map遍历(Java学习第四十一天)

方式一：通过keySet获取键，然后通过get获取值

方式二：获取每一个map对象，然后在获取其中的键和值

package day41;

import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class MapDemo01 {
    public static void main(String[] args) {
      Map map =  new HashMap();
      map.put(1,"A");
      map.put(2,"B");

        //方式一遍历map
        //获取所有的key键
        Set keySet = map.keySet();
        //增强for获取每个key值
        for(Integer key:keySet){
            System.out.println(key+","+map.get(key));
        }

        //方式二
        //获取所有的对象
        Set> entrySet = map.entrySet();
        //通过增强for来获取键和值
        for(Map.Entry me:entrySet){
            System.out.println(me.getKey()+","+me.getValue());
        }
    }
}

示例：学生类，键唯一值的代码实现。需要重写hashCode() 和equals()两个方法。注意这两个方法是自动生成的。、

package day41;

import java.util.Objects;

//学生类
public class Student {
    private  String name;
    private int age;

    public Student(){}
    public Student(String name, int age){
        this.name = name;
        this.age = age;
    }
    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;
    }
    //通过Java inteliJ 的code里面找到generate，里面找到equals和hashCode即可自动重写
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return age == student.age &&
                Objects.equals(name, student.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }
}



package day41;

import java.util.HashMap;
import java.util.Set;

public class HashMapDemo {
    public static void main(String[] args) {
        HashMap  hm = new HashMap();
        Student s1 = new Student("A",5);
        Student s2 = new Student("B",1);
        Student s3 = new Student("C",3);
        Student s4 = new Student("C",3);

        hm.put(s1,"美国");
        hm.put(s2,"日本");
        hm.put(s3,"加拿大");
        //想要相同的key,后面put的会覆盖之前的。需要Student类中重写两个方法
        hm.put(s4,"挪威");

        Set keySet = hm.keySet();
        for(Student key:keySet){
            System.out.println(key.getName()+key.getAge()+hm.get(key));
        }



    }
}