How to Create TreeMap Objects using Comparable Interface in Java?

Comparable interface is found in java.lang package which is used to order the objects of a user-defined class on the basis of a single attribute only. This interface contains a single method compareTo(Object). It is used to compare the current object with the specified objects and can be used o sort user-defined classes, wrapper class, and string objects.

Syntax:

compareTo(Object o) ;

Parameter: The specified object with which the current object is being compared.

Return Type: Integer value

• Positive integer — If the current object is greater than the specified object.
• Negative integer — If the current object is lesser than the specified object.
• Zero — If the current object is equal to the specified object.

Use of Comparable Interface in Java

TreeMap in Java, elements are stored as key-value pairs which are sorted on the basis of the key. When the Key is of String class or Wrapper Classes, it implements the Comparable interface by default and stores the elements in sorted order. However, if somebody wants to make the desired key of a particular user-defined type i.e user-defined class, we need to implement the Comparable interface to order the objects in a particular order on the basis of an attribute.

Implementation: Without using the comparable interface.

Java

// Creating TreeMap objects using
// Comparable interface in Java

// Importing all generic java utility and input
import java.io.*;
import java.util.*;

// Class - User defined named Employee
public class Employee {

    // Attributes of object of class
    int id;
    String name;

    // Parameterized constructor for user-defined class
    public Employee(int id, String name)
    {
        // This keyword refers to current object in a
        // constructor
        this.id = id;
        this.name = name;
    }
}

// Main class
class GFG {

    // Main driver method
    public static void main(String[] args) throws Exception
    {
        // Declaring and initializing a TreeMap
        TreeMap<Employee, String> tm = new TreeMap<>();

        // Employee object1
        // custom input
        Employee e1 = new Employee(1, "Pathak");

        // Employee object2
        // custom input
        Employee e2 = new Employee(2, "Anshu");

        // Put method associating specific key-value in Map
        tm.put(e1, "First");
        tm.put(e2, "Second");

        // Iterating over Map using for-each loop
        // Map with employee Key
        for (Map.Entry<Employee, String> e :
             tm.entrySet()) {

            // Print key-value pairs of TreeMap
            System.out.println(e.getKey().id + " "
                               + e.getValue());
        }
    }
}

Output: Error

The above code throws exceptions as the comparable interface has not been implemented and hence it cannot order the map elements on the basis of a key in proper order. So, in order to handle the exception

Implementation: With using the comparable interface.

Java

// Creating TreeMap objects using
// Comparable interface in Java

// Importing all generic java utility and input
import java.io.*;
import java.util.*;

// User-defined class named Employee
// implementing comparable
public class Employee implements Comparable<Employee> {

    // Attributes of object of class
    int id;
    String name;

    // Parameterized constructor for user-defined class
    public Employee(int id, String name)
    {
        // This keyword refers to
        // current object in a constructor
        this.id = id;
        this.name = name;
    }

    // Comparable interface
    public int compareTo(Employee e)
    {
        // Two instance of class can be compared
        int diff = this.id - e.id;

        // Note: Two equal employee Id will return 0
        return diff;
    }
}

// Main class
class GFG {

    // Main driver method
    public static void main(String[] args)
    {
        // Declaring and initializing a TreeMap
        TreeMap<Employee, String> tm = new TreeMap<>();

        // Employee object1 (custom input)
        Employee e1 = new Employee(1, "Pathak");

        // Employee object2 (custom input)
        Employee e2 = new Employee(2, "Anshu");

        // Put method associating specific key-value in Map
        tm.put(e1, "First");
        tm.put(e2, "Second");

        // Iterating over Map using for-each loop
        // Map with employee key
        for (Map.Entry<Employee, String> e :
             tm.entrySet()) {

            // Print key-value pairs of TreeMap
            System.out.println(e.getKey().id + " "
                               + e.getKey().name + " "
                               + e.getValue());
        }
    }
}

Output

1 Pathak First
2 Anshu Second