Java Notes

Coding Interview Questions

30+ Java coding interview questions with solutions covering strings, arrays, patterns, recursion, and data structure problems commonly asked in placements.

1. Reverse a String

Example

public String reverse(String str) { return new StringBuilder(str).reverse().toString(); } // Without StringBuilder public String reverseManual(String str) { char[] chars = str.toCharArray(); int left = 0, right = chars.length - 1; while (left < right) { char temp = chars[left]; chars[left] = chars[right]; chars[right] = temp; left++; right--; } return new String(chars); }

3. Find Duplicate Characters in String

java example

public Map<Character, Integer> findDuplicates(String str) {
    Map<Character, Integer> map = new LinkedHashMap<>();
    for (char c : str.toCharArray()) {
        map.put(c, map.getOrDefault(c, 0) + 1);
    }
    map.entrySet().removeIf(e -> e.getValue() == 1);
    return map;
}
// "programming" → {r=2, g=2, m=2}

Output

"programming" → {r=2, g=2, m=2}

4. Fibonacci Series

Example

// Iterative (O(n) time, O(1) space) public void fibonacci(int n) { int a = 0, b = 1; for (int i = 0; i < n; i++) { System.out.print(a + " "); int next = a + b; a = b; b = next; } } // Output: 0 1 1 2 3 5 8 13 21 34 // Recursive public int fib(int n) { if (n <= 1) return n; return fib(n - 1) + fib(n - 2); }

Output

0 1 1 2 3 5 8 13 21 34

5. Check if Number is Prime

Example

public boolean isPrime(int n) { if (n <= 1) return false; if (n <= 3) return true; if (n % 2 == 0 || n % 3 == 0) return false; for (int i = 5; i * i <= n; i += 6) { if (n % i == 0 || n % (i + 2) == 0) return false; } return true; }

6. Find Second Largest Element in Array

Example

public int secondLargest(int[] arr) { int first = Integer.MIN_VALUE, second = Integer.MIN_VALUE; for (int num : arr) { if (num > first) { second = first; first = num; } else if (num > second && num != first) { second = num; } } return second; } // [12, 35, 1, 10, 34, 1] → 34

7. Two Sum Problem

Example

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[]{}; } // nums=[2,7,11,15], target=9 → [0, 1]

8. Remove Duplicates from Array

Example

public int[] removeDuplicates(int[] arr) { return Arrays.stream(arr).distinct().toArray(); } // Without streams (sorted array) public int removeDuplicatesSorted(int[] arr) { if (arr.length == 0) return 0; int j = 0; for (int i = 1; i < arr.length; i++) { if (arr[i] != arr[j]) { arr[++j] = arr[i]; } } return j + 1; // new length }

9. Reverse a Linked List

java example

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

10. Check Balanced Parentheses

Example

public boolean isBalanced(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 != '(') return false; if (c == '}' && top != '{') return false; if (c == ']' && top != '[') return false; } } return stack.isEmpty(); } // "{[()]}" → true, "{[(])}" → false

Output

"{[()]}" → true, "{[(])}" → false

11. Find Missing Number in Array (1 to N)

Example

public int findMissing(int[] arr, int n) { int expectedSum = n * (n + 1) / 2; int actualSum = Arrays.stream(arr).sum(); return expectedSum - actualSum; } // [1,2,4,5,6], n=6 → 3

12. String Anagram Check

java example

public boolean isAnagram(String s1, String s2) {
    if (s1.length() != s2.length()) return false;
    char[] a = s1.toLowerCase().toCharArray();
    char[] b = s2.toLowerCase().toCharArray();
    Arrays.sort(a);
    Arrays.sort(b);
    return Arrays.equals(a, b);
}
// "listen", "silent" → true

13. Binary Search

Example

public int binarySearch(int[] arr, int target) { int left = 0, right = arr.length - 1; while (left <= right) { int mid = left + (right - left) / 2; if (arr[mid] == target) return mid; else if (arr[mid] < target) left = mid + 1; else right = mid - 1; } return -1; }

14. Find First Non-Repeating Character

java example

public char firstNonRepeating(String str) {
    LinkedHashMap<Character, Integer> map = new LinkedHashMap<>();
    for (char c : str.toCharArray()) {
        map.put(c, map.getOrDefault(c, 0) + 1);
    }
    for (Map.Entry<Character, Integer> entry : map.entrySet()) {
        if (entry.getValue() == 1) return entry.getKey();
    }
    return '\0';
}
// "aabbcdd" → 'c'

Output

"aabbcdd" → 'c'

15. Merge Two Sorted Arrays

Example

public int[] merge(int[] a, int[] b) { int[] result = new int[a.length + b.length]; int i = 0, j = 0, k = 0; while (i < a.length && j < b.length) { if (a[i] <= b[j]) result[k++] = a[i++]; else result[k++] = b[j++]; } while (i < a.length) result[k++] = a[i++]; while (j < b.length) result[k++] = b[j++]; return result; }

16. Power of Two Check

java example

public class PowerOfTwo {
    public static boolean isPowerOfTwo(int n) {
        return n > 0 && (n & (n - 1)) == 0;
    }
    
    public static void main(String[] args) {
        System.out.println(isPowerOfTwo(16));
        System.out.println(isPowerOfTwo(18));
        System.out.println(isPowerOfTwo(1024));
    }
}

Output

true
false
true

17. Count Vowels and Consonants

Example

public class VowelCount { public static void main(String[] args) { String str = "Hello World"; int vowels = 0, consonants = 0; for (char c : str.toLowerCase().toCharArray()) { if (c >= 'a' && c <= 'z') { if ("aeiou".indexOf(c) != -1) vowels++; else consonants++; } } System.out.println("Vowels: " + vowels); System.out.println("Consonants: " + consonants); } }

Output

Vowels: 3
Consonants: 7

18. Factorial (Recursive + Iterative)

Example

public class Factorial { public static long factorialRecursive(int n) { return n <= 1 ? 1 : n * factorialRecursive(n - 1); } public static long factorialIterative(int n) { long result = 1; for (int i = 2; i <= n; i++) result *= i; return result; } public static void main(String[] args) { System.out.println("5! (recursive) = " + factorialRecursive(5)); System.out.println("10! (iterative) = " + factorialIterative(10)); } }

Output

5! (recursive) = 120
10! (iterative) = 3628800

19. GCD using Euclidean Algorithm

java example

public class GCD {
    public static int gcd(int a, int b) {
        return b == 0 ? a : gcd(b, a % b);
    }
    
    public static void main(String[] args) {
        System.out.println("GCD(48, 18) = " + gcd(48, 18));
        System.out.println("GCD(56, 98) = " + gcd(56, 98));
    }
}

Output

GCD(48, 18) = 6
GCD(56, 98) = 14

20. Rotate Array by K Positions

Example

import java.util.Arrays; public class RotateArray { public static void rotate(int[] arr, int k) { int n = arr.length; k = k % n; reverse(arr, 0, n - 1); reverse(arr, 0, k - 1); reverse(arr, k, n - 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--; } } public static void main(String[] args) { int[] arr = {1, 2, 3, 4, 5, 6, 7}; rotate(arr, 3); System.out.println(Arrays.toString(arr)); } }

Output

[5, 6, 7, 1, 2, 3, 4]

21. Maximum Subarray Sum (Kadane's Algorithm)

Example

public class KadaneAlgorithm { public static int maxSubarraySum(int[] arr) { int maxSum = arr[0], current = arr[0]; for (int i = 1; i < arr.length; i++) { current = Math.max(arr[i], current + arr[i]); maxSum = Math.max(maxSum, current); } return maxSum; } public static void main(String[] args) { int[] arr = {-2, 1, -3, 4, -1, 2, 1, -5, 4}; System.out.println("Max subarray sum: " + maxSubarraySum(arr)); } }

Output

Max subarray sum: 6

22. Detect Cycle in Linked List (Floyd's Algorithm)

java example

public class LinkedListCycle {
    static class ListNode {
        int val;
        ListNode next;
        ListNode(int val) { this.val = val; }
    }
    
    public static boolean hasCycle(ListNode head) {
        ListNode slow = head, fast = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if (slow == fast) return true;
        }
        return false;
    }
    
    public static void main(String[] args) {
        ListNode head = new ListNode(1);
        head.next = new ListNode(2);
        head.next.next = new ListNode(3);
        head.next.next.next = head.next; // Create cycle
        System.out.println("Has cycle: " + hasCycle(head));
    }
}

Output

Has cycle: true

23. Stack using Two Queues

java example

import java.util.*;

public class StackUsingQueues {
    Queue<Integer> q1 = new LinkedList<>();
    Queue<Integer> q2 = new LinkedList<>();
    
    public void push(int x) {
        q2.add(x);
        while (!q1.isEmpty()) q2.add(q1.poll());
        Queue<Integer> temp = q1; q1 = q2; q2 = temp;
    }
    
    public int pop() { return q1.poll(); }
    
    public static void main(String[] args) {
        StackUsingQueues stack = new StackUsingQueues();
        stack.push(1); stack.push(2); stack.push(3);
        System.out.println(stack.pop());
        System.out.println(stack.pop());
    }
}

Output

3
2

24. Decimal to Binary Conversion

java example

public class DecimalToBinary {
    public static String toBinary(int n) {
        if (n == 0) return "0";
        StringBuilder sb = new StringBuilder();
        while (n > 0) {
            sb.append(n % 2);
            n /= 2;
        }
        return sb.reverse().toString();
    }
    
    public static void main(String[] args) {
        System.out.println("10 → " + toBinary(10));
        System.out.println("25 → " + toBinary(25));
        System.out.println("Built-in: " + Integer.toBinaryString(10));
    }
}

Output

10 → 1010
25 → 11001
Built-in: 1010

25. Intersection of Two Arrays

java example

import java.util.*;
import java.util.stream.*;

public class ArrayIntersection {
    public static int[] intersection(int[] a, int[] b) {
        Set<Integer> setA = Arrays.stream(a).boxed().collect(Collectors.toSet());
        return Arrays.stream(b).filter(setA::contains).distinct().toArray();
    }
    
    public static void main(String[] args) {
        int[] a = {1, 2, 3, 4, 5};
        int[] b = {3, 4, 5, 6, 7};
        System.out.println(Arrays.toString(intersection(a, b)));
    }
}

Output

[3, 4, 5]

26. Longest Common Prefix

Example

public class LongestCommonPrefix { public static String longestPrefix(String[] strs) { if (strs.length == 0) return ""; String prefix = strs[0]; for (int i = 1; i < strs.length; i++) { while (!strs[i].startsWith(prefix)) { prefix = prefix.substring(0, prefix.length() - 1); } } return prefix; } public static void main(String[] args) { String[] words = {"flower", "flow", "flight"}; System.out.println("Prefix: " + longestPrefix(words)); } }

Output

Prefix: fl

27. Sort Array of 0s, 1s, 2s (Dutch National Flag)

Example

import java.util.Arrays; public class DutchFlag { public static void sort012(int[] arr) { int low = 0, mid = 0, high = arr.length - 1; while (mid <= high) { if (arr[mid] == 0) { swap(arr, low++, mid++); } else if (arr[mid] == 1) { mid++; } else { swap(arr, mid, high--); } } } private static void swap(int[] arr, int i, int j) { int t = arr[i]; arr[i] = arr[j]; arr[j] = t; } public static void main(String[] args) { int[] arr = {2, 0, 1, 2, 0, 1, 0, 2, 1}; sort012(arr); System.out.println(Arrays.toString(arr)); } }

Output

[0, 0, 0, 1, 1, 1, 2, 2, 2]

28. Matrix Diagonal Sum

Example

public class DiagonalSum { public static int diagonalSum(int[][] mat) { int n = mat.length, sum = 0; for (int i = 0; i < n; i++) { sum += mat[i][i]; // Primary diagonal sum += mat[i][n - 1 - i]; // Secondary diagonal } if (n % 2 == 1) sum -= mat[n/2][n/2]; // Remove center double-count return sum; } public static void main(String[] args) { int[][] mat = {{1,2,3},{4,5,6},{7,8,9}}; System.out.println("Diagonal sum: " + diagonalSum(mat)); } }

Output

Diagonal sum: 25

29. Check Palindrome Linked List

java example

public class PalindromeLinkedList {
    static class ListNode {
        int val; ListNode next;
        ListNode(int val) { this.val = val; }
    }
    
    public static boolean isPalindrome(ListNode head) {
        ListNode slow = head, fast = head;
        while (fast != null && fast.next != null) {
            slow = slow.next; fast = fast.next.next;
        }
        ListNode rev = reverse(slow);
        while (rev != null) {
            if (head.val != rev.val) return false;
            head = head.next; rev = rev.next;
        }
        return true;
    }
    
    private static ListNode reverse(ListNode head) {
        ListNode prev = null;
        while (head != null) {
            ListNode next = head.next;
            head.next = prev; prev = head; head = next;
        }
        return prev;
    }
    
    public static void main(String[] args) {
        ListNode head = new ListNode(1);
        head.next = new ListNode(2);
        head.next.next = new ListNode(2);
        head.next.next.next = new ListNode(1);
        System.out.println("Is palindrome: " + isPalindrome(head));
    }
}

Output

Is palindrome: true

30. LRU Cache Implementation

java example

import java.util.*;

public class LRUCache extends LinkedHashMap<Integer, Integer> {
    private int capacity;
    
    public LRUCache(int capacity) {
        super(capacity, 0.75f, true); // access-order = true
        this.capacity = capacity;
    }
    
    @Override
    protected boolean removeEldestEntry(Map.Entry<Integer, Integer> eldest) {
        return size() > capacity;
    }
    
    public static void main(String[] args) {
        LRUCache cache = new LRUCache(3);
        cache.put(1, 10); cache.put(2, 20); cache.put(3, 30);
        cache.get(1); // Access 1 - makes it recently used
        cache.put(4, 40); // Evicts key 2 (least recently used)
        System.out.println("Contains 2: " + cache.containsKey(2));
        System.out.println("Contains 1: " + cache.containsKey(1));
        System.out.println("Cache: " + cache);
    }
}

Output

Contains 2: false
Contains 1: true
Cache: {3=30, 1=10, 4=40}

Summary

In this chapter, we learned about Coding Interview Questions in detail. Here are the key points:

Basic introduction to the concept and why it is needed
Syntax and structure with complete examples
Internal working and best practices
Real-world applications and use cases
Common mistakes and how to avoid them
How this topic is asked in interviews

To practice these concepts, write and run the code in your IDE and verify the output. Modify each example and experiment so that you deeply understand the concept.

Exam Focus

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Interview Use

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Java Notes

Coding Interview Questions

30+ Java coding interview questions with solutions covering strings, arrays, patterns, recursion, and data structure problems commonly asked in placements.

1. Reverse a String

Example

3. Find Duplicate Characters in String

java example

public Map<Character, Integer> findDuplicates(String str) {
    Map<Character, Integer> map = new LinkedHashMap<>();
    for (char c : str.toCharArray()) {
        map.put(c, map.getOrDefault(c, 0) + 1);
    }
    map.entrySet().removeIf(e -> e.getValue() == 1);
    return map;
}
// "programming" → {r=2, g=2, m=2}

Output

"programming" → {r=2, g=2, m=2}

4. Fibonacci Series

Example

Output

0 1 1 2 3 5 8 13 21 34

5. Check if Number is Prime

Example

6. Find Second Largest Element in Array

Example

7. Two Sum Problem

Example

8. Remove Duplicates from Array

Example

9. Reverse a Linked List

java example

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

10. Check Balanced Parentheses

Example

Output

"{[()]}" → true, "{[(])}" → false

11. Find Missing Number in Array (1 to N)

Example

public int findMissing(int[] arr, int n) { int expectedSum = n * (n + 1) / 2; int actualSum = Arrays.stream(arr).sum(); return expectedSum - actualSum; } // [1,2,4,5,6], n=6 → 3

12. String Anagram Check

java example

public boolean isAnagram(String s1, String s2) {
    if (s1.length() != s2.length()) return false;
    char[] a = s1.toLowerCase().toCharArray();
    char[] b = s2.toLowerCase().toCharArray();
    Arrays.sort(a);
    Arrays.sort(b);
    return Arrays.equals(a, b);
}
// "listen", "silent" → true

13. Binary Search

Example

14. Find First Non-Repeating Character

java example

public char firstNonRepeating(String str) {
    LinkedHashMap<Character, Integer> map = new LinkedHashMap<>();
    for (char c : str.toCharArray()) {
        map.put(c, map.getOrDefault(c, 0) + 1);
    }
    for (Map.Entry<Character, Integer> entry : map.entrySet()) {
        if (entry.getValue() == 1) return entry.getKey();
    }
    return '\0';
}
// "aabbcdd" → 'c'

Output

"aabbcdd" → 'c'

15. Merge Two Sorted Arrays

Example

16. Power of Two Check

java example

public class PowerOfTwo {
    public static boolean isPowerOfTwo(int n) {
        return n > 0 && (n & (n - 1)) == 0;
    }
    
    public static void main(String[] args) {
        System.out.println(isPowerOfTwo(16));
        System.out.println(isPowerOfTwo(18));
        System.out.println(isPowerOfTwo(1024));
    }
}

Output

true
false
true

17. Count Vowels and Consonants

Example

Output

Vowels: 3
Consonants: 7

18. Factorial (Recursive + Iterative)

Example

Output

5! (recursive) = 120
10! (iterative) = 3628800

19. GCD using Euclidean Algorithm

java example

public class GCD {
    public static int gcd(int a, int b) {
        return b == 0 ? a : gcd(b, a % b);
    }
    
    public static void main(String[] args) {
        System.out.println("GCD(48, 18) = " + gcd(48, 18));
        System.out.println("GCD(56, 98) = " + gcd(56, 98));
    }
}

Output

GCD(48, 18) = 6
GCD(56, 98) = 14

20. Rotate Array by K Positions

Example

Output

[5, 6, 7, 1, 2, 3, 4]

21. Maximum Subarray Sum (Kadane's Algorithm)

Example

Output

Max subarray sum: 6

22. Detect Cycle in Linked List (Floyd's Algorithm)

java example

public class LinkedListCycle {
    static class ListNode {
        int val;
        ListNode next;
        ListNode(int val) { this.val = val; }
    }
    
    public static boolean hasCycle(ListNode head) {
        ListNode slow = head, fast = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if (slow == fast) return true;
        }
        return false;
    }
    
    public static void main(String[] args) {
        ListNode head = new ListNode(1);
        head.next = new ListNode(2);
        head.next.next = new ListNode(3);
        head.next.next.next = head.next; // Create cycle
        System.out.println("Has cycle: " + hasCycle(head));
    }
}

Output

Has cycle: true

23. Stack using Two Queues

java example

import java.util.*;

public class StackUsingQueues {
    Queue<Integer> q1 = new LinkedList<>();
    Queue<Integer> q2 = new LinkedList<>();
    
    public void push(int x) {
        q2.add(x);
        while (!q1.isEmpty()) q2.add(q1.poll());
        Queue<Integer> temp = q1; q1 = q2; q2 = temp;
    }
    
    public int pop() { return q1.poll(); }
    
    public static void main(String[] args) {
        StackUsingQueues stack = new StackUsingQueues();
        stack.push(1); stack.push(2); stack.push(3);
        System.out.println(stack.pop());
        System.out.println(stack.pop());
    }
}

Output

3
2

24. Decimal to Binary Conversion

java example

public class DecimalToBinary {
    public static String toBinary(int n) {
        if (n == 0) return "0";
        StringBuilder sb = new StringBuilder();
        while (n > 0) {
            sb.append(n % 2);
            n /= 2;
        }
        return sb.reverse().toString();
    }
    
    public static void main(String[] args) {
        System.out.println("10 → " + toBinary(10));
        System.out.println("25 → " + toBinary(25));
        System.out.println("Built-in: " + Integer.toBinaryString(10));
    }
}

Output

10 → 1010
25 → 11001
Built-in: 1010

25. Intersection of Two Arrays

java example

import java.util.*;
import java.util.stream.*;

public class ArrayIntersection {
    public static int[] intersection(int[] a, int[] b) {
        Set<Integer> setA = Arrays.stream(a).boxed().collect(Collectors.toSet());
        return Arrays.stream(b).filter(setA::contains).distinct().toArray();
    }
    
    public static void main(String[] args) {
        int[] a = {1, 2, 3, 4, 5};
        int[] b = {3, 4, 5, 6, 7};
        System.out.println(Arrays.toString(intersection(a, b)));
    }
}

Output

[3, 4, 5]

26. Longest Common Prefix

Example

Output

Prefix: fl

27. Sort Array of 0s, 1s, 2s (Dutch National Flag)

Example

Output

[0, 0, 0, 1, 1, 1, 2, 2, 2]

28. Matrix Diagonal Sum

Example

Output

Diagonal sum: 25

29. Check Palindrome Linked List

java example

public class PalindromeLinkedList {
    static class ListNode {
        int val; ListNode next;
        ListNode(int val) { this.val = val; }
    }
    
    public static boolean isPalindrome(ListNode head) {
        ListNode slow = head, fast = head;
        while (fast != null && fast.next != null) {
            slow = slow.next; fast = fast.next.next;
        }
        ListNode rev = reverse(slow);
        while (rev != null) {
            if (head.val != rev.val) return false;
            head = head.next; rev = rev.next;
        }
        return true;
    }
    
    private static ListNode reverse(ListNode head) {
        ListNode prev = null;
        while (head != null) {
            ListNode next = head.next;
            head.next = prev; prev = head; head = next;
        }
        return prev;
    }
    
    public static void main(String[] args) {
        ListNode head = new ListNode(1);
        head.next = new ListNode(2);
        head.next.next = new ListNode(2);
        head.next.next.next = new ListNode(1);
        System.out.println("Is palindrome: " + isPalindrome(head));
    }
}

Output

Is palindrome: true

30. LRU Cache Implementation

java example

import java.util.*;

public class LRUCache extends LinkedHashMap<Integer, Integer> {
    private int capacity;
    
    public LRUCache(int capacity) {
        super(capacity, 0.75f, true); // access-order = true
        this.capacity = capacity;
    }
    
    @Override
    protected boolean removeEldestEntry(Map.Entry<Integer, Integer> eldest) {
        return size() > capacity;
    }
    
    public static void main(String[] args) {
        LRUCache cache = new LRUCache(3);
        cache.put(1, 10); cache.put(2, 20); cache.put(3, 30);
        cache.get(1); // Access 1 - makes it recently used
        cache.put(4, 40); // Evicts key 2 (least recently used)
        System.out.println("Contains 2: " + cache.containsKey(2));
        System.out.println("Contains 1: " + cache.containsKey(1));
        System.out.println("Cache: " + cache);
    }
}

Output

Contains 2: false
Contains 1: true
Cache: {3=30, 1=10, 4=40}

Summary

In this chapter, we learned about Coding Interview Questions in detail. Here are the key points:

Basic introduction to the concept and why it is needed
Syntax and structure with complete examples
Internal working and best practices
Real-world applications and use cases
Common mistakes and how to avoid them
How this topic is asked in interviews

To practice these concepts, write and run the code in your IDE and verify the output. Modify each example and experiment so that you deeply understand the concept.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Coding Interview Questions.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

Search Terms

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