Java Coding Test Course, Exploring Debugging Use Cases

Java Coding Test Course: Exploring Debugging Use Cases

Coding tests have become an essential process in many IT industries today. Every year, countless developers prepare for coding tests and try various methods to enhance their algorithm problem-solving abilities. In this course, we will focus on solving algorithm problems using Java and the application of debugging techniques.

Problem: Finding Two Sum in an Array

The following problem is to find the indices of two numbers in a given array such that their sum equals a specific target value.

Problem Description:
Given an integer array nums and an integer target target, write a function that returns the indices of the two numbers that add up to the target. Each input has exactly one solution, and you may not use the same element twice.

Function Signature: 
public int[] twoSum(int[] nums, int target)

Example

Input: nums = [2, 7, 11, 15], target = 9
Output: [0, 1]
Explanation: Since nums[0] + nums[1] = 2 + 7 = 9, the output is [0, 1].

Problem Solving Process

This problem is about finding two numbers in an array that sum to the given target value. We can consider several approaches to solve this.

1. Brute Force Method

The simplest method is to use a double loop to consider all combinations. In the worst case, this checks all elements of the array with a time complexity of O(n²) to find the two numbers that make the target.

Java Code:
public int[] twoSum(int[] nums, int target) {
    for (int i = 0; i < nums.length; i++) {
        for (int j = i + 1; j < nums.length; j++) {
            if (nums[i] + nums[j] == target) {
                return new int[] { i, j };
            }
        }
    }
    throw new IllegalArgumentException("No two sum solution");
}

2. Using HashMap

Instead of using a double loop, we can use a HashMap to store values and indices, checking if the required value exists in the HashMap. This can reduce the time complexity to O(n).

Java Code:
import java.util.HashMap;

public int[] twoSum(int[] nums, int target) {
    HashMap 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);
    }
    throw new IllegalArgumentException("No two sum solution");
}

Debugging Process

Now let’s learn how to use debugging techniques in the above code. Debugging plays a crucial role in understanding the code’s operation and solving problems. The most commonly used debugging methods in Java are as follows.

• Print Statements (System.out.println): A way to understand the state of variables by printing them at specific points in the code.

    System.out.println("Current index: " + i);
    System.out.println("Current number: " + nums[i]);
    System.out.println("Complement: " + complement);
    

• Using the IDE Debugger: Tools in IDEs like Eclipse or IntelliJ to trace code execution line by line.
• Unit Tests: Writing test cases that check inputs and expected outputs to verify the correctness of the code.

Test Cases

Here are various test cases for the function we wrote. These tests help ensure that the code works as expected.

public void testTwoSum() {
    assertArrayEquals(new int[] {0, 1}, twoSum(new int[]{2, 7, 11, 15}, 9));
    assertArrayEquals(new int[] {1, 2}, twoSum(new int[]{3, 2, 4}, 6));
    assertArrayEquals(new int[] {0, 1}, twoSum(new int[]{3, 3}, 6));
}

Conclusion

In this article, we discussed the problem of finding the sum of two numbers in an array, and explained various approaches and debugging techniques. To solve algorithm problems, it is important not only to understand the problem but also to use different algorithms and apply debugging skills in the process. I hope that by solving these problems using Java, you can further enhance your programming abilities.

If you found this course useful, I encourage you to continue studying other algorithm problems and debugging techniques. The stronger your foundation, the easier it will be to solve more complex problems.