Java Comparator Applications

Java Comparator is one of the favourite questions in Java Interviews.

This concept looks deceptively simple, and slight variations of this could be asked in the interviews.


1) Write an AlphaNumeric Comparator in Java that sorts the given strings in the human readable
    format.

 e.g., file1, file2, file 3, file4, file 10

Not like the typical comparator file1, file10

	/**
	*
	*/
	package com.core.comparator;
	import java.util.ArrayList;
	import java.util.Collections;
	import java.util.Comparator;
	import java.util.List;
	/**
	* @author kkanaparthi
	*
	* This class performs Alphanumeric Comparison for the
	* given input Strings, such that if two strings are equal
	* alphabetically, they should be sorted by their Alphabetic
	* Values, and then by their Numerical Order in increasing Order.
	*
	*/
	public class AlphanumericComparator {
	public static void main(String[] args) {
	Person person1 = new Person("Apple1",20);
	Person person2 = new Person("Apple10",3);
	Person person3 = new Person("2Google",18);
	Person person4 = new Person("Apple6",25);
	Person person5 = new Person("Apple3",21);
	Person person6 = new Person("Amazon",45);
	Person person7 = new Person("Apple2",20);
	List<Person> personsList = new ArrayList<>();
	personsList.add(person1);
	personsList.add(person2);
	personsList.add(person3);
	personsList.add(person4);
	personsList.add(person5);
	personsList.add(person6);
	personsList.add(person7);
	System.out.println(" Printing the List Elements Before sort ");
	Collections.sort(personsList,new NameComparator());
	//Collections.sort(personsList,new AgeComparator());
	System.out.println(" Printing the List Elements After sort ");
	for(Person person: personsList) {
	System.out.println(" The Person is "+person);
	}
	}
	}
	/**
	*
	* @author kkanaparthi
	*
	*/
	class NameComparator implements Comparator<Person> {
	@Override
	public int compare(Person pO1, Person pO2) {
	int compareValue = 0;
	if(pO1!=null && pO2!=null &&
	pO1.getName()!= null && pO2.getName()!= null) {
	if(areDigitsPresent(pO1.getName()) && areDigitsPresent(pO2.getName())) {
	String nonDigitString1 = getNonDigitString(pO1.getName());
	String nonDigitString2 = getNonDigitString(pO2.getName());
	System.out.println(" nonDigitString1 " + nonDigitString1
	+" nonDigitString2 "+nonDigitString2
	+" pO1.getName() "+pO1.getName()
	+" pO2.getName() "+pO2.getName());
	compareValue =
	nonDigitString1.compareToIgnoreCase(nonDigitString2);
	if(compareValue==0) {
	int digit1 = getDigitsPresent(pO1.getName());
	int digit2 = getDigitsPresent(pO2.getName());
	compareValue = digit1 - digit2;
	System.out.println("\n\n pO1.getName() "+pO1.getName()
	+" pO2.getName() "+pO2.getName()
	+" digit1 "+
	digit1
	+" digit2 "+
	digit2
	+" compareValue "+compareValue);
	}
	} else {
	compareValue = pO1.getName().compareToIgnoreCase(pO2.getName());
	}
	}
	return compareValue;
	}
	/**
	* This methods finds if the given character is Alphabetic
	* Character
	* @param input
	* @return
	*/
	private String getNonDigitString(String input) {
	String nonDigitString = "";
	StringBuilder sb = new StringBuilder();
	if(input!=null && input.length()>0) {
	for(int i=0;i<input.length();i++) {
	char charElem = input.charAt(i);
	if(Character.isLetter((charElem))) {
	sb.append(charElem);
	}
	}
	}
	nonDigitString = sb.toString();
	return nonDigitString;
	}
	/**
	* This methods finds if the given character is Alphabetic
	* Character
	* @param input
	* @return
	*/
	private int getDigitsPresent(String input) {
	int digitPresent =0;
	StringBuilder stirngBuffer = new StringBuilder();
	if(input!=null && input.length()>0) {
	for(int i=0;i<input.length();i++) {
	char charElem = input.charAt(i);
	if(Character.isDigit(charElem)) {
	stirngBuffer.append(charElem);
	}
	}
	}
	digitPresent = Integer.parseInt(stirngBuffer.toString());
	return digitPresent;
	}
	/**
	* This methods finds if the given character is Alphabetic
	* Character
	* @param input
	* @return
	*/
	public boolean areDigitsPresent(String input) {
	boolean digitPresent = false;
	if(input!=null && input.length()>0) {
	for(int i=0;i<input.length();i++) {
	char charElem = input.charAt(i);
	if(Character.isLetter((charElem))) {
	} else if(Character.isDigit(charElem)) {
	digitPresent = true;
	break;
	}
	}
	}
	return digitPresent;
	}
	}
	class AgeComparator implements Comparator<Person> {
	@Override
	public int compare(Person pO1,Person pO2) {
	if(pO1!=null && pO2!=null) {
	return pO1.getAge()-pO2.getAge();
	}
	return 0;
	}
	}
	/**
	*
	* @author kkanaparthi
	*
	*/
	class Person {
	/**
	* @param pName
	* @param pAge
	*/
	public Person(String pName, int pAge) {
	super();
	name = pName;
	age = pAge;
	}
	private String name;
	private int age;
	/**
	* @return the name
	*/
	public String getName() {
	return name;
	}
	/**
	* @param pName the name to set
	*/
	public void setName(String pName) {
	name = pName;
	}
	/**
	* @return the age
	*/
	public int getAge() {
	return age;
	}
	/**
	* @param pAge the age to set
	*/
	public void setAge(int pAge) {
	age = pAge;
	}
	/* (non-Javadoc)
	* @see java.lang.Object#toString()
	*/
	@Override
	public String toString() {
	return "Person [name=" + name + ", age=" + age + "]";
	}
	/* (non-Javadoc)
	* @see java.lang.Object#hashCode()
	*/
	@Override
	public int hashCode() {
	final int prime = 31;
	int result = 1;
	result = prime * result + age;
	result = prime * result + ((name == null) ? 0 : name.hashCode());
	return result;
	}
	/* (non-Javadoc)
	* @see java.lang.Object#equals(java.lang.Object)
	*/
	@Override
	public boolean equals(Object obj) {
	if (this == obj)
	return true;
	if (obj == null)
	return false;
	if (getClass() != obj.getClass())
	return false;
	Person other = (Person) obj;
	if (age != other.age)
	return false;
	if (name == null) {
	if (other.name != null)
	return false;
	} else if (!name.equals(other.name))
	return false;
	return true;
	}
	}

view raw AlphanumericComparator.java hosted with ❤ by GitHub

2) Chained Comparator which can be used to join Multiple Comparators


3) Given a List of Employee Objects group the Objects such that all the employees are grouped by
    Department and by Name.


4) Use Java 8 Style Comparators