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.
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