Dinesh on Java

Strategy Design Patterns

We can easily create a strategy design pattern using lambda.

To implement this pattern we will be using Interfaces and classes. The interface declares a behaviour and concrete classes define the behaviour.

If we want to introduce a new behaviour to the design, we need to add a new concrete class. So this implementation ends up with tons of concrete classes.

Example

Consider list of numbers numbers = (1,2,3,4,5,6,7,8,9,10). The requirement is to find the sum of all numbers.

1. We will introduce an Interface Strategy and add an abstract method int sum(List<Integer> numbers)

public interface Strategy 
	public int sum(List<Integer> numbers);
}

Then we will add a concrete class which will define the behaviour

import java.util.List

public class SummAll implements Strategy {
	@Override
	public int sum(List<Integer> numbers) {
		int sum = 0;
		for (Integer num : numbers) {
			sum = sum+num;
		}
		return sum;
	}
}

2. Now if we get a new requirement to get a sum of all even numbers then we need to add a new concrete class and need to define behaviour to get a sum of all even numbers.

import java.util.List


public class SummAllEven implements Strategy {

	@Override
	public int sum(List<Integer> numbers) {
		int sum = 0;
		for (Integer num : numbers) {
			if (num % 2 == 0) {
				sum = sum + num;
			}
		}
		return sum;
	}
}

For each such requirement, we need to add a new concrete class and this will end up with a lot of classes.

Using Lambda we can solve this problem very easily, let us implement the Strategy Pattern using Lambda.

Strategy Patterns using Lambda

import java.util.Arrays
import java.util.List;
import java.util.function.Predicate;


public class StrategyPatternWithLambda {
	public static int sum(final List<Integer> numbers, final Predicate<Integer> selector) {
		return numbers.stream()
				.filter(selector)
				.mapToInt(Integer::valueOf)
				.sum();
	}
	
	public static void main(String[] args) {
		List<Integer> numbers = Arrays.asList(1,2,3,4,5,6,7,8,9,10);
		//Sum of all numbers
		int sumALl = sum(numbers,e->true);
		
		//Sum of all even numbers
		int sumALlEven = sum(numbers,e->e%2==0);
		
		//Sum of all odd numbers
		int sumALlOdd = sum(numbers,e->e%2!=0);
		
		//Sum of all numbers greater thann 4
		int sumGt4 = sum(numbers,e->e>4);
	}
}