Question

我有3个类：Main，ContactLibrary和ContactInfo。 ContactLibrary包含一个名为myPhoneBook的ArrayList。 ContactInfo由一串包含名称，地址等的字符串组成。

用户想要搜索名称，例如或涉及输入的任何内容。输入是在ContactLibrary和ContactInfo类中完成的 - 两个可序列化的对象。

然而，当它到达那个点时，我得到一个NPE错误。

You have 3 entry(s) saved to disc.

Hello, and welcome to Team 6's contact list.
What would you like to do?
Enter the corresponding number of choice.

1: Add an entry to the contact list.
2: Print the entire contact list.
3: Search for a contact.
4: Exit the program.

Please enter a number from 1-4.
3
What would you like to search by?
Exception in thread "main" java.lang.NullPointerException
    at ContactLibrary.searchByCriteria(ContactLibrary.java:62)
    at Main.optionsPrompt(Main.java:62)
    at Main.main(Main.java:25)
1: Last Names.
2: Emails.
3: Zip codes.

我该怎么办？

这是我的主要内容：http://ideone.com/uvfK4U（包含顶部评论中的其他两个类）这是一个UML图：http://imgur.com/9W3TS

ContactLibrary类，请求：

/**
 * ContactLibrary, when constructed, creates an ArrayList of ContactLibrary
 * references called myPhoneBook. Every index is made to fill in objects
 * of ContactInfo, which contains entries and credentials.
 * 
 * Contains methods to create a new entry, search and print by criteria, and print list.
 */
import java.util.*;

public class ContactLibrary implements java.io.Serializable {
    private static final long serialVersionUID = 1L;
    private ArrayList<ContactInfo> myPhoneBook;
    private Scanner libraryInput = new Scanner(System.in);

    /** Constructs the ArrayList that will hold references to ContactInfo. */
    public ContactLibrary() {
        myPhoneBook = new ArrayList<ContactInfo>();
    }

    /**
     * Adds an entry to the ArrayList and utilizes the set methods in
     * ContactInfo.
     */
    public void addEntry() {
        int doAgain = 1;
        do {
            myPhoneBook.add(new ContactInfo());
            System.out.println("Would you like to enter another contact?");
            System.out.println("1: Yes.");
            System.out.println("2: No.");
            doAgain = libraryInput.nextInt();
        } while (doAgain == 1);
    }

    /**
     * Goes through every index in myPhoneBook and runs ArrayList.get() on them.
     */
    public void printList() {
        for (int i = 0; i < myPhoneBook.size(); i++) {
            System.out.println(myPhoneBook.get(i));
        }
    }

    /** Counts the number of objects within myPhoneBook and returns a string. */
    public String scanDisc() {
        int entryCount = myPhoneBook.size();
        return "You have " + entryCount + " entry(s) saved to disc.\n";
    }

    /**
     * The prompt for having the user search the database via criteria. Asks the
     * user to enter in their search criteria.
     */
    public void searchByCriteria() {
        String criteria;
        //libraryInput = new Scanner("System.in");
        int subSubMenuChoice = 0;
        System.out.println("What would you like to search by?");
        System.out.println("1: Last Names.");
        System.out.println("2: Emails.");
        System.out.println("3: Zip codes.");
        subSubMenuChoice = libraryInput.nextInt();
        switch (subSubMenuChoice) {
        case 1:
            System.out
                    .println("Please enter the last name you'd like to search for:");
            criteria = libraryInput.next();
            searchByLastName(criteria);
            break;
        case 2:
            System.out
                    .println("Please enter the e-mail you'd like to search for:");
            criteria = libraryInput.next();
            searchByEmail(criteria);
            break;
        case 3:
            System.out
                    .println("Please enter the zip code you'd like to search for:");
            criteria = libraryInput.next();
            searchByZip(criteria);
            break;
        default:
            System.out.println("Exiting");
            break;
        }
    }

    /**
     * Loops through every element in the array and returns a toString of that
     * index for comparing with the search criteria via contains().
     */
    public void searchByEmail(String criteria) {
        for (int i = 0; i < myPhoneBook.size(); i++) {
            if (criteria.compareTo((myPhoneBook.get(i)).getEmail()) == 0) {
                System.out.println(myPhoneBook.get(i));
            } else {
                System.out.print("");
            }
        }
    }

    /**
     * Loops through every element in the array and returns a toString of that
     * index for comparing with the search criteria via contains().
     */
    public void searchByLastName(String criteria) {
        for (int i = 0; i < myPhoneBook.size(); i++) {
            if (criteria.compareTo((myPhoneBook.get(i)).getLastName()) == 0) {
                System.out.println(myPhoneBook.get(i));
            } else {
                System.out.print("");
            }
        }
    }

    /**
     * Loops through every element in the array and returns a toString of that
     * index for comparing with the search criteria via contains().
     */
    public void searchByZip(String criteria) {
        for (int i = 0; i < myPhoneBook.size(); i++) {
            if (criteria.compareTo((myPhoneBook.get(i)).getZipcode()) == 0) {
                System.out.println(myPhoneBook.get(i));
            } else {
                System.out.print("");
            }
        }
    }

    /** Reorganizes the array in order by last name. */
    public void sortData() {
        Collections.sort(myPhoneBook);
    }

}

Answer 1

您正在同一inputStream上创建多个扫描程序对象，最好只创建一个并将其传递给已使用方法中的下一个类或类的构造函数。

在同一个流上使用多个扫描仪。扫描仪可以（并且将会）消耗流 - 这可能（将）导致意外的副作用。最好不要这样做。

您可以使用以下代码替换主类中的第65行：

myLibrary.searchByCriteria(menuInput);

删除该行:(在您的班级ContactLibrary中）

private Scanner libraryInput = new Scanner(System.in);

尝试获取可序列化对象的输入时出现NullPointerException

1 个答案: