Polymorphism and Method Compilation

Question

Consider the following `Book` and `AudioBook` class definitions.

```java
public class Book
{
    private int numPages;
    private String bookTitle;

public Book(int pages, String title)
    {
        numPages = pages;
        bookTitle = title;
    }

public String toString()
    {
        return bookTitle + " " + numPages;
    }

public int length()
    {
        return numPages;
    }
}

public class AudioBook extends Book
{
    private int numMinutes;

public AudioBook(int minutes, int pages, String title)
    {
        super(pages, title);
        numMinutes = minutes;
    }

public int length()
    {
        return numMinutes;
    }

public double pagesPerMinute()
    {
        return ((double) super.length()) / numMinutes;
    }
}
```

Consider the following code segment, which appears in a class other than `Book` or `AudioBook`.

```java
Line 1: Book[] books = new Book[2];
Line 2: books[0] = new AudioBook(100, 300, "The Jungle");
Line 3: books[1] = new Book(400, "Captains Courageous");
Line 4: System.out.println(books[0].pagesPerMinute());
Line 5: System.out.println(books[0].toString());
Line 6: System.out.println(books[0].length());
Line 7: System.out.println(books[1].toString());
```

Which statement best explains why the code segment will fail to compile?

Accepted Answer

Line 4 will not compile because variables of type `Book` may only call methods in the `Book` class.

Answer

Line 2 will not compile because variables of type `Book` may not refer to variables of type `AudioBook`.

Answer

Line 6 will not compile because the statement is ambiguous. The compiler cannot determine which `length` method should be called.

Answer

Line 5 will not compile because the `AudioBook` class does not have a method named `toString` declared or implemented.

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