AP Computer Science Exam Question

You are given a string s containing lowercase letters only. Write a program in Java to perform the following operations:

1. Reverse: Reverse the order of characters in the string.
2. Remove Vowels: Remove all vowels (a, e, i, o, u) from the string.
3. Count Occurrences: Count the occurrences of each character in the string and store the counts in a HashMap where the character is the key and the count is the value.

Your program should define and use the following methods:

public static String reverseString(String s)
public static String removeVowels(String s)
public static HashMap<Character, Integer> countOccurrences(String s)

Example

Input:

String s = "hellothere"

Expected Output:

Reverse: erehtolleh
Remove Vowels: hllthr
Occurrences: {h=3, e=3, l=2, o=1, t=1, r=1}

Note: In this example, the word "hellothere" is reversed to "erehtolleh". The vowels (e, o, e, e) are removed to get "hllthr". The occurrences for each character are (h=3, e=3, l=2, o=1, t=1, r=1).

Answer

import java.util.HashMap;

public class StringManipulation {
    public static void main(String[] args) {
        String s = "hellothere";
        
        System.out.println("Reverse: " + reverseString(s));
        System.out.println("Remove Vowels: " + removeVowels(s));
        System.out.println("Occurrences: " + countOccurrences(s));
    }
    
    public static String reverseString(String s) {
        StringBuilder sb = new StringBuilder(s);
        sb.reverse();
        return sb.toString();
    }
    
    public static String removeVowels(String s) {
        return s.replaceAll("[aeiou]", "");
    }
    
    public static HashMap<Character, Integer> countOccurrences(String s) {
        HashMap<Character, Integer> occurrences = new HashMap<>();
        
        for (char c : s.toCharArray()) {
            occurrences.put(c, occurrences.getOrDefault(c, 0) + 1);
        }
        
        return occurrences;
    }
}

First, we define the StringManipulation class with the main method. Inside the main method, the string to be manipulated is assigned to the s variable.

The program then calls the three methods: reverseString, removeVowels, and countOccurrences, passing the input string as a parameter to each method.

1. In the reverseString method, we create a StringBuilder object initialized with the input string. We use the reverse method of StringBuilder to reverse the string and return it as a string using the toString method.

2. In the removeVowels method, we use the replaceAll method of the String class with a regular expression [aeiou] to remove all vowels from the string. The method returns the modified string without vowels.

3. In the countOccurrences method, we create a HashMap<Character, Integer> to store the counts of each character. We iterate through each character in the input string using a for-each loop. For each character, we use the getOrDefault method of the HashMap to get the current count of the character, and increment it by 1. Finally, we return the occurrences HashMap with the counts for each character.