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Java Coding Interview Questions

Java Coding Interview Questions — Core Java.

A few short coding problems commonly asked in interviews, with solutions.

Q1. Write a program to swap two numbers without using a third variable.

java
public class SwapNumbers {
    public static void main(String[] args) {
        int a = 5, b = 10;
        a = a + b;
        b = a - b;
        a = a - b;
        System.out.println("a = " + a + ", b = " + b);
    }
}

Output:

a = 10, b = 5

Q2. Write a program to check if a string contains only digits.

java
public class DigitCheck {
    public static void main(String[] args) {
        String text = "12345";
        boolean allDigits = text.matches("[0-9]+");
        System.out.println("Contains only digits: " + allDigits);
    }
}

Output:

Contains only digits: true

Q3. Write a program to find the second largest number in an array.

java
public class SecondLargest {
    public static void main(String[] args) {
        int[] numbers = {12, 45, 7, 89, 34};
        int largest = Integer.MIN_VALUE, secondLargest = Integer.MIN_VALUE;

        for (int num : numbers) {
            if (num > largest) {
                secondLargest = largest;
                largest = num;
            } else if (num > secondLargest && num != largest) {
                secondLargest = num;
            }
        }
        System.out.println("Second largest: " + secondLargest);
    }
}

Output:

Second largest: 45

Key Points to Remember

  • Practice writing these solutions by hand, without an IDE's auto-complete, to simulate real whiteboard or online interview conditions.
  • Always consider edge cases out loud during an interview (empty arrays, negative numbers, duplicate values).
  • Interviewers often ask you to explain the time complexity of your solution after writing it.