Welcome to our programming practice lesson! Are you ready for a challenging yet exciting task involving nested loops and arrays? We will be unraveling the skill of using nested loops to search through two arrays. Brace yourself for a remarkable journey of practical learning. Let's get started!

Imagine a scenario where you are given two lists of integers. Your task is to write a function that retrieves and returns pairs of integers. The first item of the pair will be from the first list, while the second one will come from the second list. It's crucial to remember that the first element must be less than the second.

The sequence of pairs in your output should align with the order they appear in the input lists. For instance, given the lists {1, 3, 7} and {2, 8, 9}, the function should return {"1 2", "1 8", "1 9", "3 8", "3 9", "7 8", "7 9"}. It will pose a challenge if no pairs exist or if any input list is empty. Let's delve into this task step by step to uncover the solution!

Before venturing into the code, let's decode the problem. Nested looping fits perfectly here.

Start by creating an empty ArrayList named result to store our pairs.

Java
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1import java.util.ArrayList;
2
3public class Solution {
4    public static ArrayList<String> retrievePairs(ArrayList<Integer> list1, ArrayList<Integer> list2) {
5        ArrayList<String> result = new ArrayList<>();

Creating your function and data structure first is a wise strategy!

Now, the focus turns to forming the nested loops. You need to iterate over both lists, and for this, you'll need nested loops. An outer loop will select one element from the first list, and an inner loop will scan through each element of the second list.

Java
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1import java.util.ArrayList;
2
3public class Solution {
4    public static ArrayList<String> retrievePairs(ArrayList<Integer> list1, ArrayList<Integer> list2) {
5        ArrayList<String> result = new ArrayList<>();
6        for (Integer i : list1) {
7            for (Integer j : list2) {
8                // Our logic goes here
9            }
10        }
11        return result;
12    }
13}

In this setup, every element in list1 is represented by i, and for each i, j represents an element in list2.

With our loops ready, it's time to incorporate the logic. We run a check at this point: is the element i from list1 less than the element j from list2? If it's true, we insert the concatenated string {i j} into our result list.

Java
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1import java.util.ArrayList;
2import java.util.Arrays;
3
4public class Solution {
5    public static ArrayList<String> retrievePairs(ArrayList<Integer> list1, ArrayList<Integer> list2) {
6        ArrayList<String> result = new ArrayList<>();
7        
8        // Start of the outer loop: iterating through elements of list1
9        for (Integer i : list1) {
10            // Start of the inner loop: iterating through elements of list2 for each element of list1
11            for (Integer j : list2) {
12                // Check if the element from list1 is less than the element from list2
13                if (i < j) {
14                    // Add the valid pair to the result list
15                    result.add(i + " " + j);
16                }
17            }
18        }
19        // Return the final list of pairs
20        return result;
21    }
22
23    public static void main(String[] args) {
24        ArrayList<Integer> list1 = new ArrayList<>(Arrays.asList(1, 3, 7));
25        ArrayList<Integer> list2 = new ArrayList<>(Arrays.asList(2, 8, 9));
26        ArrayList<String> res = retrievePairs(list1, list2);
27        
28        // Print each pair from the result list
29        for (String pair : res) {
30            System.out.println(pair);
31        }
32        // Outputs 1 2\n1 8\n1 9\n3 8\n3 9\n7 8\n7 9\n
33    }
34}

During each execution of our inner loop, we perform this check and store the pairs that comply with our condition.

Fantastic job! You have successfully performed a complex task using nested loops to search through two lists in Java. You now possess the ability to traverse and manipulate two lists effectively for a given purpose. Keep practicing and continue challenging yourself with more tasks to solidify your understanding. In your upcoming practice sessions, you will come across similar tasks which will further sharpen your programming skills. Remember, practice is the key to mastering any concept. Happy coding!