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对不起，如果这一切似乎很难理解，但我是编程新手，我看了几本书和网站，并从我的理解，我想要做的应该工作。我正在做的任务是调用类的类。如果不把我所有的代码放在这里，我会尽量在不明确的地方尽可能具体。空指针的例外是这个代码特定行：NPE在调用equals（）
if(CDList[i].getArtist().equals(artist) == true)
// CDList是CD对象（其在另一个类中创建）的阵列
// getArtist（）是的一个方法返回的String
//艺术家按照equals（CD类）是扫描仪对象用户已经inputed，也字符串
此特定方法的要点是通过在阵列CDList搜索和比较将艺术家字符串存储到艺术家Strin中g扫描，然后相同的标题。如果找到，那么该部分数组的内容将被删除。这里是该方法的其余部分，如果它会帮助：
void delete()
{
Scanner input = new Scanner(System.in);
System.out.println("Enter artist and title to be deleted: ");
String artist = input.nextLine();
String title = input.nextLine();
for(int i = 0; i <= numOfCDs; i++)
{
if(CDList[i].getArtist().equals(artist) == true)
{
for(int j = 0; j <= numOfCDs; j++)
{
if(CDList[j].getTitle().equals(title) == true)
{
System.out.println("Found CD: " + CDList[j].getArtist() + " " +
CDList[j].getTitle());
System.out.println("Would you like to delete it? Y/1 N/0 ");
if(input.nextInt() == 1)
{
CDList[j] = null;
numOfCDs--;
}
}
else
System.out.println("CD not found.");
}
}
else
System.out.println("CD not found.");
}
}
对不起，这里是其余的代码。只是觉得它太多了，我会放弃它。
CD类：
package assignment3;
public class CD
{
public String artist;
public String title;
private tracklist listOfTracks = new tracklist();
CD(String artistName, String titleName)
{
artist = artistName;
title = titleName;
}
public String getArtist()
{
return artist;
}
public String getTitle()
{
return title;
}
public boolean addTrack(String trackInfo)
{
boolean result = false;
if(listOfTracks.add(trackInfo) == true)
result = true;
return result;
}
public int numTracks()
{
int count = listOfTracks.count();
return count;
}
public void display()
{
System.out.println(" ");
System.out.println(getArtist() + " : " + getTitle());
listOfTracks.display(7);
}
}
曲目类别：
package assignment3;
public class tracklist
{
public String[] tracks;
public int numElements;
tracklist()
{
tracks = new String[99];
numElements = 0;
}
public boolean add(String track)
{
boolean result = true;
int index = 0;
while(tracks[index] != null)
{
index++;
}
tracks[index] = track;
numElements++;
if(numElements > 99)
result = false;
return result;
}
public int count()
{
return numElements;
}
public void display(int indent)
{
for(int i = 1; i < numElements; i++)
{
System.out.print(i);
if(i >= 10)
{
for(int j = 0; j < (indent - 1); j++)
{
System.out.print(" ");
}
}
else
{
for(int j = 0; j < indent; j++)
{
System.out.print(" ");
}
}
System.out.println(tracks[i]);
}
}
}
CDList类：
package assignment3;
import java.util.Scanner;
public class CDList
{
public int numOfCDs;
private CD[] CDList;
private int front,rear;
CDList(int size)
{
CDList = new CD[size];
numOfCDs = 0;
front = 0;
rear = size - 1;
}
boolean add()
{
boolean result;
Scanner input = new Scanner(System.in);
System.out.println("Enter the Artist Name and CD Title: ");
CD userCD = new CD(input.nextLine(), input.nextLine());
System.out.println("Enter the number of tracks: ");
int trackNumber = input.nextInt();
System.out.println("Enter your track titles: ");
for(int i = 0; i <= trackNumber; i++)
{
userCD.addTrack(input.nextLine());
}
if(rear == front)
result = false;
else
{
if(CDList[rear] != null)
rear--;
else
CDList[rear] = userCD;
result = true;
}
return result;
}
void delete()
{
Scanner input = new Scanner(System.in);
System.out.println("Enter artist and title to be deleted: ");
String artist = input.nextLine();
String title = input.nextLine();
for(int i = 0; i <= CDList.length - 1; i++)
{
if((CDList[i].getArtist().equals(artist)) &&
(CDList[i].getTitle().equals(title)))
{
System.out.println("Found CD of: " + CDList[i].getArtist() + " " +
CDList[i].getTitle());
System.out.println("Would you like to delete it? Y/1 N/0 ");
if(input.nextInt() == 1)
{
CDList[i] = null;
numOfCDs--;
}
}
else
System.out.println("CD not found.");
}
}
void SortArtist()
{
CD temp = new CD(" ", " ");
for(int i = 0; i < numOfCDs; i++)
if(CDList[i].getArtist().compareTo(CDList[i + 1].getArtist()) < 0)
{
temp = CDList[i];
CDList[i] = CDList[i + 1];
CDList[i + 1] = temp;
}
}
void SortTitle()
{
CD temp = new CD(" ", " ");
for(int i = numOfCDs; i > 0; i--)
{
int x = 0;
for(int j = 1; j <= i; j++)
{
if(CDList[i].getTitle().compareTo(CDList[i + 1].getTitle()) < 0)
x = j;
}
temp = CDList[x];
CDList[x] = CDList[i];
CDList[i] = temp;
}
}
void Display()
{
for(int i = 0; i <= numOfCDs; i++)
{
while(CDList[i] == null)
i++;
CDList[i].display();
}
}
int size()
{
return numOfCDs;
}
}
不要使用'if（contition == true）'只是使用'if（condition）'。 – Pshemo 2013-03-12 02:56:37
根本不需要'== true'。 – 2013-03-12 02:56:40
请提供一些示例输入和输出。 – 2013-03-12 02:57:14