HashMap的遍历方法

使用工具

IDEA2018.2

使用说明

1. 使用迭代器，即创建Iterator对象，Iterator是一个接口，也有泛型
2. hasNext方法判断是否已经遍历完，
3. next方法取当前遍历的对象
4. HashMap用keySet()方法获得键的Set集合
5. values()方法获得值的Set集合
6. entrySet()方法获得键值对的Set集合

代码示例

Student.java

 package com.vvcat;

/**
 * @Author ꧁ʚVVcatɞ꧂
 * @Date 2019/10/13 19:37
 * @Version 1.0
 **/
public class Student {
    private Integer id;
    private String name;
    private String sex;
    private Integer age;

    public Student() {

    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Student(Integer id, String name) {
        this.id = id;
        this.name = name;
    }


    public Student(Integer id, String name, String sex, Integer age) {
        this.id = id;
        this.name = name;
        this.sex = sex;
        this.age = age;
    }

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", sex='" + sex + '\'' +
                ", age=" + age +
                '}';
    }
}

HashMap .java

package com.vvcat;

import java.util.*;

/**
 * @Author ꧁ʚVVcatɞ꧂
 * @Date 2019/10/13 19:39
 * @Version 1.0
 **/

public class HashMap {
    public static void main(String[] args) {
        //实例化3个学生
        Student stu1 = new Student(1, "张三","男",20);
        Student stu2 = new Student(2, "李四","男",21);
        Student stu3 = new Student(3, "赵五","男",22);
        // 实例化HashMap用于保存学生
        java.util.HashMap<Integer, Student> map = new java.util.HashMap<Integer, Student>();

        //建立学号与学生的映射关系
        map.put(stu1.getId(),stu1);
        map.put(stu2.getId(),stu2);
        map.put(stu3.getId(),stu3);

        // 遍历方法1.通过Map.keySet遍历key和value(经常使用，二次取值)
        Set<Integer> keys = map.keySet();
        Iterator<Integer> it1 = keys.iterator();
        System.out.println("遍历方法1，通过keySet遍历key和value获得键集合 ");
        for (Integer key : map.keySet()) {
            System.out.println("key = "+ key + " and value= " + map.get(key));
        }
        while (it1.hasNext()){
            Student st = map.get(it1.next());
            System.out.println("学号：" + st.getId() + "\t" + "姓名：" + st.getName() + "\t" + "性别：" + st.getSex() + "\t" + "年龄：" + st.getAge());
        }
        System.out.println("---------------------------");

        // 遍历方法2.通过Map.values()遍历所有的value，但不能遍历key
        Collection<Student> values = map.values();
        Iterator<Student> it2 = values.iterator();
        System.out.println("遍历方法2,通过values获得值集合");
        for (Student stu : map.values()) {
            System.out.println("value= " + stu);
        }
        while(it2.hasNext()){
            Student next = it2.next();
            System.out.println("学号：" + next.getId() + "\t"+ "姓名：" + next.getName()+ "\t" + "性别：" + next.getSex() + "\t"+ "年龄：" + next.getAge());
        }

        System.out.println("---------------------------");

        // 遍历方法3.通过Map.entrySet使用iterator遍历key和value：
        Set<Map.Entry<Integer, Student>> kys = map.entrySet();
        Iterator<Map.Entry<Integer, Student>> it3 = kys.iterator();
        System.out.println("遍历方法3，通过entrySet使用iterator遍历key和value获得键值对");
        while(it3.hasNext()){
            Student st = it3.next().getValue();
            System.out.println("学号：" + st.getId() + "\t" + "姓名：" + st.getName()+ "\t" + "性别：" + st.getSex()+ "\t" + "年龄：" + st.getAge());
        }
        System.out.println("---------------------------");

        //遍历方法4：通过Map.entrySet遍历key和value(推荐容量大时使用)
        System.out.println("通过Map.entrySet遍历key和value");
        for (Map.Entry<Integer, Student> entry : map.entrySet()) {
            Student st = entry.getValue();
            System.out.println("学号：" + entry.getKey()+ "\t" + "姓名：" + st.getName()+ "\t" + "性别：" + st.getSex()+ "\t" + "年龄：" + st.getAge());
        }
    }
}