In this example we will see how and when to use  A TreeMap is a Map implementation which provides total ordering on its elements. The elements are ordered using their natural ordering, or by a  typically provided at sorted map creation time.

A TreeMap is a Red-Black tree based NavigableMap implementation which insures log(n) time cost for the basic operations (add, remove and contains).

A TreeMap is typically used when, in a map, we want to keep the elements sorted all times. The elements are sorted by the keys in a map. The keys could also be custom objects defined with comparable/comparator to decide the attribute responsible for sorting.

Important thing to note is that the ordering maintained by a treeMap must be consistent with equals if this sorted map is to correctly implement the Map interface.

Let’s see how we can use TreeMap in different ways :

理解：(1)TreeMap是一个Map的实现，它能够对自己的元素进行排序  (2)TreeMap的基础是一个红黑树，基本的增删操作的复杂度为log(n)   (3)当我们需要map中的元素保持有序的时候，就可以用TreeMap。元素是根据键值进行排序的。键可以是用户定义的对象，可以通过compare/comparator来指定对象中的某个属性来进行排序 

1. Keys as Integers

A TreeMap guarantees that the elements inserted remains sorted on the order of keys, let’s see the example :

package com.jcg.example;import java.util.Map;import java.util.TreeMap;public class JavaTreeMapExample {        public static void main(String[] args) {                //Natural ordering of key Integer        Map integerMap = new TreeMap();        integerMap.put(1, "ABC");        integerMap.put(2, "PQR");        integerMap.put(3, "XXX");        integerMap.put(4, "YYY");                System.out.println(integerMap.toString());                    }}

{1=ABC, 2=PQR, 3=XXX, 4=YYY}

理解：当Key为Integer的时候，根据Integer的大小来进行排序

 

2. Keys as Custom Classes using Comparable

Now let’s see how we can use custom classes as keys using  For this example we will use a User class and implement Comparable.

package com.jcg.example;public class User implements Comparable {    private String firstName;    private String lastName;    private int salary;    public User(String firstName, String lastName, int salary) {        super();        this.firstName = firstName;        this.lastName = lastName;        this.salary = salary;    }    public String getFirstName() {        return firstName;    }    public void setFirstName(String firstName) {        this.firstName = firstName;    }    public String getLastName() {        return lastName;    }    public void setLastName(String lastName) {        this.lastName = lastName;    }    public int getSalary() {        return salary;    }    public void setSalary(int salary) {        this.salary = salary;    }    @Override    public String toString() {        return firstName + " " + lastName + " " + salary;    }    @Override    public int compareTo(User o) {        return this.firstName.compareTo(o.firstName);    }}

Now let’s see how to use TreeMap to get the inserted elements sorted on the basis of firstName.

package com.jcg.example;import java.util.Map;import java.util.Set;import java.util.TreeMap;public class JavaTreeMapExample {        public static void main(String[] args) {                    //Natural ordering of User        Map userMap = new TreeMap();        populateUserMap(userMap);                System.out.println(userMap.toString());                diplayMap(userMap);                    }    private static void diplayMap(Map userMap) {        Set keySet = userMap.keySet();        for (User user : keySet) {            System.out.println(user.toString());        }    }    private static void populateUserMap(Map userMap) {        userMap.put(new User("Ani","Bha",12), "My Name1");        userMap.put(new User("Cal","YYY",15), "My Name2");        userMap.put(new User("XYZ","WER",22), "My Name3");        userMap.put(new User("SSS","TER",1), "My Name4");    }}

OUTPUT :

{Ani Bha 12=My Name1, Cal YYY 15=My Name2, SSS TER 1=My Name4, XYZ WER 22=My Name3}Ani Bha 12Cal YYY 15SSS TER 1XYZ WER 22

 理解：根据User的firstname进行排序

3. Keys as Custom Classes using Comparator

Let’s say we want to sort the elements on the basis of users salaries, for this we need to implement a Comparator:

package com.jcg.example;import java.util.Comparator;public class UserSalaryComparator implements Comparator {    // This compares employees based on salaries    @Override    public int compare(User o1, User o2) {        if (o1.getSalary() >= o2.getSalary()) {            return 1;        } else {            return -1;        }    }}

Now, let’s see how to use TreeMap to get the inserted elements sorted on the basis of salary.

package com.jcg.example;import java.util.Map;import java.util.Set;import java.util.TreeMap;public class JavaTreeMapExample {        public static void main(String[] args) {                //Ordering based on Comparator on salary        Map userSalaryMap = new TreeMap(new UserSalaryComparator());        populateUserMap(userSalaryMap);        System.out.println(" *** BASED ON SALARY ***");        diplayMap(userSalaryMap);            }    private static void diplayMap(Map userMap) {        Set keySet = userMap.keySet();        for (User user : keySet) {            System.out.println(user.toString());        }    }    private static void populateUserMap(Map userMap) {        userMap.put(new User("Ani","Bha",12), "My Name1");        userMap.put(new User("Cal","YYY",15), "My Name2");        userMap.put(new User("XYZ","WER",22), "My Name3");        userMap.put(new User("SSS","TER",1), "My Name4");    }}

OUTPUT :

*** BASED ON SALARY ***SSS TER 1Ani Bha 12Cal YYY 15XYZ WER 22

So, here we see the output is sorted in increasing order of salaries of users.

理解：用Compator类根据员工的薪水高低来进行排序