我在我的代碼中有一個nullPointerException，我不知道如何解決

我在一個編程類，我正在研究一個計算和顯示您必須做多少作業問題的程序。這個程序考慮到你的教授可能會分配古怪的東西，比如只做其他奇數或奇數。這個程序必須有兩個類編寫在不同的java文件中，以加強我們在課堂上的工作。當我編譯程序時，沒有錯誤。當我嘗試運行它，這是我的錯誤：我在哪裏出錯難倒我在我的代碼中有一個nullPointerException，我不知道如何解決

顯示java.lang.NullPointerException

at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) 
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) 
at java.lang.reflect.Method.invoke(Unknown Source) 
at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:272) 


import java.util.*; 

public class HomeworkCounter 
{ 
    public void main(String[] args) 
    { 
    Operation operations = new Operation(); 
    Scanner keys = new Scanner(System.in); 
    String eoo = "", eo = ""; 
    System.out.print("Is it EOO?"); 
    keys.nextLine(); 
    eoo = keys.nextLine(); 
    if (eoo.equals("yes")) 
     operations.everyOtherOdd(); 
    System.out.print("Is it every odd?"); 
    eo = keys.nextLine(); 
    if (eo.equals("yes")) 
     operations.everyOdd(); 
    else 
     operations.allProblems(); 
    System.out.println("You have" + operations.total + "problems to finish."); 
    keys.close(); 
    } 
} 

import java.util.Scanner; 

public class Operation 
{ 
    private int start = 0; 
    private int finish = 0; 
    public int total = 0; 
    private int extras = 0; 
    Scanner keys = new Scanner(System.in); 

    public int everyOtherOdd() 
    { 
    System.out.print("Please enter your starting number: "); 
    start = keys.nextInt(); 
    total = start; 
    System.out.print("Please enter your last number: "); 
    System.out.print("How many 'extra' problems do you have?"); 
    extras = keys.nextInt(); 

    while (total <= finish) 
    { 
     System.out.println(total); 
     total = total + 4; 
    } 
    total = total + extras; 
    return total; 
    } 

    public int everyOdd() 
    { 
    System.out.print("Please enter your starting number: "); 
    start = keys.nextInt(); 
    total = start; 
    System.out.print("Please enter your last number: "); 
    System.out.print("How many 'extra' problems do you have?"); 
    extras = keys.nextInt(); 

    while (total <= finish) 
    { 
     System.out.println(total); 
     total = total + 2; 
    } 
    total = total + extras; 
    return total; 
    } 

    public int allProblems() 
    { 
    System.out.print("Please enter your starting number: "); 
    start = keys.nextInt(); 
    total = start; 
    System.out.print("Please enter your last number: "); 
    System.out.print("How many 'extra' problems do you have?"); 
    extras = keys.nextInt(); 

    while (total <= finish) 
    { 
     System.out.println(total); 
     total = total + 1; 
    } 
    total = total + extras; 
    return total; 
    } 
}

。非常感謝您提供答案。

來源

2015-02-12 Austen Clay

粘貼你的錯誤堆棧跟蹤，我們可以幫你找到位置！ – ppuskar 2015-02-12 04:35:00