【发布时间】：2014-12-15 03:22:27
【问题描述】：
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，专注于数组，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号码。添加功能完全按照它应该的方式工作，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，都会出现 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一个名字和数字，首先找到名字所在的单元格。如果要删除的名字不在数组中，则报错。现在复制最后一个非空单元格中的引用到被删除名称的单元格。将最后一个非空单元格设置为空。现在删除的PhoneEntry是垃圾，所有数组最后仍然有所有空值。”
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.");
}
}
这是我在尝试搜索不存在的名称时遇到的错误：
Exception in thread "main" java.lang.NullPointerException
at PhoneBook.search(PhoneBookComplete.java:39)
at PhoneBookComplete.main(PhoneBookComplete.java:128)
这是我尝试删除不存在的名称时遇到的错误：
Exception in thread "main" java.lang.NullPointerException
at PhoneBook.deleteEntry(PhoneBookComplete.java:73)
at PhoneBookComplete.main(PhoneBookComplete.java:193)
我知道这与我对 null 的使用有关，但我还能用什么来代替 null？另外，我应该无论如何都要使用 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，会打印一条错误消息而不会导致程序崩溃？
在回答时，请记住，我对 Java 很陌生，数组、方法和诸如此类的东西仍然让我很困惑。如果需要更多信息等，请询问，您将收到。话虽如此，任何对任何事情的帮助和解释都将受到赞赏和欢迎。谢谢！
【问题讨论】：
标签： java arrays nullpointerexception