Oracle Coding Questions 2025 - Interview Problems with Solutions

Oracle Coding Questions - Complete Problem Set

Practice 25+ Oracle coding interview questions with solutions. Oracle focuses on Java, database concepts, and clean code.

Oracle Interview Focus

Oracle values: Java expertise, Database knowledge, Enterprise software thinking, Clean OOP code.

Arrays & Strings

1. Two Sum

Java

public int[] twoSum(int[] nums, int target) {
    Map<Integer, Integer> map = new HashMap<>();
    for (int i = 0; i < nums.length; i++) {
        int complement = target - nums[i];
        if (map.containsKey(complement)) {
            return new int[]{map.get(complement), i};
        }
        map.put(nums[i], i);
    }
    return new int[]{};
}

2. Longest Substring Without Repeating Characters

Java

public int lengthOfLongestSubstring(String s) {
    Map<Character, Integer> map = new HashMap<>();
    int maxLen = 0, start = 0;
    for (int i = 0; i < s.length(); i++) {
        char c = s.charAt(i);
        if (map.containsKey(c) && map.get(c) >= start) {
            start = map.get(c) + 1;
        }
        map.put(c, i);
        maxLen = Math.max(maxLen, i - start + 1);
    }
    return maxLen;
}

3. Reverse String

Java

public void reverseString(char[] s) {
    int left = 0, right = s.length - 1;
    while (left < right) {
        char temp = s[left];
        s[left++] = s[right];
        s[right--] = temp;
    }
}

4. Valid Parentheses

Java

public boolean isValid(String s) {
    Stack<Character> stack = new Stack<>();
    for (char c : s.toCharArray()) {
        if (c == '(' || c == '{' || c == '[') {
            stack.push(c);
        } else {
            if (stack.isEmpty()) return false;
            char top = stack.pop();
            if ((c == ')' && top != '(') ||
                (c == '}' && top != '{') ||
                (c == ']' && top != '[')) {
                return false;
            }
        }
    }
    return stack.isEmpty();
}

5. Merge Sorted Arrays

Java

public void merge(int[] nums1, int m, int[] nums2, int n) {
    int i = m - 1, j = n - 1, k = m + n - 1;
    while (j >= 0) {
        if (i >= 0 && nums1[i] > nums2[j]) {
            nums1[k--] = nums1[i--];
        } else {
            nums1[k--] = nums2[j--];
        }
    }
}

Trees & Linked Lists

6. Validate BST

Java

public boolean isValidBST(TreeNode root) {
    return isValidBST(root, null, null);
}

private boolean isValidBST(TreeNode node, Integer min, Integer max) {
    if (node == null) return true;
    if (min != null && node.val <= min) return false;
    if (max != null && node.val >= max) return false;
    return isValidBST(node.left, min, node.val) &&
           isValidBST(node.right, node.val, max);
}

7. Reverse Linked List

Java

public ListNode reverseList(ListNode head) {
    ListNode prev = null;
    while (head != null) {
        ListNode next = head.next;
        head.next = prev;
        prev = head;
        head = next;
    }
    return prev;
}

8. LCA of Binary Tree

Java

public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
    if (root == null || root == p || root == q) return root;

    TreeNode left = lowestCommonAncestor(root.left, p, q);
    TreeNode right = lowestCommonAncestor(root.right, p, q);

    if (left != null && right != null) return root;
    return left != null ? left : right;
}

Dynamic Programming

9. Climbing Stairs

Java

public int climbStairs(int n) {
    if (n <= 2) return n;
    int prev2 = 1, prev1 = 2;
    for (int i = 3; i <= n; i++) {
        int curr = prev1 + prev2;
        prev2 = prev1;
        prev1 = curr;
    }
    return prev1;
}

10. Longest Common Subsequence

Java

public int longestCommonSubsequence(String text1, String text2) {
    int m = text1.length(), n = text2.length();
    int[][] dp = new int[m + 1][n + 1];

    for (int i = 1; i <= m; i++) {
        for (int j = 1; j <= n; j++) {
            if (text1.charAt(i-1) == text2.charAt(j-1)) {
                dp[i][j] = dp[i-1][j-1] + 1;
            } else {
                dp[i][j] = Math.max(dp[i-1][j], dp[i][j-1]);
            }
        }
    }
    return dp[m][n];
}

SQL Questions (Oracle-Specific)

11. Find Second Highest Salary

SELECT MAX(salary) AS SecondHighestSalary
FROM employees
WHERE salary < (SELECT MAX(salary) FROM employees);

-- Alternative using DENSE_RANK
SELECT salary AS SecondHighestSalary
FROM (
    SELECT salary, DENSE_RANK() OVER (ORDER BY salary DESC) as rnk
    FROM employees
) ranked
WHERE rnk = 2;

12. Find Duplicate Emails

SELECT email
FROM employees
GROUP BY email
HAVING COUNT(*) > 1;

13. Department Top 3 Salaries

SELECT d.name AS Department, e.name AS Employee, e.salary AS Salary
FROM employees e
JOIN departments d ON e.department_id = d.id
WHERE (
    SELECT COUNT(DISTINCT e2.salary)
    FROM employees e2
    WHERE e2.department_id = e.department_id AND e2.salary > e.salary
) < 3;

Design Questions

14. LRU Cache

Java

class LRUCache extends LinkedHashMap<Integer, Integer> {
    private int capacity;

    public LRUCache(int capacity) {
        super(capacity, 0.75f, true);
        this.capacity = capacity;
    }

    public int get(int key) {
        return super.getOrDefault(key, -1);
    }

    public void put(int key, int value) {
        super.put(key, value);
    }

    @Override
    protected boolean removeEldestEntry(Map.Entry<Integer, Integer> eldest) {
        return size() > capacity;
    }
}

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Last updated: February 2026