Question

public class BooksTestDrive {
 public static void main(String[] args) {

  Books [] myBooks = new Books[3];
  int x=0;
  myBooks[0].title = "The Grapes of Jave";
  myBooks[1].title = "The Java Gatsby";
  myBooks[2].title = "The Java Cookbook";
  myBooks[0].author = "bob";
  myBooks[1].author = "sue";
  myBooks[2].author = "ian";

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

此代码编译但在运行时，正在给出空指针异常。

Answer 1

MyBooks [3]的分配只分配数组你仍需要为数组中的每个条目分配一个“new MyBook（）”。

Answer 2

看看你的专栏：

Books [] myBooks = new Books[3];

你创建了一个数组，尽管数组中的每个元素都是一个空指针。

Answer 3

看到它，您需要初始化数组中的每个元素，在for或while

中进行尝试

Answer 4

这应该有效：

public class BooksTestDrive {
public static void main(String[] args) { 
      Books [] myBooks = new Books[3];
      // init loop
      for (int i=0;i<myBooks.length;i++) {
         myBooks[i] = new Books();
      }

      myBooks[0].title = "The Grapes of Jave";
      myBooks[1].title = "The Java Gatsby";
      myBooks[2].title = "The Java Cookbook";
      myBooks[0].author = "bob";
      myBooks[1].author = "sue";
      myBooks[2].author = "ian";

      for (Books book : myBooks) {
          System.out.printf("%s by %s\n",book.title,book.author);
      }
     }
}
}

Answer 5

您需要将图书添加到数组中。这应该有效：

class BooksTestDrive {
    public static void main(String [] args) {
        Books [] myBooks = new Books[3];
        int x = 0;
        // THIS IS WHAT WAS MISSING.
        myBooks[0] = new Books();
        myBooks[1] = new Books();
        myBooks[2] = new Books();

        myBooks[0].title = "The Grapes of Java";
        myBooks[1].title = "The Java Gatsby";
        myBooks[2].title = "The Java Cookbook";
        myBooks[0].author = "bob";
        myBooks[1].author = "sue";
        myBooks[2].author = "ian";

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

Java程序运行时错误

5 个答案: