CollectionExercise3

java">package CollectionExercise20240728;import java.util.HashMap;
import java.util.Map;
import java.util.Set;public class CollectionExercise3 {public static void main(String[] args) {// 不可变的Map集合// Map中键是不可以重复的// Map中的of方法参数是有上限的，最多20个参数（也就是10个键值对）// 键值对个数≤10Student s1 = new Student("张三", 18);Student s2 = new Student("李四", 21);Student s3 = new Student("王五", 19);Student s4 = new Student("赵六", 23);Student s5 = new Student("钱七", 17);Student s6 = new Student("周八", 30);Map<Student, String> studentMap = Map.of(s1, "重庆", s2, "四川", s3, "武汉",s4, "北京", s5, "上海", s6, "深圳");studentMap.forEach((key, value) -> System.out.println(key.getName() + " " + key.getAge() + " = " + value));// 不可变的Map无法更改studentMap.put(new Student("小明", 20), "USA"); // 无法更改// 键值对个数＞10：此时of中的参数已经不够了，需要用Map中的ofEntries方法// 创建11个键值对对象HashMap<String, String> hm = new HashMap<>();hm.put("张三", "南京");hm.put("李四", "北京");hm.put("王五", "上海");hm.put("赵六", "北京");hm.put("孙七", "深圳");hm.put("周八", "杭州");hm.put("吴九", "宁波");hm.put("郑十", "苏州");hm.put("刘一", "无锡");hm.put("陈二", "嘉兴");hm.put("aaa", "111");// 先将HashMap中的元素转换为entries集合Set<Map.Entry<String, String>> entrySet = hm.entrySet();// 创建entries数组Map.Entry<String, String>[] arr = new Map.Entry[0];// 将entries集合转换为数组，用新的数组接受Map.Entry<String, String>[] arr1 = entrySet.toArray(arr);// 将entries的数组传递给ofEntries方法Map<String, String> map = Map.ofEntries(arr1);// 不可变的Map创造成功，无法改变map.put("bbb","222");// 可以调用Map中的copyOf方法得到一个不可变的Map对象，其底层原理和上面的代码一样Map<String, String> map2 = Map.copyOf(hm);map.put("bbb","222");}
}class Student{String name;int age;public Student() {this("TestStudent", 20);}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;}}