Why i need initialize a varible class when i want use it

Question

I have class Book

private
Author author;
String title;
int noOfPages;
etc as get/set...

class Book extends class Author

public String nameOfAuthor;

My Question is : Why do I need to initialize

Author author = new Author(); // in private varible of class Book.

I'm wondering why I coded in eclipse

Author author ; // in private varible of class Book.

Eclipse doesn't return any error, but in console it looks like this:

Exception in thread "main" java.lang.NullPointerException
    at Book.setAuthorName(HelloWorld.java:25)
    at HelloWorld.main(HelloWorld.java:41)

set/get is done. Where is Author's name when I initialize

Author author;

????

Answer 1

A declaration of a field (static or non-static) of a reference type which lacks an initializer is initialized by default with null .

It's like if you did:

Author author = null;

A local variable without initializer is not initialized by default and will yield a compile time error if it used with no value.

Answer 2

You need to create an object because otherwise variable Author points to nothing or null.

// Declare the variable
Author author;
// Create an object (with the new operator) and assign the object to the variable
author = new Author();

Does this answer your question?

Answer 3

When you create an object this way

Author a;

It is just

Author a = null;`

Null is an "undefined value". How can you for example get name of nothing? Nothing won't tell you "Hi, my name is NOTHING".