Typo in http://docs.oracle.com/javase/tutorial/java/IandI/objectclass.html

Replace

public class Book {
    ...
    public boolean equals(Object obj) {
        if (obj instanceof Book)
            return ISBN.equals((Book)obj.getISBN());
        else
            return false;
    }
}

with

public class Book {
String ISBN;
public String getISBN() { return ISBN; }
    public boolean equals(Object obj) {
        if (obj instanceof Book)
            return ISBN.equals(((Book) obj).getISBN());
        else
            return false;
    }
}