我开始编写自己的类，到目前为止，我有一个作者类，它有两个字符串（名字和姓氏）和两个整数（出生和死亡年份）。现在我写了另一个类，书籍，它有一个名为bookAuthor的作者对象。所以，这是我的代码，你能帮助修复我的空指针异常。
public class Author{
String authorFirstName;
String authorLastName;
int authorBirth;
int authorDeath;
//creates a new author object with nothing set
public Author(){
}
//String l=last name, String f=frist name
//creates new author object by setting the last and first name
public Author(String l, String f){
authorLastName=l;
authroFirstName=f;
}
//returns a string that is the first name of the author
public String getFirstName(){
return authorFirstName;
}
//return a string that is the last name of the author
public String getLastName(){
return authorLastName;
}
//int b=year of birth for author
//sets the birth year of author and returns nothing
public void setDates(int b){
if (b<-2000){
authorBirth=-40000;
}else{
authorBirth=b;
}
}
//int b,d represent the authors year of birth and death respectively
//returns nothing but sets the authors year of death and birth
public void setDates(int b, int d){
if(d>=b||birth<-2000){
authorBirth=-4000;
authorDeath==-4000;
}else{
authorBirth=b;
authorDeath=d;
}
}
//returns an int that represents the birth year of the author
public int getBirth(){
return authorBirth;
}
//returns an int that represents the death year of the author
public int getDeath(){
return authorDeath;
}
//compares two authors and if they have the same first and last name then it returns 0.
//it also returns 0 if one of the authors appreviates his or her name by the first letter of their first name.
public int compareTo(Author a1){
int compareValue=1;
if(a1.getFirstName().equals(authorFirstName)&&a1.getLastName().equals(authorLastName)){
compareValue=0;
}
if(a1.charAt(0)==authorFirstName.charAt(0)&&a1.getLastName().equals(authorLastName)){
compareValue=0;
}
return compareValue;
}
//returns a String in the form of the authors name by last name, first name
public String toString(){
String name=authorLastName+", "+authorFirstName;
return name;
}
//returns a string of the last name, first name of the author
//if the birth year is known it returns last name, first name of the author but input birth by (b.____)
//if birth and death are known then returns last name, first name of the author with (birth-death)
public String printAuthorInfo(){
String name=authorLastName+", "+authorFirstName;
if(authorBirth=this.getBirth()){
name=name+" (b. "+authorBirth+")";
}
if(authorDeath=this.getDeath()){
name=authorLastName+", "+authorFirstName+"("+authorBirth+"-"+authorDeath+")";
}
return name;
}
//end of Author class
}
public class Book{
int bookYearPublish;
Author bookAuthor;
String bookNumberISBN;
String bookTitle;
public Book(){
}
//String t is the title of the book
//creats a Book object with a title
public Book(String t){
bookTitle=t;
}
//String t is the title of the book
//Author a is the author of the book
//creats a Book object with a title and author
public Book(String t, Author a){
bookTitle=t;
bookAuthor=a;
}
//String t is the title of the book
//creates a title for a Book object
//returns nothing but sets the title of the Book object
//if no title was set then the title is "TITLE NOT SET"
public void setTitle(String t){
if (t.length()==0){
bookTitle="TITLE NOT SET";
}else{
bookTitle=t;
}
}
//returns a String that represents the title of the Book
public String getTitle(){
return bookTitle;
}
//Author a is the author that will be set for the Book
//sets the author of the book
public void setAuthor(Author a){
bookAuthor=a;
}
//returns the Author of the Book
public Author getAuthor(){
return bookAuthor;
}
//int p is the year of publication of the Book
//returns nothing
//sets the year of publication for the Book
public void setYear(int p){
if (p<-2000||p>2018){
bookYearPublish=-4000;
}else{
bookYearPublish=p;
}
}
//returns an int that represents the year it was published
public int getYear(){
return bookYearPublish;
}
//String i is the ISBN number
//returns nothing but sets the ISBN for the book
//if the ISBN number is not length 13 or 10 the ISBN is "ISBN NOT SET"
public void setISBN(String i){
if(i.length()==13||i.length()==10){
bookNumberISBN=i;
}else{
bookNumberISBN="ISBN NOT SET";
}
}
//returns a String that represents the ISBN number of the book
public String getISBN(){
return bookNumberISBN;
}
//Book b is the book that is being checked if it has the same author
//returns a boolean true if a book has two authors who are the same
public boolean sameAuthor(Book b){
boolean compareValue=false;
if(b.getAuthor().compareTo(bookAuthor)==0){
compareValue=true;
}
return compareValue;
}
//Book b is the book that is being compared to by the ISBN number
//compares two books by relating thier ISBN numbers and returns 0 is they are the same
public int compareTo(Book b){
int compareValue=1;
if(bookNumberISBN.equals(b.getISBN())){
compareValue=0;
}
return compareValue;
}
//returns the title of Book
//if the author is known returns a String in the form of title. last name, first name of author
//if the year is known returns a String in the form title (year). last name, first name of author
public String toString(){
String title=bookTitle;
if(bookAuthor.equals(this.getAuthor())){
title=title+". "+bookAuthor;
}
if(bookYearPublish.equals(this.getYear())){
title=bookTitle+" ("+bookYearPublish+"). "+bookAuthor;
}
return title;
}
//ends class Book
}
.getAuthor（） 方法是 Book 类中的一个方法，它返回该书的作者我真正的问题是如何告诉我的程序"如果已经设置了某些东西，那就这样做"，例如在我的代码中，我试图告诉我的程序是否为这本书设置了标题然后返回标题......此外，如果已设置出版年份，则返回上述标题。
public String toString(){
String title= (bookTitle==null) ? "" : bookTitle;
if (getYear() != null && this.getYear().equals(bookYearPublish)) {
title +=" ("+bookYearPublish+")";
}
title += ".";
if (getAuthor() != null && this.getAuthor().equals(bookAuthor)) {
title +=" "+bookAuthor;
}
return title;
}