Java:NoSuchMethodException

Question

对于我的程序，目标是使用一堆接口(interface)并创建一个程序来进行测试并允许用户进行测试。我遇到的具体问题是 IQuestionFactory 接口(interface)，它有助于在 TestMaker 类中提出问题。我正在使用我的教授提供的类加载器，它使用我需要加载的类的名称作为参数。我不明白为什么会收到错误。

错误

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)

类加载器

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 类

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接口(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

您正在尝试创建接口(interface)的实例，这没有任何意义。您只能创建类的实例。您需要使用另一个类来实现该接口(interface)，并使用该类作为 cl.createInstance 的参数。

编辑:虽然我真的不知道你为什么在这里使用反射。你可能应该做这样的事情:

IQuestionFactory factory = new MyQuestionFactory()

其中 MyQuestionFactory 实现 IQuestionFactory。这样效率更高，然后编译器会发现你的错误。您现在正在做的是:

IQuestionFactory factory = new IQuestionFactory()

编译器会捕捉到这一点。