Java – Comparable interface

September 12, 2016 SJ Collection 0

In this article, we will discuss Comparable interface with its only important method compareTo() in detail

String & wrapper classes’ implements comparable interface and provide default natural sorting-order

1. Comparable interface:

  • Comparable interface is used to order objects of each class that implements it
  • This is part of original collection framework introduced in Java 1.2 version
  • Present in java.lang package
  • Fully qualified name is java.lang.Comparable
  • Contains only one method compareTo() which returns integer value
  • Method signature: 
public int compareTo(Object obj);
  • String & wrapper classes’ like Integer, Double, etc implements Comparable interface and provides default natural sorting-order [DNSO]
  • We can reverse natural sorting-order by overriding compareTo() method and coding our own logic
  • Based on the integer value returned by compareTo(object2) method, elements are stored/sorted,
    1. returns negative integer, if object1 less than object2 (object1 < object2)
    2. returns positive integer, if object1 greater than object2 (object1 > object2)
    3. returns Zero (0), if object1 is equal to object2 (object1 == object2)
  • Collection classes like TreeSet & TreeMap uses compareTo() method to sort elements before storing

2. Example on Comparable interface:

Customer.java

  • Customer POJO with 2 member variables of Integer and String type
  • which implements Comparable interface
  • to provide natural ordering of Customer objects on the basis of customer name
package in.bench.resources.java.collection;

public class Customer implements Comparable<Customer> {

	// member variables
	int customerId;
	String customerName;

	// 2-arg parameterized constructor
	public Customer(int customerId, String customerName) {
		super();
		this.customerId = customerId;
		this.customerName = customerName;
	}

	// override toString() method
	@Override
	public String toString() {
		return "Customer ["
				+ "customerId=" + customerId
				+ ", customerName=" + customerName
				+ "]";
	}

	// override compareTo() method
	@Override
	public int compareTo(Customer o) {
		return this.customerName.compareTo(o.customerName);
	}
}

Main class

  • This class uses above customer POJO to store objects inside TreeSet
  • prints customer objects in ascending sorting-order of customer name

CustomerNatrualOrder.java

package in.bench.resources.java.collection;

import java.util.TreeSet;

public class CustomerNatrualOrder {

	// main() method
	public static void main(String[] args) {

		// creating TreeSet object of type String
		TreeSet<Customer> ts = new TreeSet<Customer>();

		// adding elements to TreeSet object
		ts.add(new Customer(101, "Sundar Pichai"));
		ts.add(new Customer(107, "Satya Nadella"));
		ts.add(new Customer(103, "Shiv Nadar"));
		ts.add(new Customer(102, "Shantanu Narayen"));
		ts.add(new Customer(104, "Francisco D’Souza"));
		ts.add(new Customer(106, "Vishal Sikka"));
		ts.add(new Customer(105, "Chanda Kochhar"));

		System.out.println("Natural ordering of Customer Name\n");

		// natural ordering of customer name
		for(Customer cust : ts){
			System.out.println(cust.customerId + "  "
					+ cust.customerName);
		}
	}
}

Output:

Natural ordering of Customer Name

105  Chanda Kochhar
104  Francisco D’Souza
107  Satya Nadella
102  Shantanu Narayen
103  Shiv Nadar
101  Sundar Pichai
106  Vishal Sikka

Related Articles:

References:

Happy Coding !!
Happy Learning !!

Java - Comparator interface with example
Java - HashSet v/s LinkedHashSet v/s TreeSet

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