我有 3 个类：Main、ContactLibrary 和 ContactInfo。ContactLibrary 包含一个名为 myPhoneBook 的 ArrayList。ContactInfo 由一堆包含姓名、地址等的字符串组成。
例如，用户想要搜索名称或任何涉及输入的内容。输入是在 ContactLibrary 和 ContactInfo 类中完成的——这两个类都是可序列化的对象。
但是，当它达到这一点时，我会收到 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？
这是我的主要： 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，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);
}
}