Java for Testers – Interview Questions and Answers Part-7
1. What is threading?
– A thread is a lightweight subprocess, the smallest unit of processing.
– It is a separate path of execution.
– Threads are independent.
– If there occurs exception in one thread, it doesn’t affect other threads.
– It uses a shared memory area.
2. How does multi-threading is achieved?
– Multithreading can be achieved by executing two or more threads simultaneously to maximum utilization of CPU.
– Multithreaded applications execute two or more threads run concurrently.
– Each thread runs parallel to each other.
3. How to initiate a thread in Java?
Thread can be initiated by the following ways:
– Extending the Thread class
– Implementing the Runnable Interface
4. What do you mean by thread safe?
Thread-safe code is code that will work even if many Threads are executing it simultaneously.
A piece of code is thread-safe if it only manipulates shared data structures in a manner that guarantees safe execution by multiple threads at the same time.
5. What is the difference between collection and collections?
The Collection is an interface whereas Collections is a class. The Collection interface provides the standard functionality of data structure to List, Set, and Queue. However, Collections class is to sort and synchronize the collection elements.
6. What is a Collection and what are the types of collections?
A Collection is a group of individual objects represented as a single unit. Java provides Collection Framework which defines several classes and interfaces to represent a group of objects as a single unit.
Different types of Collection are:
– Set
– List
– Map
– Queue
7. What is the difference between Array and ArrayList?
– Array is static in size but ArrayList is dynamic in size.
– It is mandatory to provide the size of array during initialization while arraylist can be created without declaring its size.
– Array is faster than Arraylist.
– Array can be multidimensioanl but Arraylist is always single-dimensional.
8. What is the difference between Set and HashSet?
– Set is an interface, HashSet – implementation of interface.
– The Set interface represents a set of some objects, non-ordered, without random-element access. HashSet – implementation of the Set interface, based on the .hashCode() function.
9. What is the difference between HashMap and HashTable?
– HashMap is non synchronized and not thread safe but HashTable is synchronized and thread safe
– HashMap allows one null key and multiple null values but HashTable doesn’t allow any null key or value
– HashMap is faster than HashTable
10. What is the difference between ArrayList and LinkedList?
– Insertions are easy and fast in LinkedList as compared to ArrayList.
– An ArrayList class can act as a list only because it implements List only. List only. LinkedList class can act as a list and queue both because it implements List and Deque interfaces.
– ArrayList internally uses a dynamic array to store the elements. LinkedList internally uses a doubly linked list to store the elements.
11. Write a Java program to calculate the power of a number using a while loop?
public class Power {
public static void main(String[] args) {
int base = 3, exponent = 4;
long result = 1;
while (exponent != 0)
{
result *= base;
--exponent;
}
System.out.println("Answer = " + result);
}
}
12. Can we have duplicate key value in HashMap?
HashMap doesn’t allow duplicate keys but allows duplicate values.
13. How to fetch values from a HashMap?
The java.util.HashMap.get() method of HashMap class is used to retrieve or fetch the value mapped by a particular key mentioned in the parameter.
Hash_Map.get(Object key_element)
14. Write a Java program to verify whether a number is perfect number or not?
class PerfectNumber
{
static boolean isPerfect(int n)
{
int sum = 1;
for (int i = 2; i * i <= n; i++)
{
if (n % i==0)
{
if(i * i != n)
sum = sum + i + n / i;
else
sum = sum + i;
}
}
if (sum == n && n != 1)
return true;
return false;
}
public static void main (String[] args)
{
System.out.println("Below are all perfect" +
"numbers till 10000");
for (int n = 2; n < 10000; n++)
if (isPerfect(n))
System.out.println( n +
" is a perfect number");
}
}
15. Write a Java program for printing the Fibonacci series from 1 to 10?
class FibonacciNumber
{
// Method to print first n Fibonacci Numbers
static void printFibonacciNumbers(int n)
{
int f1 = 0, f2 = 1, i;
if (n < 1)
return;
for (i = 1; i <= n; i++)
{
System.out.print(f2+" ");
int next = f1 + f2;
f1 = f2;
f2 = next;
}
}
public static void main(String[] args)
{
printFibonacciNumbers(10);
}
}
16. Write a Java program to find the greatest of three numbers?
public class GreatestNumber {
public static void main(String args[]) {
int num1 = 10;
int num2 = 21;
int num3 = 7;
if (num1 >= num2 && num1 >= num3)
System.out.println( num1 + " is the maximum number.");
else if (num2 >= num1 && num2 >= num3)
System.out.println( num2 + " is the maximum number.");
else
System.out.println( num3 + " is the maximum number.");
}
}
17. Find and print the largest two numbers from an array of given numbers?
public class LargestTwoNumbers
{
public static void main (String[] args)
{
Scanner scn = new Scanner (System.in);
System.out.print("Enter no. of elements you want in array:");
int n = scn.nextInt();
int array[] = new int[n];
System.out.println("Enter all the elements:");
for (int i = 0; i < array.length; i++)
{
array[i] = scn.nextInt();
}
int largest1, largest2, temp;
largest1 = array[0];
largest2 = array[1];
if (largest1 < largest2)
{
temp = largest1;
largest1 = largest2;
largest2 = temp;
}
for (int i = 2; i < array.length; i++)
{
if (array[i] > largest1)
{
largest2 = largest1;
largest1 = array[i];
}
else if (array[i] > largest2 && array[i] != largest1)
{
largest2 = array[i];
}
}
System.out.println ("The First largest is " + largest1);
System.out.println ("The Second largest is " + largest2);
}
}
18. In the given String, remove the white spaces, reverse it and print only the odd position characters?
public Class PrintOddChars{
public static void main(String[] args) {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
System.out.println("Enter the string: ");
String name = br.readLine();
String odd = "";
String reversedString = reverseString(name.replaceAll("\\s",""));
for(int i=0;i<reversedString.length();i++){
if(i%2!=0){
odd = odd + reversedString.charAt(i);
}
}
System.out.println(odd);
}
public static String reverseString(String s){
String rev="";
char[] arr = s.toCharArray();
for(int i=arr.length-1;i>=0;i--)
rev = rev + arr[i];
return rev;
}
}
19. Check whether a string is anagram of another string?
public class AnagramString {
static void isAnagram(String str1, String str2) {
String s1 = str1.replaceAll("\\s", "");
String s2 = str2.replaceAll("\\s", "");
boolean status = true;
if (s1.length() != s2.length()) {
status = false;
} else {
char[] ArrayS1 = s1.toLowerCase().toCharArray();
char[] ArrayS2 = s2.toLowerCase().toCharArray();
Arrays.sort(ArrayS1);
Arrays.sort(ArrayS2);
status = Arrays.equals(ArrayS1, ArrayS2);
}
if (status) {
System.out.println(s1 + " and " + s2 + " are anagrams");
} else {
System.out.println(s1 + " and " + s2 + " are not anagrams");
}
}
}
20. In a given string, change few characters to upper case as asked?
public class ChangeCharCase {
public static void main(String[] args) throws IOException {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
System.out.println("Enter the string: ");
String name = br.readLine();
String s1 = name.substring(0, 1).toUpperCase();
String nameCapitalized = s1 + name.substring(1);
System.out.println(nameCapitalized);
}
}
21. In a given string, print the occurrence of each character?
public class FindFrequencyOfCharactersInString {
public static void main(String[] args){
Map<Character,Integer> map = new HashMap<>();
map = findFrequency("Java J2EE Java JSP J2EE");
for(Map.Entry<Character,Integer> entry: map.entrySet()){
System.out.println("Frequency of character " + entry.getKey() + " is "+ entry.getValue());
}
}
public static Map<Character,Integer> findFrequency(String input){
char[] arr = input.toLowerCase().replaceAll("\\s+","").toCharArray();
Map<Character,Integer> frequency = new HashMap<>();
for(char c: arr){
if(frequency.containsKey(c)){
frequency.put(c,frequency.get(c) + 1);
}
else{
frequency.put(c,1);
}
}
return frequency;
}
}
22. Find the duplicate strings in a given statement and remove them?
public class RemoveDuplicateWords
{
public static void main(String[] args)
{
String input="Welcome to QAScript Java Interview Question in QAScript";
String[] words=input.split(" ");
for(int i=0;i<words.length;i++)
{
if(words[i]!=null)
{
for(int j=i+1;j<words.length;j++)
{
if(words[i].equals(words[j]))
{
words[j]=null;
}
}
}
}
for(int k=0;k<words.length;k++)
{
if(words[k]!=null)
{
System.out.println(words[k]);
}
}
}
}
23. Use split method to print each word of a statement?
public class SplitStatement{
public static void main(String args[]){
String strMain ="Welcome to QAScript";
String[] arrSplit = strMain.split("\\s");
for (int i=0; i < arrSplit.length; i++){
System.out.println(arrSplit[i]);
}
}
}
24. Find and remove the duplicate characters from a given string and print ?
public static String removeDuplicates(String word){
Set<Character> chars = new HashSet<>();
StringBuilder output = new StringBuilder(word.length());
for (int i = 0; i < word.length(); i++) {
char ch = word.charAt(i);
if (chars.add(ch)) {
output.append(ch);
}
}
return output.toString();
}
25. Write a Java program to print the triangle of numbers?
public class FloydTriangle {
public static void main(String[] args){
Scanner sc = new Scanner(System.in);
System.out.println("Enter the number of rows");
int rows = sc.nextInt();
printFloydTriangle(rows);
}
public static void printFloydTriangle(int n){
int number = 1;
for(int i=0;i<n;i++){
for(int j=0;j<=i;j++){
System.out.print(number +" ");
number++;
}
System.out.println();
}
}
}
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