Java基础知识-对象的实例化

我在学习Java时遇到了这个代码片段。对此我有一个答案，但我并不完全理解它是如何工作的

我有一个班级的课本

class Books {
   String author;
   String title;
}

然后我有测试课

class TestForBook {

   Books[] myBooks = new Books[3]; // doubt : 1
   System.out.println('myBooks length is ::' + myBooks.length);//prints 3. Its correct.

  myBooks[0].title = 'Book1 title'; // This throws null pointer exception

  myBooks[0] = new Books();
  myBooks[0].title = 'Book1 title'; //works fine after above line
}

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我想了解为什么即使在声明了Books类型的数组之后，数组值也有null（疑问：注释中的1，我指的是这一行）

我不知道我遗漏了Java的什么概念。以及如何/从何处学习此类主题。任何资源或书籍建议也将是可观的。谢谢

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它不是问题id:1922677的副本。这里有解决方案（我也给出了解决方案），但我想知道为什么会这样。我的意思是，即使在声明之后，它为什么为null也是我的问题。

这是因为您刚刚创建了一个大小（长度）为3的空数组

与

---

在数组中定义一本书。因此，您可以在以后设置标题。

当您创建

Books[]myBooks=newbooks[3]

时。它将创建一个数组，该数组可以容纳Books的对象，但所有值都为null。您没有在该数组中放入任何内容。在访问myBooks[0]时，title实际上是在null值上调用title，因此引发null指针异常

在第二个场景中，您分配book对象，然后调用该对象的标题。 所以主要的一点是，当您对空对象调用方法或属性时，它将抛出异常。 myBooks[0]=新书（）；

---

myBooks[0]。标题='Book1 title'

创建对象数组时，请记住，

new

命令适用于数组，该数组最初包含用于容纳对象的请求插槽，但不会由任何特定对象填充。这意味着为了拥有一个包含3本书的数组，您需要

• 创建一个可以容纳3本书的数组对象

  新书[3]

• 创建3个图书对象

  newbook（）

  ，并将它们放入数组中（每个插槽中一个）

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通过内联注释来理解它
类TestForBook{

   Books[] myBooks = new Books[3]; // here you have initialized an array of size 3, but it is currently empty, but three Book object can fit in it
   System.out.println('myBooks length is ::' + myBooks.length);//prints 3. Its correct. // since you have specified the length of the array, this will return 3 despite it being empty

  myBooks[0].title = 'Book1 title'; // This throws null pointer exception because your books array is empty, myBooks[0] = null, hence the null pointer exception.

  myBooks[0] = new Books(); //now you have added a book object in the empty book array at index 0
  myBooks[0].title = 'Book1 title'; //since myBooks[0] now contains a books object that your created above, this will return that object instead of null and will work. However if you try this with myBooks[1] that will be null as you still have not put a book object at index 1
}

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我想了解为什么即使在声明了Books类型的数组之后，数组值也有空值-因为在创建数组时，数组的每个元素都有该类型的默认值。类的默认值为null。您已声明了数组，但没有声明数组中的元素，因此它们有默认值，为null谢谢Edwin。明白了。

myBooks[0] = new Books();

   Books[] myBooks = new Books[3]; // here you have initialized an array of size 3, but it is currently empty, but three Book object can fit in it
   System.out.println('myBooks length is ::' + myBooks.length);//prints 3. Its correct. // since you have specified the length of the array, this will return 3 despite it being empty

  myBooks[0].title = 'Book1 title'; // This throws null pointer exception because your books array is empty, myBooks[0] = null, hence the null pointer exception.

  myBooks[0] = new Books(); //now you have added a book object in the empty book array at index 0
  myBooks[0].title = 'Book1 title'; //since myBooks[0] now contains a books object that your created above, this will return that object instead of null and will work. However if you try this with myBooks[1] that will be null as you still have not put a book object at index 1
}