Java Interview Programming Questions

to be continue...

How to reverse a string in java?

//Using StringBuffer class
StringBuffer sbf = new StringBuffer("MyJava");

System.out.println(sbf.reverse());    //Output : avaJyM



//Using iterative method
String str = "MyJava";
 
char[] strArray = str.toCharArray();
 
for (int i = strArray.length - 1; i >= 0; i--)
{
    System.out.print(strArray[i]);     //Output : avaJyM
}

//Using recursive method.
static String recursiveMethod(String str)
{
     if ((null == str) || (str.length() <= 1))
     {
            return str;
     }
 
     return recursiveMethod(str.substring(1)) + str.charAt(0);
}

/*
1st Call —>   recursiveMethod(「MyJava」)
2nd Call —>   recursiveMethod(「yJava」) + 「M」
3rd Call —>  (recursiveMethod(「Java」) + 「y」) + 「M」
4th call —>  ((recursiveMethod(「ava」) + 「J」)+」y」) + 「M」
5th Call —>  (((recursiveMethod(「va」) + 「a」) + 「J」)+」y」) + 「M」
6th Call —>  ((((recursiveMethod(「a」) + 「v」) + 「a」) + 「J」)+」y」) + 「M」
*/

How to find duplicate characters in a string in java?

/*
Problem :
Write a java program to find duplicate characters and their count in a given string? 
For example, in a string 「Better Butter」, duplicate characters and their count is t : 4, e : 3, r : 2 and B : 2.
*/
import java.util.HashMap;
import java.util.Set;
 
class DuplicateCharactersInString
{
    static void duplicateCharCount(String inputString)
    {
        //Creating a HashMap containing char as key and it's occurrences as value
 
        HashMap<Character, Integer> charCountMap = new HashMap<Character, Integer>();
 
        //Converting given string to char array
 
        char[] strArray = inputString.toCharArray();
 
        //checking each char of strArray
 
        for (char c : strArray)
        {
            if(charCountMap.containsKey(c))
            {
                //If char is present in charCountMap, incrementing it's count by 1
 
                charCountMap.put(c, charCountMap.get(c)+1);
            }
            else
            {
                //If char is not present in charCountMap,
                //putting this char to charCountMap with 1 as it's value
 
                charCountMap.put(c, 1);
            }
        }
 
        //Getting a Set containing all keys of charCountMap
 
        Set charsInString = charCountMap.keySet();
 
        System.out.println("Duplicate Characters In "+inputString);
 
        //Iterating through Set 'charsInString'
 
        for (Character ch : charsInString)
        {
            if(charCountMap.get(ch) > 1)
            {
                //If any char has a count of more than 1, printing it's count
 
                System.out.println(ch +" : "+ charCountMap.get(ch));
            }
        }
    }
 
    public static void main(String[] args)
    {
       duplicateCharCount("JavaJ2EE");
 
       duplicateCharCount("Fresh Fish");
 
       duplicateCharCount("Better Butter");
    }
}

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