Java数组对象编译问题

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 9 年前。

下面的代码在编译时出错。我知道它缺少一些东西，但我就是不知道是什么。

public class Books {
    String title;
    String author;

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        Books[] myBooks = new Books[3];

        myBooks[0].title = "Learn Java";
        myBooks[1].title = "Java Development";
        myBooks[2].title = "Java Today!";
        myBooks[0].author = "S";
        myBooks[1].author = "Mr. S";
        myBooks[2].author = "SN";

        int x = 0;
        while (x < myBooks.length) {
            x = x + 1;
            System.out.print(myBooks[x].title);
            System.out.print(" by ");
            System.out.println(myBooks[x].author);
        }

    }

}

Answer 1

改变

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

至

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

考虑当 x=2 位于循环的 srat 时的情况。首先变成3，然后books[3]会抛出异常。

除此之外，您只是定义了一个数组，它只是引用。您需要初始化所有这些。

for(Books book : myBooks ) {
   book = new Books();
}

如果您没有初始化数组，它们只是指向 null，如果您对其执行任何操作，例如 myBooks[i].title，您将得到 NPE。