Java Basics - Instantiation of objects

I was learning Java and came across this code snippet. Have an answer for that but I am not exactly understanding how it works.

I have a class Books

class Books {
   String author;
   String title;
}

Then I have test class

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
}

I want to understand why even after declaring array of type Books, array values have null(doubt : 1 in comments, I am referring to that line).

I do not know what concept of Java I am missing. And how/from where I can learn these kind of topics. Any resource or books suggestion also would be appreciable. Thanks.

It is not duplicate of question id : 1922677. Solution is available there(I have also given the solution) but I wanted to know why it is like that. I mean even after declaring, why it has null was my question.