HashMap

1. 

    

    

2. 

    

    

3. 练习1：

    //需求：创建一个HashMap集合，键是学生对象Student，值是籍贯，储存3个键值对对象并遍历，要求同性别，同年龄位同一个学生
           //核心点：如果是键存储自定义对象，需要重写equal和Hashcode方法
           //创建一个map集合
           HashMap<Student,String> hashMap=new HashMap<>();
           //创建对象
           Student student1 = new Student("张三", "25");
           Student student2= new Student("李四", "35");
           Student student3 = new Student("王五", "45");
           //添加元素
           hashMap.put(student1,"江西");
           hashMap.put(student2,"万年");
           hashMap.put(student3,"苏桥");
           //键值对遍历
           Set<Map.Entry<Student, String>> entries = hashMap.entrySet();
           for (Map.Entry<Student, String> entry : entries) {
               Student key = entry.getKey();
               String value = entry.getValue();
               System.out.println(key+value);
           }
           System.out.println("-------------------------------------------------------------");
           //键遍历
           Set<Student> students = hashMap.keySet();
           for (Student key : students) {
               String value = hashMap.get(key);
               System.out.println(value+key);
           }
           System.out.println("-------------------------------------------------------------");
           //lambda表达式遍历
           hashMap.forEach(new BiConsumer<Student, String>() {
               @Override
               public void accept(Student student, String s) {
                   System.out.println(student+s);
               }
           });
   
   
       }
   }
   class Student{
       private String name;
       private String age;
   
       public Student() {
       }
   
       public Student(String name, String age) {
           this.name = name;
           this.age = age;
       }
   
       public String getName() {
           return name;
       }
   
       public void setName(String name) {
           this.name = name;
       }
   
       public String getAge() {
           return age;
       }
   
       public void setAge(String age) {
           this.age = age;
       }
       //关于值，重写方法
   
       @Override
       public boolean equals(Object o) {
           if (this == o) return true;
           if (o == null || getClass() != o.getClass()) return false;
           Student student = (Student) o;
           return Objects.equals(name, student.name) &&
                   Objects.equals(age, student.age);
       }
   
       @Override
       public int hashCode() {
           return Objects.hash(name, age);
       }
   
       @Override
       public String toString() {
           return "Student{" +
                   "name='" + name + '\'' +
                   ", age='" + age + '\'' +
                   '}';
       }
   }

4. 练习2;

    

    

5. 代码实现：

   import java.util.*;
   
   public class map集合练习2 {
       public static void main(String[] args) {
           //定义一个数组记录集合
           String[] array={"A","B","C","D"};
           //定义一个集合记录景点次数
           ArrayList<String> arrayList=new ArrayList<>();
           //利用随机数模拟80个同学投票，并且把投票结果存储起来
           Random random=new Random();
           for (int i = 0; i <80 ; i++) {
               int j = random.nextInt(array.length);
               arrayList.add(array[j]);
   
           }
          //将投票的次数放入到HashMap中
           HashMap<String,Integer> map=new HashMap<>();
           //遍历集合中的元素
           for (String name : arrayList) {
               //判断当前景点在集合中是否存在
               if (map.containsKey(name)){
                  //存在
   
                   //获取当前景点被投票的次数
                  int count = map.get(name);
                   //代表当前景点又被投了一次
   count++;
                   //将新的投票结果加入
                   map.put(name,count);
   
               }else {
                   //不存在
                   map.put(name,1);
               }
           }
           //判断最大值
           //记录最小值
           int max=0;
           Set<Map.Entry<String, Integer>> entries = map.entrySet();
           for (Map.Entry<String, Integer> entry : entries) {
               int count=entry.getValue();
               if (count>max){
                   max=count;
               }
           }
           System.out.println(map);
           System.out.println(max);
   
   //将最大票的景点打印出来
           for (Map.Entry<String, Integer> entry : entries) {
               int count=entry.getValue();
               if (count==max){
                   System.out.println(entry.getKey());
               }
           }
       }
   }