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What is Top-Down and Bottom-Up Approach in Java?

In Java, Top-down and bottom-up are two different approaches to building a comprehensive Software development pattern. They are applicable in various contexts and programming languages, including Java. Let us explore how they differ from each other with appropriate examples.

Top-Down Approach

  • In the top-down approach, it starts with the high-level overview where it focuses on overall functionality.
  • And then it breaks down the larger problems into smaller bits, more detailed components.
  • Before processing into the implementation part, it focuses on defining the interfaces and relationships between these detailed components.
  • This approach mainly works on creating recursive function calls to solve the sub-modules.
  • It focuses more on emphasizing the larger module at the beginning and later it refines that broader part into refined components.
  • The primary advantage of the top-down approach is that it helps in understanding the overall functionality and structure easily.

Example:

Let’s see with a problem statement how can we implement this approach.

Problem Statement: Implementing an algorithm for sorting an array of integers.

Below are the steps to implement the top-down approach in Java.

1. Considering the overall functionality and defining:

  • Taking array of integers as input.
  • Sorting the elements of the array.
  • Return the array once the sorting is completed.

2. Choosing scenario how to convert it into smaller modules:

  • Opting the relevant algorithm for sorting.
  • Understanding the algorithm’s functionality.
  • Implementing different functions to dividing the array and comparing withe the elements inside the array and finally merging the sub-arrays.

3. Implementation of Breaking the overall functionality into smaller modules:

  • Implement the sorting algorithm and make sure the functionality of it works fine.
  • Implement functions for merging and comparing elements for the array.
  • Now integrate these smaller modules into the main sorting function for sorting.

Program to Implement Top-Down approach in Java

Below is the implementation of Top-Down Approach in Java:

Java

// Java Program to implement TopDownApproach
import java.util.*;
  
public class TopDownApproach {
  
    // merge function to merge two subarrays
    public static void merge(int[] array, int left, int mid, int right) {
        int n1 = mid - left + 1;
        int n2 = right - mid;
  
        // create temporary arrays
        int[] leftArr = new int[n1];
        int[] rightArr = new int[n2];
  
        // copy data to temporary arrays
        for (int i = 0; i < n1; i++) {
            leftArr[i] = array[left + i];
        }
        for (int j = 0; j < n2; j++) {
            rightArr[j] = array[mid + 1 + j];
        }
  
        // merge the temporary arrays
        int i = 0, j = 0, k = left;
        while (i < n1 && j < n2) {
            if (leftArr[i] <= rightArr[j]) {
                array[k] = leftArr[i];
                i++;
            } else {
                array[k] = rightArr[j];
                j++;
            }
            k++;
        }
  
        // copy remaining elements of leftArr[] if any
        while (i < n1) {
            array[k] = leftArr[i];
            i++;
            k++;
        }
  
        // copy remaining elements of rightArr[] if any
        while (j < n2) {
            array[k] = rightArr[j];
            j++;
            k++;
        }
    }
  
    // main function to implement Merge Sort
    public static void mergeSort(int[] array, int left, int right) {
        if (left < right) {
            int mid = (left + right) / 2;
            // sort first and second halves
            mergeSort(array, left, mid);
            mergeSort(array, mid + 1, right);
            // merge the sorted halves
            merge(array, left, mid, right);
        }
    }
  
    // driver method to test above functions
    public static void main(String[] args) {
        int[] array = {16, 15, 13, 11, 18, 37, 12, 24};
        mergeSort(array, 0, array.length - 1);
        // print the sorted array
        for (int j : array) {
            System.out.print(j + " ");
        }
    }
}

Output

11 12 13 15 16 18 24 37

Output:

TopDown approach Output

Explanation of the above Program:

  • For top-down approach, merge sort algorithm is implemented.
  • At the beginning of the program itself the larger view of the program can be viewed.
  • The program break downs the larger problems into smaller bits, more detailed components.
  • It basically, merges and the sorts into halves and recursively builds the solution.

Bottom-Up Approach

  • The Bottom-up Approach starts with the implementation of lower-level overview.
  • It starts building smaller modules/components and combine them to get the bigger picture of the whole functionality in the end.
  • This approach focuses more on the smaller modules first and all the system complexities are merged once the smaller modules are done with implementation.
  • As this approach starts with smaller modules the communication between these modules is necessary.
  • The Bottom-up approach is well suited for java programming language as it is object oriented the communication between the classes is required.
  • In testing, implementation of this approach helps in enhancing the integration of modules more efficiently.

Example

Let’s see with a problem statement how can we implement this approach.

Problem Statement: Implementing an algorithm for sorting an array of integers.

Below are the steps to implement the bottom-up approach in Java.

1. Defining reusable building blocks:

  • Implementing functions for swapping the elements.
  • Logic implementation for comparing the elements.
  • Based on the pivot element make sure to implement functions for dividing the array.

2. Combining the blocks for sorting:

  • Implement any sorting algorithm by considering the functions which are defined for swapping and comparing.
  • Implement the function which used for dividing the array for iterative implementation of the sorting algorithm.

3. Testing Functionality:

  • After all the implementations test the overall functionality with a relevant input.

Program to Implement Bottom-Up approach in Java

Below is the implementation of the Bottom-Up approach in Java:

Java

// Java Program to implement BottomUp Approach
import java.util.*;
  
public class BottomUpApproach {
  
    // function to swap elements
    public static void swap(int[] array, int i, int j) {
        int temp = array[i];
        array[i] = array[j];
        array[j] = temp;
    }
  
    // function to perform Bubble Sort
    public static void bubbleSort(int[] array) {
        for (int i = 0; i < array.length - 1; i++) {
            boolean swap = false;
            for (int j = 0; j < array.length - i - 1; j++) {
                if (array[j] > array[j + 1]) {
                    swap(array, j, j + 1);
                    swap = true;
                }
            }
            if (!swap) {
                break;
            }
        }
    }
  
    // main function to test above functions
    public static void main(String[] args) {
        int[] array = {6, 5, 3, 1, 8, 7, 2, 4};
        // call the Bubble Sort function
        bubbleSort(array);
        // print the sorted array
        for (int i : array) {
            System.out.print(i + " ");
        }
    }
}

Output

1 2 3 4 5 6 7 8

Output:

BottomUp Approach Output

Explanation of the above Program:

In the above example,

  • Bubble sort algorithm is implemented.
  • At the beginning itself the smaller components are built.
  • For swapping and comparing two functions are implemented.
  • So, in an iterative mode the array sorting can be reused.

Conclusion

In conclusion, choosing the type of approach depends upon the context of the requirement. In the above program examples merge sort is implemented for top-down approach and bubble-sort for bottom-up approach. Merge sort algorithm is basically used for larger/complex datasets and bubble sort algorithm is used for simple sorting requirement. So, choosing the type of approach completely depends upon the context of your requirement.


 


Reffered: https://www.geeksforgeeks.org

Java

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