练习题
练习1
题目
请使用LinkedList来模拟一个堆栈或者队列数据结构。
答案
堆栈
import java.util.LinkedList;
public class MyStack<E> {
private LinkedList<E> list=new LinkedList<>();
public LinkedList<E> getList() {
return list;
}
public void setList(LinkedList<E> list) {
this.list = list;
}
public void push(E e){
this.list.addFirst(e);
}
public E pop(){
return this.list.pollFirst();
}
@Override
public String toString() {
return "MyStack{" +
"list=" + list +
'}';
}
public static void main(String[] args){
MyStack<String> myStack = new MyStack<>();
myStack.push("刘备");
myStack.push("关羽");
myStack.push("张飞");
System.out.println("myStack = " + myStack);
System.out.println("myStack.pop() = " + myStack.pop());
System.out.println("myStack = " + myStack);
}
}
队列
import java.util.LinkedList;
public class Queue<E> {
private LinkedList<E> list = new LinkedList<>();
public LinkedList<E> getList() {
return list;
}
public void setList(LinkedList<E> list) {
this.list = list;
}
public void add(E e){
list.addLast(e);
}
public E poll() {
return list.pollFirst();
}
@Override
public String toString() {
return "Stack [list=" + list + "]";
}
public static void main(String[] args){
Queue<String> stack = new Queue<>();
stack.add("刘东月");
stack.add("徐培杰");
stack.add("刘博");
System.out.println(stack);
System.out.println("---------");
System.out.println(stack.poll());
System.out.println(stack);
System.out.println("---------");
System.out.println(stack.poll());
System.out.println(stack);
}
}
练习2
题目:
用TreeSet存储以下数据：
“xiaoqiang”、 “zhangsan”、“lisi”、“xiaohua”、 “ruhua”、 “wangcai”
要求按照字符串的长度进行排序后再存储（如果长度相同，则按字符串的自然顺序排序）
答案：
@Test
public void test1() {
TreeSet<String> set = new TreeSet<>(new Comparator<String>() {
@Override
public int compare(String o1, String o2) {
int i = o1.length() - o2.length();
int j = o1.compareTo(o2);
return i==0?j:i;
}
});
set.add("xiaoqiang");
set.add("zhangsan");
set.add("lisi");
set.add("xiaohua");
set.add("ruhua");
set.add("wangcai");
System.out.println("set = " + set);
}
练习3
题目
List对象中存放多个Person对象(此对象包含，名字，年龄、id)。按Person的年龄从小到大排序，假设年龄相等的话再按名字的大小来排序。求出年龄最大的那个学生信息。
答案
public class Person {
private int id;
private String name;
private int age;
@Override
public String toString() {
return "Person{" +
"id=" + id +
", name='" + name + '\'' +
", age=" + age +
'}';
}
public Person(int id, String name, int age) {
this.id = id;
this.name = name;
this.age = age;
}
public Person() {
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
}
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
public class Test {
public static void main(String[] args){
ArrayList<Person> list = new ArrayList<>();
list.add(new Person(1,"tom",20));
list.add(new Person(2,"aom",20));
list.add(new Person(3,"bom",18));
/* Collections.sort(list, new Comparator<Person>() {
@Override
public int compare(Person o1, Person o2) {
int i = o1.getAge() - o2.getAge();
int j = o1.getName().compareTo(o2.getName());
return i==0?j:i;
}
});*/
list.sort(new Comparator<Person>() {
@Override
public int compare(Person o1, Person o2) {
int i = o1.getAge() - o2.getAge();
int j = o1.getName().compareTo(o2.getName());
return i==0?j:i;
}
});
System.out.println(list);
}
}
练习4
题目
“chenhao”
“zhangsan”
“zhangsan”
“chenhao”
“lisi”
“wangwu”
“zhaoliu”
“xiaoqiang”
“haha”
打印输出各个字符串出现的次数（使用HashMap实现，键保存字符串，值保存保存出现的次数）
public static void main(String[] args){
String[] arr=new String[]{"chenhao","zhangsan","zhangsan","chenhao"
,"lisi","wangwu","zhaoliu","xiaoqiang","haha"};
HashMap<String, Integer> map = new HashMap<>();
for (String s : arr) {
Integer count = map.get(s);
if (count==null){
map.put(s,1);
}else {
map.put(s,map.get(s)+1);
}
}
for (Map.Entry<String, Integer> entry : map.entrySet()) {
System.out.println(entry.getKey()+"="+entry.getValue());
}
}
练习5
题目
“asdascveasrgdfsdf”获取该字符串中，每一个字母出现的次数。
要求：打印结果是：a(2)b(1)…
答案
@Test
public void test2() {
char[] chars = "asdascveasrgdfsdf".toCharArray();
TreeMap<Character, Integer> map = new TreeMap<>();
for (char c : chars) {
Integer count = map.get(c);
if (count==null){
map.put(c,1);
}else{
map.put(c,map.get(c)+1);
}
}
for (Map.Entry<Character, Integer> entry : map.entrySet()) {
System.out.print(entry.getKey() +"("+ entry.getValue()+")");
}
}
练习6
题目
随机产生50个30到35的整数，统计每个数字出现的次数（TreeMap实现），输出时按照数字的降序排列，并且统计出现次数最多的数字和它的次数。
PS：如果有两个数字出现的次数一样，则只需输出其中一个。
答案
@Test
public void test1() {
Random random = new Random();
TreeMap<Integer, Integer> map = new TreeMap<>(new Comparator<Integer>() {
@Override
public int compare(Integer o1, Integer o2) {
return o2.compareTo(o1);
}
});
for (int i = 0; i < 50; i++) {
int n = random.nextInt(6) + 30;
Integer count = map.get(n);
if (count==null){
map.put(n,1);
}else{
map.put(n,count+1);
}
}
Integer maxKey=0;
Integer maxValue=0;
for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
System.out.println(entry.getKey()+":"+entry.getValue());
if (entry.getValue()>maxValue){
maxValue=entry.getValue();
maxKey=entry.getKey();
}
}
System.out.println("出现次数最多的数是:"+maxKey+",出现了"+maxValue+"次");
}
练习7
题目
创建购物车实体类，模拟购物车功能需求：
- 添加商品到购物车（输入商品的编号和数量）
- 删除商品（删除购物车中的指定购物项）
- 修改商品（修改商品的数量）
- 显示所购买的商品信息（按购买商品的总价进行升序显示）
答案
Product类
public class Product {
private int id;
private String name;// 名称
private int count;//数量
private double price;//单价
public double getPrice() {
return price;
}
public void setPrice(double price) {
this.price = price;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getCount() {
return count;
}
public void setCount(int count) {
this.count = count;
}
public Product() {
}
public Product(int id, String name, int count) {
this.id = id;
this.name = name;
this.count = count;
}
public Product(int id, String name, int count, double price) {
this.id = id;
this.name = name;
this.count = count;
this.price = price;
}
@Override
public String toString() {
return id+"----"+name+"----"+count;
}
}
Cart类
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
public class Cart {
private Map<Integer, CartItem> map=new HashMap<>();
private double total;
public double getTotal() {
return total;
}
public void setTotal(double total) {
this.total = total;
}
//得到购物车的集合
public Collection<CartItem> getCartItems(){
return map.values();
}
//添加购物车
public void add(CartItem cartItem){
Product product = cartItem.getProduct();//得到购买的商品对象
int id = product.getId();//得到商品的id
//到map取得 id对应的购物项
CartItem cartItem1 = map.get(id);
if (cartItem1!=null){//原来买过
cartItem1.setCount(cartItem1.getCount()+cartItem.getCount());//原来的count+新买的count
}
else {//原来没买过
map.put(id,cartItem);//直接往里放
}
setTotal(total+cartItem.getSubTotal());//原来的总价+新购物的小计
}
//删除购物车
public void remove(Integer id){
CartItem cartItem = map.remove(id);
setTotal(total-cartItem.getSubTotal());//原来的总价-删除的购物项的小计
}
//清空购物车
public void clear(){
map.clear();
setTotal(0);
}
public void update(int id, int count) {
CartItem cartItem = map.get(id);
if (cartItem!=null){
setTotal(total-cartItem.getSubTotal());
cartItem.setCount(count);
setTotal(total+cartItem.getSubTotal());
}
}
}
CartItem类
public class CartItem {
private Product product;//购买的商品
private int count;//购买的数量
private double subTotal;//小计
@Override
public String toString() {
return product.getId() +"\t"+product.getName()+"\t"+product.getPrice()+"\t"+count+"\t"+getSubTotal();
}
public CartItem(Product product, int count) {
this.product = product;
this.count = count;
}
public CartItem() {
}
public Product getProduct() {
return product;
}
public void setProduct(Product product) {
this.product = product;
}
public int getCount() {
return count;
}
public void setCount(int count) {
this.count = count;
}
public double getSubTotal() {
return product.getPrice()*count;
}
public void setSubTotal(double subTotal) {
this.subTotal = subTotal;
}
}
ProductNotEnoughException类
package a00;
public class ProductNotEnoughException extends Exception {
public ProductNotEnoughException(String message) {
super(message);
}
public ProductNotEnoughException() {
}
}
ProductNotExistException类
public class ProductNotExistException extends Exception {
public ProductNotExistException(String message) {
super(message);
}
public ProductNotExistException() {
}
}
Shop类
import java.util.Collection;
public class Shop {
private Cart cart=new Cart();
private Product[] products=new Product[]{
new Product(1,"黄瓜",10,2),
new Product(2,"西红柿",10,3),
new Product(3,"菠萝",10,2),
new Product(4,"苹果",10,5),
new Product(5,"桔子",10,3),
};
public void buy(int id, int count) throws ProductNotExistException, ProductNotEnoughException {
//遍历数组判断用户输入id在商品数组中,不存在抛异常:商品不存在
boolean flag=false;
for (Product product : products) {
if (product.getId()==id){
flag=true;
break;
}
}
if (flag==false){
throw new ProductNotExistException("商品不存在");
}
//遍历数组判断商品数量是否<=库存的商品数量,如果抛出商品库存不够异常
for (Product product : products) {
if (product.getId()==id && count>product.getCount()){
throw new ProductNotEnoughException("商品库存不够");
}
}
//将数组对应的商品数量减购买数量
for (Product product : products) {
if (product.getId()==id ){
product.setCount(product.getCount()-count);
System.out.println("购买成功");
//添加购物车的入口
CartItem cartItem = new CartItem();
cartItem.setProduct(product);
cartItem.setCount(count);
cart.add(cartItem);
showCartList();
break;
}
}
}
//show()显示商品列表
public void show(){
System.out.println("请选择商品序号:");
for (Product product : products) {
System.out.println(product);
}
}
public void menu(){
System.out.println("1.添加商品到购物车");
System.out.println("2.删除商品");
System.out.println("3.修改商品");
System.out.println("4.查看购物车");
}
//show()显示购物车列表
public void showCartList(){
System.out.println("购物车列表:");
Collection<CartItem> cartItems = cart.getCartItems();
TreeSet<CartItem> set = new TreeSet<>(new Comparator<CartItem>() {
@Override
public int compare(CartItem o1, CartItem o2) {
return Double.compare(o1.getSubTotal(),o2.getSubTotal());
}
});
set.addAll(cartItems);
System.out.println("id\tname\tprice\tcount\tsubtotal");
for (CartItem cartItem : set) {
System.out.println(cartItem);
}
System.out.println("总价格:"+cart.getTotal());
}
public void remove(int id) {
cart.remove(id);
}
public void update(int id, int count) {
cart.update(id,count);
}
}
TestMain类
import java.util.Scanner;
public class TestMain {
public static void main(String[] args) {
//创建商店对象
Shop shop = new Shop();
//创建用户对象
User user = new User(shop);
//创建Scanner对象
Scanner scanner = new Scanner(System.in);
while (true){
shop.menu();
System.out.println("请输入功能编号:");
int i = scanner.nextInt();
switch(i){
case 1:
shop.show();
System.out.println("请输入商品id:");
int id = scanner.nextInt();
System.out.println("请输入购买数量:");
int count = scanner.nextInt();
//调用user对象的buy()
user.buy(id,count);
shop.show();
break;
case 2:
System.out.println("请输入商品id:");
int id2 = scanner.nextInt();
shop.remove(id2);
break;
case 3:
System.out.println("请输入商品id:");
id = scanner.nextInt();
System.out.println("请输入购买数量:");
count = scanner.nextInt();
shop.update(id,count);
break;
package a00;
import java.util.Scanner;
public class TestMain {
public static void main(String[] args) {
//创建商店对象
Shop shop = new Shop();
//创建用户对象
User user = new User(shop);
//创建Scanner对象
Scanner scanner = new Scanner(System.in);
while (true){
shop.menu();
System.out.println("请输入功能编号:");
int i = scanner.nextInt();
switch(i){
case 1:
shop.show();
System.out.println("请输入商品id:");
int id = scanner.nextInt();
System.out.println("请输入购买数量:");
int count = scanner.nextInt();
//调用user对象的buy()
user.buy(id,count);
shop.show();
break;
case 2:
System.out.println("请输入商品id:");
int id2 = scanner.nextInt();
shop.remove(id2);
break;
case 3:
System.out.println("请输入商品id:");
id = scanner.nextInt();
System.out.println("请输入购买数量:");
count = scanner.nextInt();
shop.update(id,count);
break;
case 4:
shop.showCartList();
break;
default:
break;
}
}
}
}
User类
public class User {
private Shop shop;
public User() {
}
public User(Shop shop) {
this.shop = shop;
}
public void buy(int id,int count){
try {
shop.buy(id,count);
} catch (ProductNotExistException e) {
System.out.println(e.getMessage());
} catch (ProductNotEnoughException e) {
System.out.println(e.getMessage());
}
}
}
Map
poolFirstEntry
移除并返回与此映射中的最小键关联的键-值映射关系；如果映射为空，
则返回 null。
pollLastEntry
移除并返回与此映射中的最大键关联的键-值映射关系；如果映射为空，
则返回 null。
subMap(K fromKey,boolean fromInclusive,K to Key,boolean toInclusive)
subMap(K fromKey,K toKey)
数组也能实现内部比较器和外部比较器
Arrays工具类
Arrays.sort()
Collections工具类
Collections.sort()
线程安全的集合版本
- synchronizedCollection
- synchronizedMap
- synchronizedSortedMap
- synchronizedSortedSet
集合总结
可以使用比较器的方法、类或接口
- TreeMap
- TreeSet
- list.sort
- Arrays.sort
- Collections.sort
垃圾回收
1.什么是垃圾
2.什么是垃圾回收
如何定位垃圾
引用计数算法
可达性分析算法
垃圾回收算法
标记清除算法
复制算法
标记整理算法
分代收集算法
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