMethodException

Question

For my program the goal is to use a bunch of interfaces and create a program that makes a test and allows the user to take it. The specific issue I have is with the interface IQuestionFactory, which assists in making questions in the TestMaker class. I'm using a class loader given by my professor, which uses the name of the class I need to load as a parameter. I cannot figure out why I'm getting the error.

Error

java.lang.NoSuchMethodException: test.api.IQuestionFactory.<init>()
    at java.lang.Class.getConstructor0(Unknown Source)
    at java.lang.Class.getConstructor(Unknown Source)
    at test.util.MyClassLoader.createInstance(MyClassLoader.java:37)
    at test.maker.TestMaker.main(TestMaker.java:13)
Exception in thread "main" java.lang.NullPointerException
    at test.maker.TestMaker.main(TestMaker.java:14)

ClassLoader

package test.util;

import java.lang.reflect.Constructor;

public class MyClassLoader {

    //instead of using the constructor, we provide a single instance of MyClassLoader
    public static MyClassLoader instance = new MyClassLoader();

    //by making the constructor private, we ensure that it can't be called.
    private MyClassLoader() {

    }

    /**
     * Load class of the given className and create an object of that class.
     * @param className the name of the class including its package. e.g.  test.impl.QuestionFactory
     * @return return the object created.
     */
    public Object createInstance(String className) {
        try {
            ClassLoader classLoader = this.getClass().getClassLoader();
            Class loadedMyClass = classLoader.loadClass(className);
            Constructor constructor = loadedMyClass.getConstructor();
            Object myClassObject = constructor.newInstance();
            return myClassObject;
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }
}

TestMaker Class

package test.maker;

import test.api.IQuestion;
import test.api.IQuestionFactory;
import test.api.ITestSet;
import test.util.MyClassLoader;

public class TestMaker {
    public static void main (String[] args){

        MyClassLoader cl = MyClassLoader.instance;

        IQuestionFactory factory = (IQuestionFactory) cl.createInstance("test.api.IQuestionFactory");
        ITestSet testset = factory.makeEmptyTestSet();

        String question = "Which of the following people was not a US president?";
        String[] choices = {"George Washington", "Benjamin Franklin", "Andrew Jackson", "Mr. Rodgers"};
        int answer = 1;

        IQuestion q = factory.makeMultipleChoice(question, choices, answer);
        testset.add(q);

        factory.save(testset,"Questions");

    }
}

IQuestionFactory Interface

package test.api;

import java.io.IOException;


public interface IQuestionFactory {
    /**
     * @param question The question
     * @param choices The choices as an array
     * @param answer The index to the answer in the choices array
     * @return An instance of a multiple choice question.
     */
    public IQuestion makeMultipleChoice(String question, String[] choices, int answer);


    /**
     * @param question The question.
     * @param answer The answer
     * @return an instance of a true-false question
     */
    public IQuestion makeTrueFalse(String question, boolean answer);

    /** 
     * @param question The question, including the blanks
     * @param answers Array of answers to the blanks
     * @return an instance of a fill-in-the-blank question
     */
    public IQuestion makeFillInBlank(String question, String [] answers);

    /**
     * @param question The question.
     * @param keywords The answers as a list of key words.  
     * @return an instance of a short answer question.
     */
    public IQuestion makeShortAnswer(String question, String[] keywords);

    /**
     * @param filename The file containing the test set.
     * @return A Test set
     * @throws IOException if can't load the file.
     */
    public ITestSet load(String filename);


    /**
     * @param testSet The test set to be stored.
     * @param filename  The filename to be used.
     * @return true if save is successful
     * @throws IOException if unable to save the test set.
     */
    public boolean save(ITestSet testSet, String filename);


    /**
     * Create an empty test set.
     * 
     * @return an empty test set.
     */
    public ITestSet makeEmptyTestSet();


}

Answer 1

You are trying to create an instance of an interface, which doesn't make any sense. You can only create an instance of a class. You need to implement the interface with another class and use that class as the parameter for cl.createInstance.

Edit: Although I honestly have no idea why you're using reflection here at all. You should probably be doing something like this:

IQuestionFactory factory = new MyQuestionFactory()

Where MyQuestionFactory implements IQuestionFactory. It's more efficient like that and then the compiler would catch your mistakes. What you are doing right now is:

IQuestionFactory factory = new IQuestionFactory()

And the compiler would catch that.