Java Program to find Duplicate Character In String ?

Below example show how to count repeated characters from String.

import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class DuplicateCharacterFromString {

public static void method(String str){
Map<Character, Integer> map = new HashMap<>();
for(int i=0;i<str.length();i++){
if(map.containsKey(str.charAt(i))){
map.put(str.charAt(i), map.get(str.charAt(i))+1);
}
else{
map.put(str.charAt(i), 1);
}
}
Set<Character> set = map.keySet();
for(Character c : set){
if(map.get(c) > 1){
System.out.println(c+":"+map.get(c));
}
}
}
public static void main(String[] args) {
DuplicateCharacterFromString.method("abca");
}


}
Program Output :-

a:2

Java Programs to Reverse a Numbers ?

Below example shows how to reverse a number using numeric operations. 

public class ReverseNumbers {

public static void reverse(int num){
int reverse = 0;
while(num != 0){
reverse = (reverse*10)+(num%10);
num = num/10;
}
System.out.println("REVERSE : "+reverse);
}
public static void main(String[] args) {
ReverseNumbers.reverse(123);
ReverseNumbers.reverse(1425);
}

}
Program Output :-
REVERSE : 321
REVERSE : 5241

Java Program to find First Non Repeated Character In A String ?

If the word "stress" is input  then it should print  't'   as output .
If the word "teeter" is input  then it should print  'r'   as output .

import java.util.HashMap;
import java.util.Map;

public class FirstNonRepeatedCharacter {
    
public static Character getFirstCha(String str){
Map<Character, Integer> map = new HashMap<>();
for(int i=0;i<str.length();i++){
if(map.containsKey(str.charAt(i))){
map.put(str.charAt(i), map.get(str.charAt(i))+1);
}
else{
map.put(str.charAt(i), 1);
}
}
for(int i=0;i<str.length();i++){
if(map.get(str.charAt(i)) == 1){
return str.charAt(i);
}
}
return null;
}
public static void main(String[] args) {
System.out.println("FIRST NON REPEATED CHARACTER : "+FirstNonRepeatedCharacter.getFirstCha("teeter"));
System.out.println("FIRST NON REPEATED CHARACTER : "+FirstNonRepeatedCharacter.getFirstCha("stress"));
}
} 

Program Output :-
FIRST NON REPEATED CHARACTER : r

FIRST NON REPEATED CHARACTER : t

Java Code Showing Use of String Tokenizer

String Tokenizer belongs to Util Package. And it is use to break String using delimiter such as space , comma(,) , slace either forward(/) or backword(\) or you can use any special symbol or character. 

public class StringTokenizerExample {

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

String str="India is a Great Country.";
StringTokenizer token = new StringTokenizer(str, " ");
while(token.hasMoreTokens()){
System.out.println(token.nextToken());
}
}
}

Java Code Showing use of Comparable Interface

class Employee implements Comparable<Employee>{
private String name;
private int id;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
@Override
public int compareTo(Employee emp) {

return (this.id < emp.id) ? -1 : (this.id > emp.id) ? 1 : 0;
}
}
public class ComparableExample {

public static void main(String[] args) {
Employee e = new Employee();
e.setId(10);
Employee e1 = new Employee();
e1.setId(-1);
Employee e2 = new Employee();
e2.setId(12);
List<Employee> list = new ArrayList<Employee>();
list.add(e);
list.add(e1);
list.add(e2);
Iterator<Employee> it = list.iterator();
Employee emp = null;
System.out.println("BEFORE LIST");
while(it.hasNext()){
emp = it.next();
System.out.println(emp.getId());
}
Collections.sort(list);
Iterator<Employee> it1 = list.iterator();
Employee emp1 = null;
System.out.println("AFTER LIST");
while(it1.hasNext()){
emp1 = it1.next();
System.out.println(emp1.getId());
}
}
}

Java Code To Reverse Each word of String

public class ReverseWord {

public static void main(String[] args) {
String s1="Java Concept Of The Day";
String[] s2=s1.split(" ");
for(int j=0;j<s2.length;j++){
String token=s2[j];
for(int i=token.length()-1;i>=0;i--){
System.out.print(token.charAt(i));
}
System.out.print(" ");
}
}
}

Java Code to Reverse String With White Spaces

public class ReverseStringWithSpaces {

public static void main(String[] args) {
String s1="I Am Not String";
char[] s2=s1.toCharArray();
char[] resultArray=new char[s2.length];
for(int i=0;i<s2.length;i++){
if(s2[i] == ' '){
resultArray[i]=' ';
}
}
int count=resultArray.length-1;
for(int j=0;j<s2.length;j++){
if(s2[j] != ' '){
if(resultArray[count] == ' '){
count--;
}
resultArray[count]=s2[j];
count--;
}
}
System.out.println(s1);
System.out.println(resultArray);
}
}

Java Code to Reverse String

Case 1 > Using For Loop

static void method_1(){
String s1="abc";
for(int i=s1.length()-1;i>=0;i--){
System.out.print(s1.charAt(i));
}
System.out.println();

}

Case 2 > Using String Buffer

static void method_2(){
String s1="abc";
StringBuffer sb=new StringBuffer(s1);
sb.reverse();
System.out.println(sb);
}

Case 3 >  Using Recursive Method

static String recursive_method(String s1){
if(s1==null || s1.length() <=1){
return s1;
}
return recursive_method(s1.substring(1))+s1.charAt(0);
}

Java Code to Remove White Spaces from String

Case 1 > Using Predefined Method

static void method_1(){
String s1="abc d e f";
System.out.println("OUTPUT M1 = "+s1.replaceAll("\\s", ""));

}

Case 2 > Using charAt() Method

static void method_2(){
String s1="abc d e f";
StringBuffer sb=new StringBuffer();
for(int i=0;i<s1.length();i++){
if(s1.charAt(i)!=' ' ){
sb.append(s1.charAt(i));
}
}
System.out.println("OUTPUT M2 = "+sb);

}

Java Code to Check Palindrome

Case 1 > String Palindrome

public static void stringPalindrome(){
String s1="Malayalam";
String s2="";
for(int i=s1.length()-1;i>=0;i--){
s2+=s1.charAt(i);
}
if(s1.equalsIgnoreCase(s2)){
System.out.println("String is Palindrome");
}
else{
System.err.println("String is Not Palindrome");
}
}

Case 2 > Int Palindrome

public static void intPalindrome(){
int n=121;
int palindrome = n;
int reverse=0;
while(palindrome != 0) {
int rem=palindrome % 10;
reverse = reverse * 10 + rem;
palindrome = palindrome / 10;
}
if(n == reverse){
System.out.println("Int is Palindrome");
}
else{
System.err.println("Int is Not Palindrome");
}
}

Java Code to Create Immutable Class

Rule 1 : Make Class Final
Rule 2 : Make Data Members Final
Rule 3 : Make Parameterized Constructor
Rule 4 : Make Only Getters Not Setters

final class ImmutableCheck{
private final String variable;

public ImmutableCheck(String variable) {
this.variable=variable;
}
public String getVariable() {
return variable;
}

}

Java Code to Remove All Spaces

Case 1 > Using Predefined Method

static void method_1(){
String s1="abc d e f";
System.out.println("OUTPUT M1 = "+s1.replaceAll("\\s", ""));
}

Case 2 > Using charAt() Method

static void method_2(){
String s1="abc d e f";
StringBuffer sb=new StringBuffer();
for(int i=0;i<s1.length();i++){
if(s1.charAt(i)!=' ' ){
sb.append(s1.charAt(i));
}
}
System.out.println("OUTPUT M2 = "+sb);
}

Java Code to Count Numbers of Characters From String

public class CountNumberOfCharacters {

public static void main(String[] args) {
String s1="abcb";
Map<Character, Integer> map=new HashMap<>();
for(int i=0;i<s1.length();i++){
char ch=s1.charAt(i);
if(map.containsKey(ch)){
map.put(ch, map.get(ch)+1);
}
else{
map.put(ch, 1);
}
}
Set<Character> set=map.keySet();
for(Character c:set){
System.out.println("CHARACTER = "+c+" , OCCURENCE = "+map.get(c));
}
}

}