不理解我的代码中的空指针异常（java）

Question

我知道什么是空指针异常，但我不明白为什么我会得到它。

以下是引发错误的行：

IUserAction pinResponse = ATM.getUserResponse("Please enter your 4-digit PIN.",    ActionType.STRING); 

IUserAction是一个公共接口类，ActionType是一个枚举。

这很奇怪，因为我被添加了代码，这条线被赋予了我，我认为它应该可以工作，而我不熟悉Java，我不是新的面向对象编程，所以我不确定为什么这不起作用。

这里有更多代码：

package com.atm;

import com.atm.IUserAction.ActionType;

/**
 * The ATM implementation class.
 * 
 * @author mshields
 * 
 */
public class AtmImpl {

    private static final IAtmHelper ATM = AtmUtils.getAtmHelper();

    /**
     * This method is invoked when a bank card is inserted into the ATM. The card
     * is ejected immediately upon completion of this method, which represents the
     * user's ATM session.
     * 
     * @param args
     *          the bank card number is the only argument.
     */
    public static void main(String args[]) {

        IUserAction pinResponse = ATM.getUserResponse("Please enter your 4-digit PIN.", ActionType.STRING);

        if (ActionType.STRING.equals(pinResponse.getActionType())
                && ATM.verifyPIN(args[0], pinResponse.getStringEntry())) 
        {
            // todo: ...

继承IUserAction的开始

package com.atm;

import java.math.BigDecimal;

/**
 * Interface that defines a user action at the ATM.
 * 
 * @author mshields
 * 
 */
public interface IUserAction {

    public enum ActionType {
        /**
         * Indicates that the user typed a series of characters or digits (for
         * example, a PIN).
         */
        STRING,

和ATM

package com.atm;

import java.math.BigDecimal;
import java.util.List;

import com.atm.IUserAction.ActionType;

/**
 * Interface that defines ATM utility methods.
 * 
 * @author mshields
 * 
 */
public interface IAtmHelper {