尚学堂java 参考答案 第九章

一.选择题

1.AC

解析:A.注意题目是Collections不是 Collection,前者是一个until下的类,后者才是接口

C.Set中的数据是无序且不能重复的

2.A

解析:将发生数组越界错误

3.D

解析:A和B都是list,有序,可以重复,C是使用二叉树实现的,也是有序的

4.C

解析:Set不可重复,重复会发生覆盖key,相当于没有添加

5.C

解析:重复会覆盖,但不会报错

二.简答题

1.数组和集合的比较

数组不是面向对象的，存在明显的缺陷，集合弥补了数组的缺点，比数组更灵活更实用，而且不同的集合框架类可适用不同场合。如下：

· 1：数组存放基本数据类型和对象，而集合类存放的都是对象的引用，而非对象本身！

· 2：数组长度固定无法动态改变，集合类容量动态改变。 

· 3：数组无法判断其中实际存有多少元素，length只告诉了数组的容量，定义了多大的长度后，即使没有存放那么多的元素，没有存放的元素的空间就浪费了，而集合的size()可以确切知道元素的个数 

· 4：数组仅采用顺序表方式， 集合有多种实现方式和不同适用场合。

· 5：集合以类的形式存在，具有封装、继承、多态等类的特性，通过简单的方法和属性即可实现各种复杂操作，大大提高了软件的开发效率

2.https://blog.csdn.net/an341221/article/details/50514664

3.Arraylist底层是数组,所以每次扩容都是创建新数组,很耗资源.LinkedList底层是双向链表,所以删添很方便

4.哈希表其实就是数组+链表,用hashCode的值作为数组的编号(KEY),然后每个数组内部放入链表(VALUE)

5.泛型可以让程序在编写时灵活转换类型,提高封装性,并且在编译时就实行类型检查,更安全

三.程序

1.

package ch9_1;

import java.util.*;

public class ch9_1 {
    public static void main(String[] args) {
        List<Book> listBook = new ArrayList<Book>() ;
        Map<String, Book> mapBook = new HashMap();

        listBook.add(new Book("0000001","老人与海",18.80,"上海教育出版社"));
        listBook.add(new Book("0000002","编程思想",180,"机械印刷出版社"));
        listBook.add(new Book("0000003","梦的解析",50,"清华出版社"));

//注意map的存储是散列的,是按hash值来算的,所以取出数据顺序不一定和我们写的时候一样,存同理
        mapBook.put("0000000",new Book("0000000","红与黑",30.5,"清华出版社"));
        mapBook.put("0000001",new Book("0000001","老人与海",18.8,"上海教育出版社"));
        mapBook.put("0000002",new Book("0000002","编程思想",180,"机械印刷出版社"));
        mapBook.put("0000003",new Book("0000003","梦的解析",50,"清华出版社"));

        for (Book books: listBook) {
            System.out.println("ID:"+books.getId()+"\tname:"+books.getName()+"\t price:"+books.getPrice()+"\t publishingHouse:"+books.getPublishingHouse());
        }
        System.out.println("++++++++++++++++++++++++++++++++++++++++++");
//        for (Iterator<Map.Entry<String,Book>> it = mapBook.entrySet().iterator();it.hasNext();){
//            Map.Entry e = (Map.Entry) it.next();
//            System.out.println(e);
//            Book printBook = (Book) e.getValue();
//            System.out.println("ID:"+printBook.getId()+"\tname:"+printBook.getName()+"\t price:"+printBook.getPrice()+"\t publishingHouse:"+printBook.getPublishingHouse());
//        }
        Set<String> keySet = mapBook.keySet();
        for (String id : keySet) {
            System.out.println("ID:"+mapBook.get(id).getId()+"\tname:"+mapBook.get(id).getName()+"\t price:"+mapBook.get(id).getPrice()+"\t publishingHouse:"+mapBook.get(id).getPublishingHouse());
        }

    }

}

2.

package ch9_2;

import java.util.HashSet;
import java.util.TreeSet;

public class ch9_2 {

    public static void main(String[] args) {
        HashSet<Book> hashSet = new HashSet<Book>();
        TreeSet<Book> treeSet = new TreeSet<Book>();

        //hashSet需要重写hashcode方法和equals()方法
        System.out.println("这里添加hashSet元素!");
        hashSet.add(new Book("0000000","红与黑",30.5,"清华出版社"));
        hashSet.add(new Book("0000000","老人与海",18.80,"上海教育出版社"));
        hashSet.add(new Book("0000002","编程思想",180,"机械印刷出版社"));
        hashSet.add(new Book("0000003","梦的解析",50,"清华出版社"));
        System.out.println("hashSet:\n"+hashSet);

        //treeSet需要实现Comparable<T>接口
        System.out.println("+++++++++++++++++++++++++++++++++++++++++\n这里添加treeSet元素!");
        treeSet.add(new Book("0000000","红与黑",30.5,"清华出版社"));
        treeSet.add(new Book("0000000","老人与海",18.8,"上海教育出版社"));
        treeSet.add(new Book("0000003","编程思想",180,"机械印刷出版社"));
        treeSet.add(new Book("0000002","梦的解析",50,"清华出版社"));
        System.out.println("treeSet:\n"+treeSet);

    }
}

package ch9_2;

public class Book  implements  Comparable<Book> {
    private String id;
    private String name;
    private  double price;
    private  String publishingHouse;

    public Book(String id, String name, double price, String publishingHouse) {
        this.id = id;
        this.name = name;
        this.price = price;
        this.publishingHouse = publishingHouse;
    }

    @Override
    public int compareTo(Book b1){
        System.out.println("这里调用了compareTo()方法");
        return Integer.parseInt( this.id)-Integer.parseInt(b1.id);
    }

    @Override
   public int  hashCode(){

        System.out.println("我调用了hashcode()函数!");
        return this.id.hashCode();
    }

    @Override
    public boolean equals(Object obj) {
        System.out.println("我调用了equals()函数!");
        if (this == obj)
        {
            return true;
        }
        if (obj == null)
        {
            return false;
        }
        if (getClass() != obj.getClass())
        {
            return false;
        }
        Book other = (Book) obj;
        if (id != other.id)
        {
            return false;
        }
        if(id == other.id){
            return  true;
        }
        if (name == null)
        {
            if (other.name != null)
            {
                return false;
            }
        } else if (!name.equals(other.name))
        {
            return false;
        }
        if (publishingHouse == null)
        {
            if (other.publishingHouse != null)
            {
                return false;
            }
        } else if (!publishingHouse.equals(other.publishingHouse))
        {
            return false;
        }
        if (Double.doubleToLongBits(price) != Double
                .doubleToLongBits(other.price))
        {
            return false;
        }
        return true;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) {
        this.price = price;
    }

    public String getPublishingHouse() {
        return publishingHouse;
    }

    public void setPublishingHouse(String publishingHouse) {
        this.publishingHouse = publishingHouse;
    }

    @Override
    public String toString() {
        return "Book{" +
                "id='" + id + '\'' +
                ", name='" + name + '\'' +
                ", price=" + price +
                ", publishingHouse='" + publishingHouse + '\'' +
                '}'+'\n';
    }
}

3.