Algorithms in Java Interviews


In this post, we will see algorithm problems with their solutions which are asked during Java interviews.

How to check if a number is Palindrome?

void checkPalindrome(int n){
  int temp, sum = 0;
  int input=n;

  while(n>0) {
     temp = n%10;
     sum = (sum*10) + temp;
     n = n/10;
  }

  if(input == sum){
   System.out.println("Palindrome");
  } else {
   System.out.println("Not Palindrome");
  }
}

How to check if a number is Prime in Java8?

void checkPrime(int n) {
if(n > 1 && IntStream.range(2, n).noneMatch(i -> i%n==0)) {
System.out.println("Prime");
} else {
System.out.println("Non-Prime");
}
}

How to sort objects in reverse order in Java8?

Student student1 = new Student(372,"Venkat",1);
Student student2 = new Student(2,"Sachin",4);
Student student3 = new Student(2345,"Ganguly",6);
Student student4 = new Student(72,"Karthik",2);
List studlist = new CopyOnWriteArrayList();
studlist.add(student1);
studlist.add(student2);
studlist.add(student3);
studlist.add(student4);

// Iterate in Java8
studlist.forEach(s -> System.out.println(s.name));

// Sort by Ids
studlist.sort((Student s1,Student s2) -> s1.getId() - s2.getId());

// Sort by Rank in reverse Order
studlist.sort((Student s1,Student s2) -> s2.getRank() - s1.getRank());

Find second highest number in an Array?

int arr[] = {45,89, 29,1, 9, 100};
int highest = 0, secondHighest = 0;

for(int i=0; i<arr.length;i++) {   if(arr[i] > highest) {
     highest = arr[i];
  } else if(arr[i] > secondHighest) {
     secondHighest = arr[i];
  }
}

Find Nth highest Salary from a SQL Table?

SELECT MIN(SALARY) FROM EMPLOYEE
       WHERE SALARY IN (SELECT DISTINCT TOP N 
                               FROM EMPLOYEE ORDER BY SALARY desc);

Print Only Numerics from a String?

String sampleStr = "fdsha3430d3kdjafl0737434833";
String numericsOnlyStr = sampleStr.replaceAll("[^0-9]", "");

Print Duplicates in an Array?

for(int i=0;i<arr.length;i++) {
  for(int j=i+1; j< arr.length; j++) {
     if(arr[i] == arr[j]) {
           System.out.println(arr[j]);
     }
  }
}

Fetch Frequency of Elements repeated in an Array?

  Map<Integer, Integer> mp = new HashMap<>(); 
  
        // Iterating through array elements 
        for (int i = 0; i < n; i++) 
        { 
            if (mp.containsKey(arr[i]))  { 
                mp.put(arr[i], mp.get(arr[i]) + 1); 
            } else { 
                mp.put(arr[i], 1); 
            } 
        } 
        
        // Iterating through Map and Printing frequencies 
        for (Map.Entry<Integer, Integer> entry : mp.entrySet()) { 
            System.out.println(entry.getKey() + " " + entry.getValue()); 
        }

Find Triplets in an array whose sum is equal to n?

public class Triplets {
public static List<List> findTriplets(int[] numbers, int sum) {
List<List> tripletsCombo = new ArrayList<List>();
HashSet set = new HashSet();
List triplets = new ArrayList();

if (numbers.length == 0 || sum <= 0) {
   return tripletsCombo;
}

Arrays.sort(numbers);

for (int i = 0; i < numbers.length - 2; i++) {
int j = i + 1;
int k = numbers.length - 1;

while (j < k) {
   if (numbers[i] + numbers[j] + numbers[k] == sum) {
      String str = numbers[i] + "," + numbers[j] + "," +       numbers[k];
      // Check for the unique Triplet
      if (!set.contains(str)) {
               triplets.add(numbers[i]);
               triplets.add(numbers[j]);
               triplets.add(numbers[k]);
               tripletsCombo.add(triplets);
               triplets = new ArrayList();
               set.add(str);
     }
     j++;
     k--;
} else if (numbers[i] + numbers[j] + numbers[k] < sum) {    j++; } else { // numbers[i] + numbers[j] + numbers[k] > sum
   k--;
}
}
}

return tripletsCombo;
}

public static void main(String[] args) {
int[] numbers = { 2, 3, 1, 5, 4 };
int sum = 9;
List<List> triplets = findTriplets(numbers, sum);

if (triplets.isEmpty()) {
   System.out.println("No triplets are found");
} else {
   System.out.println(triplets);
}
}
}

How to check if two strings are Anagrams?

Two strings are called Anagrams if they contain same set of characters but in different order.  Examples:  “Astronomer – Moon starer”, “A gentleman – Elegant man”, “Dormitory – Dirty Room”, “keep – peek”.

void isAnagram(String input1, String input2) {
   //Removing all white spaces from s1 and s2
   String s1_nonSpaces = input1.replaceAll("\\s", "");
   String s2_nonSpaces = input2.replaceAll("\\s", "");

   boolean status = true;
   if(s1_nonSpaces.length() != s2_nonSpaces.length()) {
      status = false;
   } else {
      char[] s1Array = s1_nonSpaces.toLowerCase().toCharArray();
      char[] s2Array = s2_nonSpaces.toLowerCase().toCharArray();
      Arrays.sort(s1Array); 
      Arrays.sort(s2Array); 
      status = Arrays.equals(s1Array, s2Array);
   }
   System.out.print(status?"Anagrams":"Non-Anagrams");
}

Swap numbers without using temp/third variable?

void swapWithoutTemp(int a, int b) {
 a = a+b;
 b = a-b;
 a = a-b;
}

Find number of combinations for Sum of Two Elements from two arrays is equal to N?

We have two arrays of numbers, suppose we take one element from first array and another element from second array. Their sum should be equal to N(given number).

sumOfTwoElementsInTwoArrays() {
  int arr1[] = {4,8,10,12,7};
  int arr2[] = {6,90,34,45};

  int sumValue = 44; 
  HashSet complements = new HashSet();
  int pairCount = 0;

  for(int i=0;i<arr1.length;i++) {
    complements.add(arr1[i] - sumValue);
  }

  for(int j=0;j<arr1.length;j++) {
    if(complements.contains(arr2[j])) {
      pairCount++;
    }
 }

System.out.print("Number of pairs is "+pairCount);
}

First non repeated character in a String?

String str = "BANANA";
char firsNonRepeatedCharacter;
HashMap<Character, Integer> hmp = new HashMap<Character, Integer>();

for(int z=0;z<s.length();z++) {
  if(hmp.containsKey(str.charAt(z))) {
    hmp.put(str.charAt(z), hmp.get(str.charAt(z))+1);
  } else {
     hmp.put(str.charAt(z), 1);
  }
}

Set characterSet = hmp.keySet();
for(Character c:characterSet){
  if(hmp.get(c).toString()equals("1")) {
    firsNonRepeatedCharacter = c;
    break;
  }
}

Find the number of occurrence of an element in an array using Java8?

int b[] = {1,2,34,1};

List bList = Arrays.stream(b).boxed().collect(Collectors.toList());

System.out.println(bList.stream().filter(z -> z.toString().equalsIgnoreCase("1")).count());

100 doors toggle open/close

There are 100 doors in a row, all doors are initially closed. A person walks through all doors multiple times and toggle (if open then close, if close then open) them in following way:

In first walk, the person toggles every door, In second walk, the person toggles every second door, i.e., 2nd, 4th, 6th, 8th, …, In third walk, the person toggles every third door, i.e. 3rd, 6th, 9th, …

Find in nth walk, what will be the status of all doors

doorsOpenClosed(int no_of_walks) {
  int door_id, walk_id;
  int doors[] = new int[101];
  for(int i=0;i<100;i++) {
   doors[i] = 0;
  }

for (walk_id = 1; walk_id <= no_of_walks; walk_id++) {
  for (door_id = walk_id; door_id <= 100; door_id += walk_id) {
    if(door_id%walk_id == 0) {
      doors[door_id]=(doors[door_id] == 0)?1:0;
    }
  }
}

for (int j = 0; j <= 100; j++) {
 if(doors[j] == 1) {
   System.out.println("Open Door number::::"+j);
 }
}

}

Core Java and Java 8 Concepts


In this post, you will see some important Core Java/Java 8 concepts related to Collections, Exception Handling, Multi-threading, Concurrency etc.

Comparable Vs Comparator

Comparable Comparator
Comparable provides a single sorting sequence. In other words, Sorting of  collection is based on a single property of a class such as ID, ItemName or quantity etc. The Comparator provides multiple sorting sequences. In other words, sorting of collection can based of multiple properties such as ID, ItemName, and quantity etc.
Comparable affects the original class, i.e., the actual class is modified. Comparator doesn’t affect the original class, i.e., the actual class is not modified.
Comparable provides compareTo() method to sort elements. Comparator provides compare() method to sort elements.
Comparable is from  java.lang package. A Comparator is from java.util package.
Sorting list of Objects-Comparable type can be done using Collections.sort(List) method. Sorting list of Objects-Comparator type by Collections.sort(List, Comparator) method.

JVM Architecture

Different types of Class Loaders?

  • Bootstrap class Loader
  • Extensions class Loader
  • System class Loader

Boostrap class loader loads the classes from jdk/jre/lib/rt.jar. Extension class loader loads the classes from jdk/lib/ext folder jars. System class loader loads the classes from CLASSPATH.

Difference between ClassNotFoundException and NoClassDefFoundError

  • ClassNotFoundException is an Exception, while NoClassDefFoundError is an Error.
  • ClassNotFoundException occurs when CLASSPATH does not get updated with required JAR files while NoClassDefFoundError occurs when required class definition is not present at runtime.

Example for NoClassDefFoundError :

class Shape {
  public void draw() {
     System.out.println("Drawing Shape!");
  }
}

public class DrawingApp {
  public void draw() {
     System.out.println("Drawing Shape!");
  }
  public static void main(String[] args) {
     Shape shape = new Shape();
     shape.draw();
  }
}

After compilation, Shape.class and DrawApp.class are generated, If Shape.class is deleted and DrawApp is run then NoClassDefFoundError is thrown.

Difference between ConcurrentHashMap and SynchronizedMap

  • ConcurrentHashMap is designed for concurrency and improves performance while Collections.synchronizedMap(map) which is non-synchronized by sort can be synchronized by applying a wrapper using Collections.synchronizedMap(map).
  • ConcurrentHashMap doesn’t support null keys or null values while synchronized HashMap supports one null key.
  • Locking in SynchronizedMap is at object level, so read/write operations performance is slower.
  • Locking in ConcurrentHashMap is at a much finer granularity at a hashmap bucket level.

Differences betwen equals() and hashcode() methods

equals() and hashCode() are methods present in Object class and hashCode method should not be used to check if two object references are same. Reason: hashCode just returns int value for an Object, even two different objects can have same hashCode integer. The value returned by hashCode() is the object’s hash code, which is the object’s memory address in hexadecimal. equals() checks if the two object references are same. If two objects are equal then their hashCode must be the same, but the reverse is not true.

O(1) vs O(n) vs O(log n)

These are measures of time complexity of running a piece of code.

O(1) – if execution time is constant, it requires the same amount of time regardless of the size. Example:  array – accessing any element int i = a[0];

O(n) – if execution time is directly proportional to the size.  Example: Linear search for an element has a time complexity of O(n).

O(log n) – if execution time is proportional to the logarithm of the input size. Example: Performing Binary Search on array of elements

Changes to HashMap in Java8

  • In case of Hash collision entry objects are stored as a node in a LinkedList and equals() method is used to compare keys. That comparison to find the correct key with in a linked-list is a linear operation so in a worst case scenario the complexity becomes O(n).
  • To address this issue, Java 8 hash elements use Balanced Tree instead of LinkedList after a certain threshold is reached. Which means HashMap starts with storing Entry objects in linked list but after the number of items in a hash becomes larger than a certain threshold, the hash will change from using a LinkedList to a Balanced Tree, which will improve the worst case performance from O(n) to O(log n).

Fail Fast Vs Fail Safe Iterators

Fail-Fast Iterators Fail-Safe Iterators
Fail-Fast iterators doesn’t allow  modifications of a collection while iterating over it. Fail-Safe iterators allow modifications of a collection while iterating over it.
Concurrent Modification Exception is thrown if a collection is modified while iterating over it. These iterators don’t throw any exceptions if a collection is modified while iterating over it.
They use original collection to traverse over the elements of the collection. They use copy of the original collection to traverse over the elements of the collection.
These iterators don’t require extra memory. These iterators require extra memory to clone the collection.
Ex : Iterators returned by ArrayList, Vector, HashMap. Ex : Iterator returned by CopyOnWriteArrayList, ConcurrentHashMap.

Difference between map() and flatmap() in Java8

Lets suppose we are applying map and flatmap on stream of streams. Example given below

Stream<List<Character>> stream = Stream.of({'a','b'},{'c','d'})

with map:  For input Stream of two lists {‘a’,’b’} and {‘c’,’d’}, output will be {{‘a’,’b’},{‘c’,’d’}} .Here two lists are placed inside a list, so the output will be list containing lists

With flat map: For input Stream of two lists {‘a’,’b’} and {‘c’,’d’}, output will be {{a,b,c,d}} .Here two lists are flattened and only the values are placed in list, so the output will be list containing only elements

What are Functional interfaces how we can define them?

Functional interfaces are interfaces which have only one single abstract method in it. Example:  Runnable Interface since it has only single abstract method, run().

From Java8, we can use @FunctionalInterface to define a functional interface. Although this annotation is optional, once it is used then declaring more than one abstract method will throw compile time error.

Rules of Method Overloading and Method Overriding

There are specific rules while we implement method overloading and overriding in Java with regards to increasing/decreasing visibility of methods of parent class in child class and throwing Checked Exceptions in child class. Complete rules are posted in this below link

https://malliktalksjava.com/2020/05/29/rules-of-method-overloading-and-overriding/

Exception Handling flow having return statements in try/catch/finally blocks

  • Once try block encounters a return statement, the flow immediately transfers to finally block. Let say,it prints “print statement from finally”.
  • Upon the completion of finally block execution, control goes back to the return statement in the try block and returns “returning from try block”.
  • If finally block has a return statement, then the return statements from try/catch blocks will be overridden.

Exception Handling flow while exceptions thrown in catch/finally blocks

  • If the catch block completes normally, then the finally block is executed. Then there is a choice:
  • If the finally block completes normally, then the try statement completes normally. If the finally block completes abruptly for any reason, then the try statement completes abruptly for the same reason.
  • If the catch block completes abruptly for reason R, then the finally block is executed. Then there is a choice:
    If the finally block completes normally, then the try statement completes abruptly for reason R.
    If the finally block completes abruptly for reason S, then the try statement completes abruptly for reason S (and reason R is discarded).

FixedThreadPool vs CachedThreadPool vs ScheduledThreadPool

  • newCachedThreadPool(): creates an expandable thread pool executor. New threads are created as needed, and previously constructed threads are reused when they are available. Idle threads are kept in the pool for one minute. This executor is suitable for applications that launch many short-lived concurrent tasks.
  • newFixedThreadPool(int n): creates an executor with a fixed number of threads in the pool. This executor ensures that there are no more than n concurrent threads at any time. If additional tasks are submitted when all threads are active, they will wait in the queue until a thread becomes available. If any thread terminates due to failure during execution, it will be replaced by a new one. The threads in the pool will exist until it is explicitly shutdown. Use this executor if you and to limit the maximum number of concurrent threads.
  • newScheduledThreadPool(int corePoolSize): creates an executor that can schedule tasks to execute after a given delay, or to execute periodically. Consider using this executor if you want to schedule tasks to execute concurrently.

What is ThreadLocal?

ThreadLocal class provides thread-local variables. It enables you to create variables that can only be read and write by the same thread. If two threads are executing the same code and that code has a reference to a ThreadLocal variable then the two threads can’t see the local variables of each other.

Diffence Volatile vs AtomicInteger?

volatile keyword is used on variables to solve the visibility problem in multi-threaded environment.  AtomicInteger is used if we perform compound operations(incrementing(i++) decrementing(i–)) on variables.

volatile is used on boolean flags, AtomicInteger is used for counters.

 

Differences between yield, join, & sleep

yield() method pauses the currently executing thread temporarily for giving a chance to the remaining waiting threads of the same priority to execute. If there is no waiting thread or all the waiting threads have a lower priority then the same thread will continue its execution. The yielded thread when it will get the chance for execution is decided by the thread scheduler whose behavior is vendor dependent.

join() If any executing thread t1 calls join() on t2 i.e; t2.join() immediately t1 will enter into waiting state until t2 completes its execution.

sleep() Based on our requirement we can make a thread to be in sleeping state for a specified period of time

Differences between Runnable and Callable

  • Runnable object does not return a result whereas a Callable object returns a result.
  • Runnable object cannot throw a checked exception wheras a Callable object can throw an exception.
  • The Runnable interface has been around since Java 1.0 whereas Callable was only introduced in Java 1.5.
class ThreadA implements Runnable {
@Override
public void run() { }
}

public class ThreadB implements Callable<String> {
@Override
public String call() throws Exception {
return "Thread B ran Successfully";
}
}

What is Semaphore in concurrency?

Semaphore is used to restrict the entry to a service to a fixed number of threads at a given time. This is generally used on slow services to make it available for fixed number of requests.

Semaphore semaphore = new Semphore(no_of_permits);

In run() method of a thread, we can use semaphore.acquire() before accessing the slow service and semaphore.release() after to ensure fixed number (defined as no_of_permits) of threads are eligible to access it.

Difference between CyclicBarrier and CountDownLatch?

Both CyclicBarrier and CountDownLatch are used in Multi threading scenario where one Thread waits for one or more Thread to complete their job before it continues processing but main difference between two is that, you can not reuse same CountDownLatch instance once count reaches to zero and latch is open, on the other hand, CyclicBarrier can be reused by resetting Barrier, Once barrier is broken.

  • Initialization of countdownlatch is CountDownLatch latch = new CountDownLatch(4);
  • Method used to countdown (generally used inside run method of thread at a specific point) is latch.countDown()
  • Method used to await a specific thread till countdown number completes is latch.await()
  • Phaser can be used either to perform functionality of both CyclicBarrier and CountDownLatch

References:
https://docs.oracle.com/en/java/javase/
https://stackoverflow.com/
https://dzone.com/

Rules of method overloading and overriding


In this post we will see the rules which needs to adhered while implementing method overriding and overloading with regards to increasing/decreasing visibility of methods of parent class in child class and throwing Checked Exceptions in child class.

Method Overriding rules

For terminology, original method is known as overridden method and new method is known as overriding method. Below rules must be followed to override a methods in Java :

  • Overriding method cannot throw checked exception which is higher in hierarchy than the checked Exception thrown by overridden method. For example if overridden method throws IOException which is checked Exception, than overriding method can not throw java.lang.Exception because it comes higher in type hierarchy.
    "Exception 'Exception' is not compatible with throws clause in" 
    
    **** Overriding method can have Runtime Exceptions declared even if Overridden method does not throw any type of Exceptions.
  • Overriding method can not reduce access of overridden method. It means if overridden method is defined as public than overriding method can not be protected or package private. Similarly if original method is protected then overriding method cannot be package-private. You can see what happens if you violate this rule in Java,
     "You cannot reduce visibility of inherited method of a class".
  • Overriding method can increase access of overridden method. This is opposite of earlier rule.
    ****According to this if overridden method is declared as protected then overriding method can be protected or public
  • private, static, final methods can not be overridden.
    "Cannot override the final method from Parent"
  • Return type of overriding method must be same as overridden method. Changing return type of method in child class will throw compile time error
    "return type is incompatible with parent class method"
    

Method Overloading rules

Here is the list of rules which must be followed to overload a method:

  • First rule to overload a method is to change method signature. method signature is made of number of arguments, type of arguments and order of arguments if they are of different types.  One can change any of these or combinations of them to overload a method in Java.
  • Return type of method is not part of method signature, hence changing the return type alone will not overload a method in Java.  In fact, it will result in compile time error.

Difference between ClassNotFoundException and NoClassDefFoundError


1. java.lang.ClassNotFoundException :  This exception indicates that the class was not found on the classpath. This indicates that we were trying to load the class definition, and the class did not exist on the classpath.

2. java.lang.NoClassDefFoundError :  This exception indicates that the JVM looked in its internal class definition data structure for the definition of a class and did not find it. This is different than saying that it could not be loaded from the classpath. Usually this indicates that we previously attempted to load a class from the classpath, but it failed for some reason – now we’re trying to use the class again (and thus need to load it, since it failed last time), but we’re not even going to try to load it, because we failed loading it earlier (and reasonably suspect that we would fail again). The earlier failure could be a ClassNotFoundException or an ExceptionInInitializerError (indicating a failure in the static initialization block) or any number of other problems. The point is, a NoClassDefFoundError is not necessarily a classpath problem.

How to read a URL using Java?


  • Below program consists of steps to read data from the URL
package in.malliktalksjava;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.URL;
import java.net.URLConnection;

/**
* @author malliktalksjava
*
*/
public class URLConnectionReader {

public static void main(String[] args) throws Exception {

URL oracle = new URL("http://malliktalksjava.in/");
URLConnection yc = oracle.openConnection();
BufferedReader in = new BufferedReader(new InputStreamReader(
yc.getInputStream()));
String inputLine;
while ((inputLine = in.readLine()) != null)
System.out.println(inputLine);
in.close();
}
}

Example Program: Search Word in Folder files and print output


To Search a word in in list of files available in Folder, you need to find the list of files first and then scan each and every for required word. Below is the sample program to find the a given word Java in D:\\test folder of files.

package in.javatutorials;

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;
import java.util.regex.MatchResult;

/**
 * Search for the files in a folder and prints all file details.
 */
public class WordCrawlerInFolder {

private static String directoryPath = "D://test";
private static String searchWord = "Java";

public WordCrawlerInFolder() {
super();
}

public static void main(String[] args) {
   WordCrawlerInFolder crawler = new WordCrawlerInFolder();
    File directory = new File(directoryPath);

    if (directory == null || !directory.exists()) {
           System.out.println("Directory doesn't exists!!!");
           return;
    }
    crawler.directoryCrawler(directory, searchWord);
}

/**
* Gets all the file and directories and prints accordingly
* @param directory
* Directory path where it should search
*/
public void directoryCrawler(File directory, String searchWord) {

// Get List of files in folder and print
File[] filesAndDirs = directory.listFiles();

// Print the root directory name
//System.out.println("-" + directory.getName());

// Iterate the list of files, if it is identified as not a file call
// directoryCrawler method to list all the files in that directory.
for (File file : filesAndDirs) {

if (file.isFile()) {
searchWord(file, searchWord);
//System.out.println(" |-" + file.getName());
} else {
directoryCrawler(file, searchWord);
}
}
}

/**
* Search for word in a given file.
* @param file
* @param searchWord
*/
private void searchWord(File file, String searchWord){
Scanner scanFile;
try {
scanFile = new Scanner(file);
while (null != scanFile.findWithinHorizon("(?i)\\b"+searchWord+"\\b", 0)) {
MatchResult mr = scanFile.match();
System.out.printf("Word found : %s at index %d to %d.%n", mr.group(),
mr.start(), mr.end());
}
scanFile.close();
} catch (FileNotFoundException e) {
System.err.println("Search File Not Found !!!!! ");
e.printStackTrace();
}
}
}

We have used some escape characters in above class searchWord() method, below is the notation for the same.

  1. (?i) turn on the case-insensitive switch
  2. \b means a word boundary
  3. java is the string searched for
  4. \b a word boundary again.

If search term contain special characters, it would be suggested to use \Q and \E around the string, as it quotes all characters in between. Make sure the input doesn’t contain \E itself.

Other Useful Links:

Javac/Java searching algorithm for other classes

Example program to reverse a Number in Java

How to find count of duplicates in a List

Threads Interview questions in Java

Example Java Program to Search Files in a Folder


Below Java Program lists the file names and directory names available in given folder. To do this implementation , we can get the files list in a folder using File class available in Java API. Iterate the files one by one and write the file name on to console.

If it is identified as a directory instead of a file, then iterate the process as mentioned in directoryCrawler() method in the below class.

package in.javatutorials;

import java.io.File;

/**
* @author malliktalksjava
*
* Search for the files in a folder and prints all file details.
*
*/
public class FolderCrawler {

private static String directoryPath = “D://test”;

/**
* Creating constructor
*/
public FolderCrawler() {
super();
}

/**
* main method
*
* @param ags
*/
public static void main(String[] args) {
FolderCrawler crawler = new FolderCrawler();

File directory = new File(directoryPath);

if (directory == null || !directory.exists()) {
System.out.println(“Directory doesn’t exists!!!”);
return;
}

crawler.directoryCrawler(directory);
}

/**
* Gets all the file and directories and prints accordingly
*
* @param directory
* Directory path where it should search
*/
public void directoryCrawler(File directory) {

// Get List of files in folder and print
File[] filesAndDirs = directory.listFiles();

// Print the root directory name
System.out.println(“-” + directory.getName());

// Iterate the list of files, if it is identified as not a file call
// directoryCrawler method to list all the files in that directory.
for (File file : filesAndDirs) {

if (file.isFile()) {
System.out.println(” |-” + file.getName());
} else {
directoryCrawler(file);

}
}// end of for

}// End of directory Crawler
}

 

Other Useful Links:

Javac/Java searching algorithm for other classes

Example program to reverse a Number in Java

How to find count of duplicates in a List

Threads Interview questions in Java

Merge Sort Example in Java


package in.malliktalksjava;

/**
* @author malliktalksjava
*
* Sorts the given array using merge sort algorithm
*/
public class MergeSortExample {

private int[] array;
private int[] tempMergArr;
private int length;

/**
* @param args
*
* main method.
* Pass the array to sort method and prints the output array into console.
*/
public static void main(String args[]){

int[] inputArr = {12,25,36,95,86,21,14,52,85,32};
MergeSortExample mergeSorter = new MergeSortExample();
mergeSorter.sort(inputArr);
for(int sortedValue:inputArr){
System.out.print(sortedValue);
System.out.print(” “);
}
}

/**
* @param inputArr
*
* Checks given array is null of not, then pass it to mergeSort Method.
*/
public void sort(int[] inputArr) {

if (inputArr == null || inputArr.length == 0) {
System.out.println(“Given Array is null or does not contain any elements”);
return;
}

this.array = inputArr;
this.length = inputArr.length;
this.tempMergArr = new int[length];
mergeSort(0, length – 1);
}

/**
* @param lowerIndex
* @param higherIndex
*
* Sorts the array using mergesort algorithm.
*/
private void mergeSort(int lowerIndex, int higherIndex) {

if (lowerIndex < higherIndex) {
int middle = lowerIndex + (higherIndex – lowerIndex) / 2;
// sort the left side of the array
mergeSort(lowerIndex, middle);
// sort the right side of the array
mergeSort(middle + 1, higherIndex);
// merge both sides
mergeParts(lowerIndex, middle, higherIndex);
}
}

/**
* @param lowerIndex
* @param middle
* @param higherIndex
*
*
*/
private void mergeParts(int lowerIndex, int middle, int higherIndex) {

for (int i = lowerIndex; i <= higherIndex; i++) {
tempMergArr[i] = array[i];
}
int i = lowerIndex;
int j = middle + 1;
int k = lowerIndex;
while (i <= middle && j <= higherIndex) {
if (tempMergArr[i] <= tempMergArr[j]) {
array[k] = tempMergArr[i];
i++;
} else {
array[k] = tempMergArr[j];
j++;
}
k++;
}
while (i <= middle) {
array[k] = tempMergArr[i];
k++;
i++;
}
}
}

 

Other Useful Links:

Quick Sort Example Program in Java

Insertion Sort Example program in JAVA

Selection Sort Example Program in Java

Bubble Sort Example in JAVA

Quick Sort Example Program in Java


package in.malliktalksjava;

/**
* @author malliktalksjava
*
* Sort the given array using QuickSort Algorithm
*/
public class QuickSortExample {

private int array[];
private int length;

/**
* @param args
*
* Main Method. Calls the sort method and print the output into
* console.
*/
public static void main(String args[]) {
QuickSortExample quickSorter = new QuickSortExample();
int[] input = { 15, 42, 14, 100, 85, 1, 13, 6, 9 };
quickSorter.sortArray(input);
for (int count : input) {
System.out.print(count);
System.out.print(” “);
}
}

/**
* @param inputArr
*
* Checks given array is empty or null, if not calls quickSort
* method.
*/
public void sortArray(int[] inputArr) {

if (inputArr == null || inputArr.length == 0) {
return;
}
this.array = inputArr;
length = inputArr.length;
quickSort(0, length – 1);
}

/**
* @param lowerIndex
* @param higherIndex
*
* Sorts the given array
*/
private void quickSort(int lowerIndex, int higherIndex) {

int temp1 = lowerIndex;
int temp2 = higherIndex;
// calculate pivot number, here pivot is middle index number
int pivot = array[lowerIndex + (higherIndex – lowerIndex) / 2];
// Divide into two arrays
while (temp1 <= temp2) {
/**
* In each iteration, we will identify a number from left side which
* is greater then the pivot value, and also we will identify a
* number from right side which is less then the pivot value. Once
* the search is done, then we exchange both numbers.
*/
while (array[temp1] < pivot) {
temp1++;
}
while (array[temp2] > pivot) {
temp2–;
}
if (temp1 <= temp2) {
exchangeNumbers(temp1, temp2);
// move index to next position on both sides
temp1++;
temp2–;
}
}
// call quickSort() method recursively
if (lowerIndex < temp2)
quickSort(lowerIndex, temp2);
if (temp1 < higherIndex)
quickSort(temp1, higherIndex);
}

/**
* @param param1
* @param param2
*
* Exchange the numbers and set to instance array variable
*/
private void exchangeNumbers(int param1, int param2) {
int temp = array[param1];
array[param1] = array[param2];
array[param2] = temp;
}
}

 

Other Useful Links:

Bubble Sort Example in JAVA

Selection Sort Example Program in JAVA

Insertion Sort Example program in JAVA

Merge Sort Example in Java