JavaSEDemo26集合与垃圾回收机制

桃扇骨 2021-09-25 07:56 307阅读 0赞

### 文章目录 ###

*  练习题
 *   *  练习1
     *  练习2
     *  练习3
     *  练习4
     *  练习5
     *  练习6
     *  练习7
     *   *  Product类
         *  Cart类
         *  CartItem类
         *  ProductNotEnoughException类
         *  ProductNotExistException类
         *  Shop类
         *  TestMain类
         *  User类
     *  Map
     *  poolFirstEntry
     *  pollLastEntry
     *  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
     *  Collections工具类
     *  线程安全的集合版本
 *  集合总结
 *  可以使用比较器的方法、类或接口
 *  垃圾回收

# 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：

1.  添加商品到购物车（输入商品的编号和数量）
2.  删除商品（删除购物车中的指定购物项）
3.  修改商品（修改商品的数量）
4.  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()