Question

得到一个空指针异常，我不懂，因为我是java的新手。

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);

最后一行是我得到错误的地方。当我说

时，我认为它已经初始化了

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

IAtmHelper基本上是空的：

public class AtmUtils {

    public static IAtmHelper getAtmHelper() 
    {
        // this dummy impl is just included to satisfy compile-time errors
        return null;
    }
}

这是使用的ATMHelper类的顶部：

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 {

这是函数

public IUserAction getUserResponse(String argMessage,
            ActionType... argValidActionTypes);

他们并没有真正做过anthing，因为这不是一个真正的项目，但更像是一个测试。我基本上理解如何做所有这些，但我不能在主要的开头绕过这个空指针。感谢

Answer 1

它正在抛出一个空指针，因为你要返回一个 null 对象：

public static IAtmHelper getAtmHelper() 
    {
        // this dummy impl is just included to satisfy compile-time errors
        return null; // <-- I am causing your NPE
    }

您可能希望从函数中返回AtmHelper的实例，如下所示：

  public static IAtmHelper getAtmHelper() 
    {
        return new AtmHelper();
    }

获取空指针异常，Java新手

1 个答案: