Design Patterns

But now Makes Sense

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Design Patterns is, without any doubt one of the most hard to grasp topics in SE. One of the reason is, even after understanding patterns theoretically, it's very hard to track down it's usecase in real life applications. Design Patterns are often misunderstood/misused, just to create more problems than solving them. Our attempt here is to provide very good, yet easy to understand real life applications of design patterns, so that one may grasp the concept properly as well as understand it's practical usage with merits and demerits.

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Introduction

Types of Design Patterns

• Creational
• Structural
• Behavioral

Creational Design Patterns

• Simple Factory
• Factory Method
• Abstract Factory Method

Structural Design Patterns

• Adapter
• Decorator
• Facade

Behavioral Design Patterns

• Strategy
• Chain of Responsibility
• Iterator
• Observer

✨ Strategy

Real World Problem

Sasha is learning various sorting algoritms. She's learning selection sort now, but she knows in future, she's going to learn insertion sort, quick sort, merge sort etc. But right now, she's also building a library that can help her with sorting integer numbers. So, she wants to build the library in such a way that she can change the underlying sorting algorithm without changing the other parts of codebase where the sorting functionality is being called.

To put Simply

Strategy pattern provides a way to select an algorithm's behavior at runtime.

Code example

Consider an interface SortingStrategy that provides a method for sorting integer array.

public interface SortingStrategy {
    public Integer[] apply(Integer[] array);
}

And a class SortUtil that's provide sorting facility, but it takes an object of type SortingStrategy as input which is responsible for sorting.

public class SortUtil {
    private SortingStrategy sort;

    public SortUtil(SortingStrategy sort) {
        this.sort = sort;
    }

    public Integer[] sortIntegerArray(Integer[] array) {
        return this.sort.apply(array);
    }
}

Now consider classes two different strategy insertion sort and selection sort where both classes implements SortingStrategy.

Here is the implementation of SelectionSort:

// selection sort
@Slf4j
public class SelectionSort implements SortingStrategy {
    public void swap(Integer[] arr, int i, int j) {
        Integer temp = arr[j];
        arr[j] = arr[i];
        arr[i] = temp;
    }
    public Integer[] apply(Integer[] array) {
        log.info("Sorting strategy: Selection Sort");
        // performing selection sort
        Integer[] buf = Arrays.copyOf(array, array.length);
        for(int i=0; i<buf.length; ++i) {
            int x = i;
            for(int j=i+1; j<buf.length; ++j) {
                if(buf[j] < buf[x]) x = j;
            }
            swap(buf, i, x);
        }
        return buf;
    }
}

Here is the implementation of InsertionSort:

// insertion sort
@Slf4j
public class InsertionSort implements SortingStrategy {
    public Integer[] apply(Integer[] array) {
        log.info("Sorting strategy: Insertion Sort");
        // performing insertion sort
        Integer[] buf = Arrays.copyOf(array, array.length);
        for(int i=0; i<buf.length; ++i) {
            int j = i-1, key = buf[i];
            while(j >= 0 && key < buf[j]) {
                buf[j+1] = buf[j];
                --j;
            }
            buf[j+1] = key;
        }
        return buf;
    }
}

Now, one can easily switch the strategy they choose:

Integer[] array = {1, 9, 8, 7, 4, 2, 0};

SortUtil sortUtil = new SortUtil(new SelectionSort());
Integer[] sortedArray = sortUtil.sortIntegerArray(array);
log.info("Sorted Array: " + Arrays.toString(sortedArray));

sortUtil = new SortUtil(new InsertionSort());
sortedArray = sortUtil.sortIntegerArray(array);
log.info("Sorted Array: " + Arrays.toString(sortedArray));

You can find the full example code here.

License

This repository is published under MIT License. To know more about license please visit this link.

Contributing

I'll continue to improve this repository. So, watch this repo. If you want to contribute, please read this guide.