TreeMap容器的使用

image-20220502150822905-16514753038931

TreeMap和HashMap同样实现了Map接口，所以，对于API的用法来说是没有区别的。HashMap效率高于TreeMap；TreeMap是可以对键进行排序的一种容器，在需要对键排序时可选用TreeMap。TreeMap底层是基于红黑树实现的。

在使用TreeMap时需要给定排序规则：

  • 元素自身实现比较规则
  • 通过比较器实现比较规则

元素自身实现比较规则

public class Users implements Comparable<Users>{
private String username;
private int userage;


public Users(String username, int userage) {
this.username = username;
this.userage = userage;
}


public Users() {
}


@Override
public boolean equals(Object o) {
System.out.println("equals...");
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;


Users users = (Users) o;


if (userage != users.userage) return false;
return username != null ? username.equals(users.username) : users.username == null;
}


@Override
public int hashCode() {
int result = username != null ? username.hashCode() : 0;
result = 31 * result + userage;
return result;
}


public String getUsername() {
return username;
}


public void setUsername(String username) {
this.username = username;
}


public int getUserage() {
return userage;
}


public void setUserage(int userage) {
this.userage = userage;
}


@Override
public String toString() {
return "Users{" +
"username='" + username + '\'' +
", userage=" + userage +
'}';
}


//定义比较规则
//正数：大，负数：小，0：相等
@Override
public int compareTo(Users o) {
if(this.userage < o.getUserage()){
return 1;
}
if(this.userage == o.getUserage()){
return this.username.compareTo(o.getUsername());
}
return -1;
}
}

 

public class TreeMapTest {
public static void main(String[] args) {
//实例化TreeMap
Map<Users,String> map = new TreeMap<>();
Users u1 = new Users("oldlu",18);
Users u2 = new Users("admin",22);
Users u3 = new Users("sxt",22);
map.put(u1,"oldlu");
map.put(u2,"admin");
map.put(u3,"sxt");
Set<Users> keys = map.keySet();
for(Users key :keys){
System.out.println(key+" --------- "+map.get(key));
}
}
}

 

public class TreeMapTest {
public static void main(String[] args) {
//实例化TreeMap
Map<Users,String> map = new TreeMap<>();
Users u1 = new Users("oldlu",18);
Users u2 = new Users("admin",22);
Users u3 = new Users("sxt",22);
map.put(u1,"oldlu");
map.put(u2,"admin");
map.put(u3,"sxt");
Set<Users> keys = map.keySet();
for(Users key :keys){
System.out.println(key+" --------- "+map.get(key));
}
}
}

通过比较器实现比较规则

public class Student {
private String name;
private int age;


public Student(String name, int age) {
this.name = name;
this.age = age;
}


public Student() {
}


@Override
public String toString() {
return "Student{" +
"name='" + name + '\'' +
", 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 boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;


Student student = (Student) o;


if (age != student.age) return false;
return name != null ? name.equals(student.name) : student.name == null;
}


@Override
public int hashCode() {
int result = name != null ? name.hashCode() : 0;
result = 31 * result + age;
return result;
}
}

 

public class StudentComparator implements Comparator<Student> {


//定义比较规则
@Override
public int compare(Student o1, Student o2) {
if(o1.getAge() > o2.getAge()){
return 1;
}
if(o1.getAge() == o2.getAge()){
return o1.getName().compareTo(o2.getName());
}
return -1;
}
}

 

public class TreeMapTest {
public static void main(String[] args) {
Map<Student,String> treeMap = new TreeMap<>(new StudentComparator());
Student s1 = new Student("oldlu",18);
Student s2 = new Student("admin",22);
Student s3 = new Student("sxt",22);
treeMap.put(s1,"oldlu");
treeMap.put(s2,"admin");
treeMap.put(s3,"sxt");
Set<Student> keys1 = treeMap.keySet();
for(Student key :keys1){
System.out.println(key+" ---- "+treeMap.get(key));
}
}
}

实时效果反馈

1.TreeMap可以对容器中的排序?

A 对Key排序；

B 对Value排序；

C 对Key与Value同时排序；

D 对指定的Key与Value进行排序；

答案

1=>A