Question

运行java程序时出现的错误：" java.lang.NullPointerException”

代码是：
class Books {
String title;
String author;
}

public class BooksTestDrive {
public static void main(String [] args) {
Books [] myBooks = new Books[3];
int x = 0;
myBooks[0].title = "the Grapes of Java";
myBooks[1].title = "the Grapes of Java2";
myBooks[2].title = "the Grapes of Java3";
myBooks[0].author = "bob";
myBooks[1].author = "bob2";
myBooks[2].author = "bob3";

while(x < 3) {
System.out.print(myBooks[x].title);
System.out.print("by");
System.out.println(myBooks[x].author);
x = x + 1;
}
}
}

用javac编译通过，但是用java运行时就报错了：
Exception in thread "main" java.lang.NullPointerException
at BooksTestDrive.main(BooksTestDrive.java:10)
不知道是哪里出错了啊？？？
希望高手能指出改进方案，谢谢了

Answer 1

你好，你的程序有如下问题：
1、Books [] myBooks = new Books[3] 这句只是声明并实例化了一个数组，但是数组里的对象并没有被实例化，因此下面要对每个数组里的对象进行实例化:myBooks[0] = new Books();
2、这样改程序就可以运行了，但是你这样写破坏了Books类的封装性，不是面向对象编程所提倡的，不建议你这样写。你应该在Books类中加入setter，getter方法用于给title, author属性赋值和获取值。

修改后的完整代码如下：

public class Books {
public String title;
public String author;
/**
* @return the title
*/
public String getTitle() {
return title;
}
/**
* @param title the title to set
*/
public void setTitle(String title) {
this.title = title;
}
/**
* @return the author
*/
public String getAuthor() {
return author;
}
/**
* @param author the author to set
*/
public void setAuthor(String author) {
this.author = author;
}

}

public class BooksTestDrive {
public static void main(String[] args) {
Books[] myBooks = new Books[3];
int x = 0;
myBooks[0] = new Books();
myBooks[1] = new Books();
myBooks[2] = new Books();
myBooks[0].setTitle("the Grapes of Java");
myBooks[1].setTitle("the Grapes of Java2");
myBooks[2].setTitle("the Grapes of Java3");
myBooks[0].setAuthor("bob");
myBooks[1].setAuthor("bob2");
myBooks[2].setAuthor("bob3");

while (x < 3) {
System.out.print(myBooks[x].getTitle());
System.out.print(" by ");
System.out.println(myBooks[x].getAuthor());
x = x + 1;
}
}
}

Answer 2

class Books {
public String title;
public String author;
}

public class BooksTestDrive {
public static void main(String [] args) {
Books [] myBooks = new Books[3];
for(int i=0;i<myBooks.length;i++){
myBooks[i]=new Books();
}
int x = 0;
myBooks[0].title = "the Grapes of Java";
myBooks[1].title = "the Grapes of Java2";
myBooks[2].title = "the Grapes of Java3";
myBooks[0].author = "bob";
myBooks[1].author = "bob2";
myBooks[2].author = "bob3";

while(x < 3) {
System.out.print(myBooks[x].title);
System.out.print(" "+"by"+" ");
System.out.println(myBooks[x].author);
x = x + 1;
}
}
}
这样写就行了，你只是new了数组。但是数组里的对象也必须实例化！

Answer 3

new Books[3]; 这只是产生一个数组 这个数组的大小为3 里面没存有对象

Answer 4

Books [] myBooks = new Books[3];
这个时候只是单纯的创建了数组对象本身
而没有对数组中每个对象进行实例化

解决办法
Books [] myBooks = new Books[3];
myBooks[0] = new Books();
myBooks[0].title = "the Grapes of Java";
myBooks[0].author = "bob";
依次就可以了