java - 来自空数组单元的 NullPointerException

Question

使用Java而不是IDE，而是使用命令行，我应该编写一个基本的电话簿程序，重点关注数组，其中用户有三个选项:在数组中搜索名称并打印名称和电话号码(如果找到姓名)，则根据用户输入将新姓名和电话号码添加到数组中，并根据用户输入从数组中删除姓名和电话号码。添加功能完全正常工作，但搜索和删除功能无法正常工作。我的代码可以编译并运行，但每当我尝试搜索或删除数组中不存在的名称时，都会收到 NullPointerException 运行时错误。

这些是程序“删除条目”部分的具体说明:

“要从这样的数组中删除姓名和号码，首先找到该姓名的单元格。如果要删除的姓名不在数组中，则报告错误。现在将引用复制到最后一个非空单元格中到删除的名字的单元格。将最后一个非空单元格设置为空。现在删除的PhoneEntry是垃圾，并且所有数组的末尾仍然有所有空值。”

这是我的代码:

import java.util.*;

class PhoneEntry
{
    String name;    // name of a person
    String phone;   // their phone number

    PhoneEntry( String n, String p )
    {
        name = n; phone = p;
    }
}

class PhoneBook
{
    PhoneEntry[] phoneBook;

    PhoneBook()    // constructor
    {
        phoneBook = new PhoneEntry[ 10 ] ;

        phoneBook[0] = new PhoneEntry( "James Barclay", "(418) 665-1223" );
        phoneBook[1] = new PhoneEntry( "Grace Dunbar",  "(860) 399-3044" );
        phoneBook[2] = new PhoneEntry( "Paul Kratides", "(815) 439-9271" );
        phoneBook[3] = new PhoneEntry( "Violet Smith",  "(312) 223-1937" );
        phoneBook[4] = new PhoneEntry( "John Wood",     "(913) 883-2874" );
        phoneBook[5] = new PhoneEntry( null, null );
        phoneBook[6] = new PhoneEntry( null, null );
        phoneBook[7] = new PhoneEntry( null, null );
        phoneBook[8] = new PhoneEntry( null, null );
        phoneBook[9] = new PhoneEntry( null, null );
    }

    PhoneEntry search( String targetName )
    {
        for ( int i = 0 ; i < phoneBook.length ; i++ )
        {
            System.out.println(i);  // debug
            if ( phoneBook[i] != null && phoneBook[i].name.equals( targetName ) )
            {
                return phoneBook[i];
            }
        }
        return null;
    }

    PhoneEntry addEntry( String addName, String addNumber )
    {
        boolean found = false;
        for ( int i = 0 ; i < phoneBook.length ; i++ )
        {
            System.out.println(i);  // debug
            if ( phoneBook[i].name == null )
            {
                phoneBook[i] = new PhoneEntry( addName, addNumber );
                found = true;
                    break;
            }
        }
        if ( !found )
        {
            System.out.println("Phone book is full! Delete an entry first!");
        }
        return null;
    }

    PhoneEntry deleteEntry( String deleteName )
    {
        boolean found = false;
        for ( int i = 0 ; i < phoneBook.length ; i++ )
        {
            System.out.println(i);  // debug
            if ( phoneBook[i] != null && phoneBook[i].name.equals( deleteName ) )
            {
                System.out.println( phoneBook[i].name + " found");  // debug
                phoneBook[i] = new PhoneEntry( null, null );
                found = true;
                    break;
            }
        }
        if ( !found )
        {
            System.out.println( "Entry not found." );
        }
        return null;
    }
}

class PhoneBookComplete
{
    public static void main ( String[] args )
    {
        PhoneBook pb = new PhoneBook();
        Scanner scan = new Scanner( System.in );
        String tempName, tempNumber, tempDelName;

            // INITIAL WELCOME MESSAGE START

        System.out.println();
        System.out.println("**********************");
        System.out.println("***** PHONE BOOK *****");
        System.out.println("**********************");
        System.out.println();
        System.out.println(" 1  Search for an entry");
        System.out.println(" 2  Add a new entry");
        System.out.println(" 3  Delete an entry");
        System.out.println(" 4  Quit program");
        System.out.println();
        System.out.print("Enter a command: ");

        String userInput = scan.nextLine();
        PhoneEntry entry;
        System.out.println();

            // INITIAL WELCOME MESSAGE END

        while ( userInput != "quit" )
        {
            if ( userInput.equals("1") )        //  NAME SEARCH
            {
                    System.out.println("    NAME SEARCH START");
                System.out.println("**********************");
                System.out.println("***** NAME SEARCH ****");
                System.out.println("**********************");
                System.out.println();
                System.out.print("Enter a name: ");
                userInput = scan.nextLine();
                entry = pb.search( userInput );

                if ( entry != null )
                {
                    System.out.println();
                    System.out.println( "  " + entry.name + ": " + entry.phone );
                    System.out.println();
                }
                else if ( userInput.equals("quit") )
                {
                    break;
                }
                else
                {
                    System.out.println("Name not found.");
                    System.out.println();
                }
                    System.out.println("    NAME SEARCH END");
                    System.out.println();
            }
            else if ( userInput.equals("2") )   //  ADD ENTRY
            {
                    System.out.println("    ADD ENTRY START");

                System.out.println("**********************");
                System.out.println("***** ADD ENTRY ******");
                System.out.println("**********************");
                System.out.println();

                System.out.print("Enter full name: ");
                tempName = scan.nextLine();

                    if ( userInput.equals("quit") )
                    {
                        break;
                    }

                System.out.print("Enter phone number: ");
                tempNumber = scan.nextLine();

                    if ( userInput.equals("quit") )
                    {
                        break;
                    }

                entry = pb.addEntry( tempName, tempNumber );

                System.out.println();
                System.out.println( "Entry for " + tempName + " successfully added." );
                //System.out.println("Phone book is full! Delete an entry first!");

                    System.out.println("    ADD ENTRY END");
                    System.out.println();
            }
            else if ( userInput.equals("3") )   //  DELETE ENTRY
            {
                    System.out.println("    DELETE ENTRY START");

                System.out.println("**********************");
                System.out.println("**** DELETE ENTRY ****");
                System.out.println("**********************");
                System.out.println();

                System.out.print("Enter full name: ");
                tempDelName = scan.nextLine();
                entry = pb.deleteEntry( tempDelName );

                if ( entry == null )
                {
                    System.out.println();
                    System.out.println( "Entry for " + tempDelName + " successfully deleted." );
                    System.out.println();
                }
                else if ( userInput.equals("quit") )
                {
                    break;
                }
                else
                {
                    System.out.println();
                }

                    System.out.println("    DELETE ENTRY END");
                    System.out.println();
            }
            else if ( userInput.equals("4") )   //  QUIT PROGRAM
            {
                    System.out.println();
                break;
            }

            //System.out.println("Select a command:");
            System.out.println("**********************");
            System.out.println("**********************");
            System.out.println("**********************");
            System.out.println();
            System.out.println(" 1  Search for an entry");
            System.out.println(" 2  Add a new entry");
            System.out.println(" 3  Delete an entry");
            System.out.println(" 4  Quit program");
            System.out.println();
            System.out.print("Enter a command: ");
            userInput = scan.nextLine();
        }
        System.out.println("Goodbye.");
    }
}

以下是当我尝试搜索不存在的名称时出现的错误:

Exception in thread "main" java.lang.NullPointerException
        at PhoneBook.search(PhoneBookComplete.java:39)
        at PhoneBookComplete.main(PhoneBookComplete.java:128)

这是当我尝试删除不存在的名称时出现的错误:

Exception in thread "main" java.lang.NullPointerException
        at PhoneBook.deleteEntry(PhoneBookComplete.java:73)
        at PhoneBookComplete.main(PhoneBookComplete.java:193)

我知道这与我使用 null 有关，但是除了 null 我还能用什么来代替呢？另外，我应该无论如何都使用null。

所以我想我的问题是:应该在我的代码中修改什么，以便当用户尝试删除或搜索不存在的名称时，打印错误消息而不使程序崩溃？

在回答时，请记住我对 Java 还很陌生，数组、方法之类的东西对我来说仍然很困惑。如果需要更多信息等，请询问，您将收到。话虽如此，任何帮助和解释都值得赞赏和欢迎。谢谢!

Answer 1

检查 name 是否不为 null，这

if (phoneBook[i] != null && phoneBook[i].name.equals(targetName))

应该是这样的

if (phoneBook[i] != null && targetName.equals(phoneBook[i].name))

和deleteEntry类似

if (phoneBook[i] != null && deleteName.equals(phoneBook[i].name))

因为当phoneBook[i].name为null时，它会导致NullPointerException调用.equals() 关于它。您还可以在 .name 上添加 null 测试，但随后您需要另一个 && ，例如

if (phoneBook[i] != null && phoneBook[i].name != null &&
    phoneBook[i].name.equals(targetName))