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，或任何涉及输入的内容。输入是从ContactLibrary和ContactInfo类（可序列化的对象）中完成的。
但是，达到该点时会出现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.
我是什么做的？
这是我的主目录：http://ideone.com/uvfK4U（在顶部注释中包含其他两个类）
这是一个UML图：http://imgur.com/9W3TS
ContactLibrary类，根据要求：
/**
* 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);
}
}
最佳答案
您将在同一个inputStream上创建多个Scanner对象，最好仅创建一个并将其传递给使用的方法或该类的构造函数中的下一个类。
在同一流上使用多个扫描仪。扫描程序可以（并且将）消耗流-这可能（将）导致意外的副作用。最好不要这样做。
您可以将主类中的第65行替换为：
myLibrary.searchByCriteria(menuInput);
删除行：（在您的类ContactLibrary中）
private Scanner libraryInput = new Scanner(System.in);