JAVA在代码中已经覆写了toString方法 为什么输出还是对象的地址

package com.xyh.Demo;

import java.util.Arrays;
import java.util.Comparator;

class BC implements Comparator{

@Override
public int compare(Book o1, Book o2) {
    // TODO Auto-generated method stub
    if(o1.getDate()<o2.getDate()){
        return -1;
    }else if(o1.getDate()>o2.getDate()){
        return 1;
    }
    return 0;
}

}
class Book {
private int date;
private String author;

/**
 * @param date
 * @param author
 */
public Book(int date, String author) {
    super();
    this.date = date;
    this.author = author;
}

public int getDate() {
    return date;
}

public void setDate(int date) {
    this.date = date;
}

public String getAuthor() {
    return author;
}

public void setAuthor(String author) {
    this.author = author;
}

@Override
public String toString() {
    return "Book [date=" + date + ", author=" + author + "]";
}

}

public class Test {

public static void main(String[] args) {
    // TODO Auto-generated method stub
     Book []book={
             new Book(100,"XX"),
             new Book(90,"YY")
             };

     Arrays.sort(book,new BC());
     System.out.println(book);
}

}