Java中Set转List 和 TreeMap中实现自定义类作为key值

Map转Set & Set转List

        //构造Map数据
    Map<String, String> map = new HashMap<String, String>();
    map.put("ele1", "小樱");
    map.put("ele2", "若曦");
    map.put("ele3", "晴川");
    Set<String> set = map.keySet();

    //Set转List,方法一 ： ArrayList(Collection<?> c) 
    List<String> list1 = new ArrayList<String>(set);
    for(int i = 0; i < list1.size(); i++){
    System.out.println("list1(" + i + ") --> " + list1.get(i));
    }

    //Set转List,方法二：List实现类（ArrayList/LinkedList）的方法  -- addAll(Collection<?> c) 
    List<String> list2 = new ArrayList<String> ();
    list2.addAll(set);
    for(String elem : list2){
    System.out.println(elem);
    }

在TreeMap中，如果key值是java中的基本类型，TreeMap会自身帮我们排序，如果是自定义类型，如定义一个Student类作为key值，我们如果不告知排序规则的话，运行就会报错。所以，如何实现自定义排序呢？解决方案有2个，二者选一即可：

• 在Student类中实现Comparable，重写compareTo方法
• 在构造函数中new Comparator,匿名内部类，重写compare 方法

代码如下：

public class Student implements Comparable<Student>{
    private String name;
    private int age;

    public Student() {
        super();
    }
    public Student(String name, int age) {
        super();
        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;
    }
    @Override
    public String toString() {
        return "Student [name=" + name + ", age=" + age + "]";
    }

    @Override
    public int compareTo(Student o) {
        int num1 = this.getAge()-o.getAge();
        int num2 = num1==0?this.getName().compareTo(o.getName()):num1;
        return num2;
    }


public class TestTreeMap {
    public static void main(String[] args) {
        //1.创建集合
        TreeMap<Student, String> map = new TreeMap<Student, String>(new Comparator<Student>() {
            //按照年龄来排序,年龄相同按照姓名来排序
            @Override
            public int compare(Student o1, Student o2) {
                if(o1.getAge()==o2.getAge()){
                    return o1.getName().compareTo(o2.getName());
            }
            return o1.getAge()-o2.getAge();
            }
        //2.创建学生对象并往集合中增加
        Student s1 = new Student("张三",27);
        Student s2 = new Student("李四",29);
        Student s3 = new Student("王五",16);
        Student s4 = new Student("张三",27);
        map.put(s1, "2001");
        map.put(s2, "2002");
        map.put(s3, "2003");
        map.put(s4, "2004");

        //3.遍历集合 ，排序完成
        Set<Student> set = map.keySet();
        for(Student student : set){
            String value = map.get(student);
            System.out.println(student.getName()+"=="+student.getAge()+"=="+value);
        }
    }
}