Java Program to Sort Keys in TreeMap by using Comparator with User Defined Objects

04 Oct, 2021
The TreeMap in Java is used to implement Map interface and NavigableMap along with the AbstractMap Class. The map is sorted according to the natural ordering of its keys, or by a Comparator provided at map creation time, depending on which constructor is used. To sort keys in TreeMap by using a comparator with user-defined objects in Java we have to create a class that implements the Comparator interface to override the compare method.
// AccordingMarks class that implements the 
// comparator interface to override compare method

class AccordingMarks implements Comparator<Student> {
    public int compare(Student s1, Student s2) {
        return s1.getMarks().compareTo(s2.getMarks());
    }
}
In the below code, we are passing a custom object as a key in TreeMap i.e Student user-defined class. In this case, we need to pass the comparator AccordingMarks in the constructor, while creating the TreeMap object that sorts the TreeMap according to marks of the student.
Example 1: Sort keys in ascending order of the marks
Java
// Java program to demonstrate how to sort TreeMap of custom// class objectsimport java.util.*; // Custom classclass Student {     private String name;    private int marks;     public Student(String name, Integer marks)    {        this.name = name;        this.marks = marks;    }     public String getName() { return this.name; }     public Integer getMarks() { return this.marks; }    // override toString method    public String toString()    {        return this.name + ": " + marks;    }} // Comparator that sort elements according to marks in// Ascending orderclass AccordingMarks implements Comparator<Student> {    public int compare(Student s1, Student s2)    {        return s1.getMarks().compareTo(s2.getMarks());    }} // Driver Codepublic class GFG {    public static void main(String[] args)    {         // New TreeMap of custom class Student        TreeMap<Student, Integer> map            = new TreeMap<>(new AccordingMarks());         // Add elements to TreeMap        map.put(new Student("Akshay", 400), 1);        map.put(new Student("Bina", 500), 2);        map.put(new Student("Chintu", 300), 3);         System.out.println(            "TreeMap keys sorting in ascending order of the marks:");         // Print map using Entry        for (Map.Entry<Student, Integer> entry :             map.entrySet()) {            System.out.println("Key : (" + entry.getKey()                               + "), Value : "                               + entry.getValue());        }    }}
Output:
TreeMap keys sorting in ascending order of the marks:
Key : (Chintu: 300), Value : 3
Key : (Akshay: 400), Value : 1
Key : (Bina: 500), Value : 2
Example 2: Sort keys into descending order of the marks
Java
// Java program to demonstrate how to sort TreeMap of custom// class objectsimport java.util.*; // Custom classclass Student {     private String name;    private int marks;     public Student(String name, Integer marks)    {        this.name = name;        this.marks = marks;    }     public String getName() { return this.name; }     public Integer getMarks() { return this.marks; }    // override toString method    public String toString()    {        return this.name + ": " + marks;    }} // Comparator that sort elements according to marks in// Descending orderclass AccordingMarks implements Comparator<Student> {    public int compare(Student s1, Student s2)    {        return s2.getMarks().compareTo(s1.getMarks());    }} // Driver Codepublic class GFG {    public static void main(String[] args)    {         // New TreeMap of custom class Student        TreeMap<Student, Integer> map            = new TreeMap<>(new AccordingMarks());         // Add elements to TreeMap        map.put(new Student("Akshay", 400), 1);        map.put(new Student("Bina", 500), 2);        map.put(new Student("Chintu", 300), 3);         System.out.println(            "TreeMap Keys sorted in descending order of the marks: ");         // Print map using Entry        for (Map.Entry<Student, Integer> entry :             map.entrySet()) {            System.out.println("Key : (" + entry.getKey()                               + "), Value : "                               + entry.getValue());        }    }}
Output:
TreeMap Keys sorted in descending order of the marks: 
Key : (Bina: 500), Value : 2
Key : (Akshay: 400), Value : 1
Key : (Chintu: 300), Value : 3
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