！空指针错误
日期：2014-05-20 浏览次数：20908 次
求助！空指针异常！
执行时控制台输入一个字符串可以，但是输入两个三个就会抛出空指针异常，怎么解决呢？
------解决方案--------------------
改成下面试试：
------解决方案--------------------
public class Course {
public static void main(String[] args) {
if(args.length == 1)new Student(args[0]);
if(args.length == 2)new Student(args[0],args[1]);
if(args.length == 3)new Student(args[0],args[1],args[2]);
}
}
class Student{
String studentNumber ;
Book book;
Book book2;
public Student(String s){
this.studentNumber = s;
System.out.println(s);
}
public Student(String s,String s2){
this.studentNumber = s;
this.book.bookName = s2;
System.out.println( s + "chooses" + s2);
}
public Student(String s,String s1,String s2){
this.studentNumber = s;
this.book.bookName = s1;
this.book2.bookName = s2;
System.out.println(s + "chooses" + s1 +"and" + s2);
}
}
class Book{
String bookName;
}
执行时控制台输入一个字符串可以，但是输入两个三个就会抛出空指针异常，怎么解决呢？
------解决方案--------------------
public Student(String s,String s2){
this.studentNumber = s;
this.book.bookName = s2; //book没有初始化
System.out.println( s + "chooses" + s2);
}
public Student(String s,String s1,String s2){
this.studentNumber = s;
this.book.bookName = s1; //book没有初始化
this.book2.bookName = s2; //book2没有初始化
System.out.println(s + "chooses" + s1 +"and" + s2);
}
改成下面试试：
public class Course {
public static void main(String[] args) {
if(args.length == 1)new Student(args[0]);
if(args.length == 2)new Student(args[0],args[1]);
if(args.length == 3)new Student(args[0],args[1],args[2]);
}
}
class Student{
String studentNumber ;
Book book = new Book();
Book book2 = new Book();
public Student(String s){
this.studentNumber = s;
System.out.println(s);
}
public Student(String s,String s2){
this.studentNumber = s;
this.book.bookName = s2;
System.out.println( s + "chooses" + s2);
}
public Student(String s,String s1,String s2){
this.studentNumber = s;
this.book.bookName = s1;
this.book2.bookName = s2;
System.out.println(s + "chooses" + s1 +"and" + s2);
}
}
class Book{
String bookName;
}
------解决方案--------------------
//楼上正解，
public Student(String s,String s2){
this.studentNumber = s;
this.book.bookName = s2;//如果你debug的话，可以看出this.book==null,this.book.bookName当然会报错
System.out.println( s + "chooses" + s2);
}
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