1. Write a Java program to reverse a string.
The provided Java program demonstrates various techniques to reverse a given string. It offers three methods for string reversal:
package com.javainfotech;
public class StringReverse {
public static void main(String[] args) {
//Using StringBuffer
String res = reverseUsingStringBuffer("JavaInfotech");
System.out.println("Reverse String - "+res);
String result = reverseUsingIterate("JavaInfotech");
System.out.println("Reverse String - "+result);
String results = reverseUsingRecursive("JavaInfotech");
System.out.println("Reverse String - "+results);
}
//Using StringBuffer
public static String reverseUsingStringBuffer(String str){
StringBuffer sb = new StringBuffer(str);
return sb.reverse().toString();
}
//Using Iterating
public static String reverseUsingIterate(String str){
char[] ch = str.toCharArray();
String s ="";
for(int i=ch.length-1;i>=0;i--){
s = s+ch[i];
}
return s;
}
//Using Recursive
public static String reverseUsingRecursive(String str){
if((str==null) || (str.length()<=1)){
return str;
}
return reverseUsingRecursive(str.substring(1))+str.charAt(0);
}
}
2. How do I reverse the number?
package com.javainfotech.program;
public class ReverseNumber {
public static void main(String[] args) {
reverseNumber(54321);
}
public static void reverseNumber(int n) {
int reversed = 0;
while (n > 0) {
int lastDigit = n % 10;
if (lastDigit == 0 && reversed == 0) {
n /= 10;
continue;
}
reversed = reversed * 10 + lastDigit;
n /= 10;
}
System.out.println("Reverse Number :"+reversed);
}
}3. Write a program in Java that counts the occurrences of each character in the string and returns a Map<String, Long>, where the key is a String representing a character, and the value is a Long representing the number of times that character appears in the string.
package com.javainfotech.program;
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.stream.Collectors;
public class FindDuplicate {
public static void main(String args[]) {
String str = "javainfotech";
//Solution 1: Basic
Map<Character,Integer> map = new HashMap<Character,Integer>();
char[] ch = str.toCharArray();
for(int i=0;i<ch.length;i++) {
if(map.containsKey(ch[i])) {
map.put(ch[i], map.get(ch[i])+1);
}else {
map.put(ch[i], 1);
}
}
for(Entry<Character, Integer> e : map.entrySet()) {
System.out.println(e.getKey()+" -> "+e.getValue());
}
//Solution 2: Using Java 1.8 with Stream API
Map<String,Long> mapResult = str.chars()
.mapToObj(c-> String.valueOf((char) c))
.collect(Collectors.groupingBy(c->c, Collectors.counting()));
mapResult.forEach((st,n)-> System.out.println(st+" -> "+n));
}
}4. How do I create an immutable class in Java?
package com.javainfotech.String;
public final class ImmutableExample {
private final int id;
private final String name;
public ImmutableExample(int id, String name) {
super();
this.id = id;
this.name = name;
}
public int getId() {
return id;
}
public String getName() {
return name;
}
}5. Can you write the output of the below progaram?
package com.javainfotech.String;
public class StringConcatation {
public static void main(String[] args){
System.out.println("Test"+12+10);
System.out.println(12+10+"Test");
System.out.println("Test"+12+10+"Example");
System.out.println("Test"+12+"Example"+10);
System.out.println(10+12+"Test"+12+"Example"+10);
}
}6. Van Eck’s sequence is a number sequence where each term is defined based on the previous terms. The sequence starts as follows:
a(1) = 0a(2) = 0- For
n > 2,a(n)is the number of terms since the last occurrence of the valuea(n-1).
For example:
a(1) = 0a(2) = 0a(3) = 1(because0last occurred at position 1)a(4) = 2(because0last occurred at position 2)a(5) = 1(because1last occurred at position 3)a(6) = 1(because2last occurred at position 4)
Write a Java program that calculates and prints the Nth term of Van Eck’s sequence. The program should take an integer N as input and return the Nth term of the sequence.
Input:
- An integer
N(1 <= N <= 10^5)
Output:
- The Nth term of Van Eck’s sequence.
Example:
For N = 5, the output should be 1.
For N = 6, the output should be 1.
package com.javainfotech.sequence;
public class VanECKSequence {
static int MAX = 25;
static int[] seq = new int[MAX];
public static void vanEckSequence(){
for(int i=0;i<MAX-1;i++){
for(int j=i-1;j>=0;j--){
if(seq[j]==seq[i]){
seq[i+1]=i-j;
break;
}
}
}
}
public static void main(String[] args) {
vanEckSequence();
for(int i=0;i<seq.length-1;i++){
System.out.print(seq[i]+", ");
}
}
}
Output:
0, 0, 1, 0, 2, 0, 2, 2, 1, 6, 0, 5, 0, 2, 6, 5, 4, 0, 5, 3, 0, 3, 2, 9, 7. Can you write progaram to sort the number given in an array?
package com.javainfotech;
public class SortArray {
public static void main(String[] args) {
int[] arr = {1,3,5,8,6,5,24,87,65,43,22,12};
int temp, count = arr.length;
for(int i=0;i<count;i++){
for(int j=i+1;j<count;j++){
if(arr[i]>arr[j]){
temp = arr[i];
arr[i] = arr[j];
arr[j] = temp;
}
}
System.out.print(arr[i]+", ");
}
}
}
Output:
1, 3, 5, 5, 6, 8, 12, 22, 24, 43, 65, 878. Given an array of integers num and an integer target, find two numbers in the array that add up to target and return their indices.
- Input: num = [1, 4, 9, 12], target = 5
- Output: [0, 1]
- Explanation: Because num[0] + num[1] == 5, we return [0, 1].
package com.javainfotech.arrays;
import java.util.Arrays;
public class TwoSum {
public static void main(String[] args) {
//Create array
int[] num = {1, 4, 9, 12};
int target = 5;
int[] result = getTwoSum(num,target);
System.out.println("Result indices : "+Arrays.toString(result));
}
//Method create to get the indices
public static int[] getTwoSum(int[] num, int target) {
int[] ans = new int[2];
for(int i=0;i<num.length;i++) {
for(int j=i+1;j<num.length;j++) {
if(num[i]+num[j]==target) {
ans[0]=i;
ans[1]=j;
return ans;//Return indices
}
}
}
return new int[] {-1,-1};
}
}
