黑马程序员_集合HashMap练习：获取Map集合元素的两种方式：keySet()和entrySet()方法

今天学习了Map集合。

Map是一种以键值对的形式存在的集合,其中每个键映射到一个值，几乎所有通用 Map 都使用哈希映射。位于java.util包中，其子类有HashMap,TreeMap。HashMap缺省的情况下是线程非安全的;当多线程访问时可能需要提供同步机制，key和value的值允许为null,键值允许重复，没有顺序。

对于map集合遍历元素的两种方式进行了练习，结果如下：

package com.itpractice;import java.util.HashMap;import java.util.Iterator;import java.util.Map;import java.util.Set;public class StudentMapDemo {public static void main(String[] args) {HashMap<Student, String> hm = new HashMap<Student, String>();hm.put(new Student("jinglt", 27), "三河");hm.put(new Student("yangq", 26), "临沂");hm.put(new Student("liul", 25), "武汉");hm.put(new Student("jinglt", 24), "三河");// 第一种取出方式：keySet()// Set<Student> keySet = hm.keySet();Iterator<Student> it1 = hm.keySet().iterator();while (it1.hasNext()) {Student s = it1.next();System.out.println("学生姓名：" + s.getName() + "  ***  " + "学生年龄："+ s.getAge() + "  ***  " + "学生地址：" + hm.get(s));}System.out.println("%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%");// 第二种取出方式：entrySet()//Set<Map.Entry<Student, String>> entrySet = hm.entrySet();Iterator<Map.Entry<Student, String>> it2 = hm.entrySet().iterator();while (it2.hasNext()) {Map.Entry<Student, String> me = it2.next();System.out.println("学生姓名：" + me.getKey().getName() + "---"+ "学生年龄：" + me.getKey().getAge() + "---" + "学生地址："+ me.getValue());}}}/** * 定义一个学生类 *  * @author Administrator *  */class Student implements Comparable<Student> {/** * 学生姓名 */private String name;/** * 学生年龄 */private int age;/** * 无参构造函数 */public Student() {super();}/** * 构造函数 */public Student(String name, int age) {this.name = name;this.age = age;}/** * getXxx()和setXxx()方法 */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;}/** * 重写toString()方法 */@Overridepublic String toString() {return "student [name=" + name + ", age=" + age + "]";}/** * 重写hashCode()方法 */@Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result + age;result = prime * result + ((name == null) ? 0 : name.hashCode());return result;}/** * 重写equals()方法 */@Overridepublic boolean equals(Object obj) {if (this == obj)return true;if (obj == null)return false;if (getClass() != obj.getClass())return false;Student other = (Student) obj;if (age != other.age)return false;if (name == null) {if (other.name != null)return false;} else if (!name.equals(other.name))return false;return true;}/** * 覆盖父类的compareTo()方法 */@Overridepublic int compareTo(Student s) {int num = new Integer(this.age).compareTo(s.age);if (num == 0)return this.name.compareTo(s.name);return num;}}