Java String and Array Interview Questions

Here’s a detailed guide on Java String and Array interview questions, complete with explanations and examples:

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1. Reverse a String

Reversing a string is one of the most commonly asked interview questions.

Example:

public class ReverseString {
    public static void main(String[] args) {
        String input = "hello";
        String reversed = new StringBuilder(input).reverse().toString();
        System.out.println("Reversed String: " + reversed);
    }
}

Explanation:

• StringBuilder.reverse(): Efficiently reverses a string.
• Alternative: Use a loop to reverse character by character.

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2. Check if a String is a Palindrome

A string is a palindrome if it reads the same backward as forward.

Example:

public class PalindromeCheck {
    public static void main(String[] args) {
        String str = "madam";
        String reversed = new StringBuilder(str).reverse().toString();
        if (str.equals(reversed)) {
            System.out.println("The string is a palindrome.");
        } else {
            System.out.println("The string is not a palindrome.");
        }
    }
}

Explanation:

• Reverse the string and compare it with the original.

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3. Find Duplicate Characters in a String

Example:

import java.util.*;

public class DuplicateCharacters {
    public static void main(String[] args) {
        String str = "programming";
        Map<Character, Integer> charCount = new HashMap<>();

        for (char c : str.toCharArray()) {
            charCount.put(c, charCount.getOrDefault(c, 0) + 1);
        }

        System.out.println("Duplicate Characters:");
        charCount.forEach((key, value) -> {
            if (value > 1) System.out.println(key + ": " + value);
        });
    }
}

Explanation:

• Use a HashMap to store character frequencies.
• Print characters with a count greater than 1.

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4. Remove Vowels from a String

Example:

public class RemoveVowels {
    public static void main(String[] args) {
        String str = "beautiful";
        String result = str.replaceAll("[aeiouAEIOU]", "");
        System.out.println("String without vowels: " + result);
    }
}

Explanation:

• replaceAll: Removes all vowels using a regex.

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5. Check if Two Strings are Anagrams

Two strings are anagrams if they have the same characters in the same frequency.

Example:

import java.util.Arrays;

public class AnagramCheck {
    public static void main(String[] args) {
        String str1 = "listen";
        String str2 = "silent";

        char[] arr1 = str1.toCharArray();
        char[] arr2 = str2.toCharArray();

        Arrays.sort(arr1);
        Arrays.sort(arr2);

        if (Arrays.equals(arr1, arr2)) {
            System.out.println("The strings are anagrams.");
        } else {
            System.out.println("The strings are not anagrams.");
        }
    }
}

Explanation:

• Sort both strings and compare the sorted arrays.

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6. Find the First Non-Repeated Character

Example:

import java.util.LinkedHashMap;
import java.util.Map;

public class FirstNonRepeated {
    public static void main(String[] args) {
        String str = "swiss";
        Map<Character, Integer> charCount = new LinkedHashMap<>();

        for (char c : str.toCharArray()) {
            charCount.put(c, charCount.getOrDefault(c, 0) + 1);
        }

        for (Map.Entry<Character, Integer> entry : charCount.entrySet()) {
            if (entry.getValue() == 1) {
                System.out.println("First non-repeated character: " + entry.getKey());
                break;
            }
        }
    }
}

Explanation:

• Use a LinkedHashMap to maintain insertion order.

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7. Rotate an Array

Example:

import java.util.Arrays;

public class RotateArray {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 4, 5};
        int k = 2; // Number of rotations

        rotate(arr, k);
        System.out.println("Rotated Array: " + Arrays.toString(arr));
    }

    private static void rotate(int[] arr, int k) {
        k %= arr.length;
        reverse(arr, 0, arr.length - 1);
        reverse(arr, 0, k - 1);
        reverse(arr, k, arr.length - 1);
    }

    private static void reverse(int[] arr, int start, int end) {
        while (start < end) {
            int temp = arr[start];
            arr[start] = arr[end];
            arr[end] = temp;
            start++;
            end--;
        }
    }
}

Explanation:

1. Reverse the entire array.
2. Reverse the first k elements.
3. Reverse the remaining elements.

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8. Find the Maximum Subarray Sum (Kadane’s Algorithm)

Example:

public class MaxSubarraySum {
    public static void main(String[] args) {
        int[] arr = {-2, 1, -3, 4, -1, 2, 1, -5, 4};
        System.out.println("Maximum Subarray Sum: " + maxSubArraySum(arr));
    }

    private static int maxSubArraySum(int[] arr) {
        int maxSoFar = arr[0];
        int maxEndingHere = arr[0];

        for (int i = 1; i < arr.length; i++) {
            maxEndingHere = Math.max(arr[i], maxEndingHere + arr[i]);
            maxSoFar = Math.max(maxSoFar, maxEndingHere);
        }

        return maxSoFar;
    }
}

Explanation:

• Use a single loop to calculate the maximum sum ending at each index.

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9. Find the Intersection of Two Arrays

Example:

import java.util.*;

public class ArrayIntersection {
    public static void main(String[] args) {
        int[] arr1 = {1, 2, 2, 3};
        int[] arr2 = {2, 3, 4};

        Set<Integer> set1 = new HashSet<>();
        for (int num : arr1) set1.add(num);

        Set<Integer> result = new HashSet<>();
        for (int num : arr2) {
            if (set1.contains(num)) result.add(num);
        }

        System.out.println("Intersection: " + result);
    }
}

Explanation:

• Use a HashSet to store elements from one array and check for matches in the other.

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10. Merge Two Sorted Arrays

Example:

import java.util.Arrays;

public class MergeSortedArrays {
    public static void main(String[] args) {
        int[] arr1 = {1, 3, 5};
        int[] arr2 = {2, 4, 6};

        int[] merged = merge(arr1, arr2);
        System.out.println("Merged Array: " + Arrays.toString(merged));
    }

    private static int[] merge(int[] arr1, int[] arr2) {
        int[] result = new int[arr1.length + arr2.length];
        int i = 0, j = 0, k = 0;

        while (i < arr1.length && j < arr2.length) {
            if (arr1[i] < arr2[j]) {
                result[k++] = arr1[i++];
            } else {
                result[k++] = arr2[j++];
            }
        }

        while (i < arr1.length) result[k++] = arr1[i++];
        while (j < arr2.length) result[k++] = arr2[j++];

        return result;
    }
}

Explanation:

• Use two pointers to merge elements in sorted order.

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Pro Tips for Interview:

1. String Pool: Be ready to explain how strings are stored in the string pool and why they are immutable.
2. Array vs. ArrayList:
   • Arrays are fixed in size, while ArrayList is dynamic.
3. Edge Cases: Always discuss edge cases like empty strings, single-element arrays, etc.
4. Optimize: Use algorithms like Kadane’s for array problems to optimize performance.

Let me know if you'd like further explanations or additional examples! 😊