In this post I would like to show how the strategy pattern can be implemented as an enum with lambdas.

The Strategy Pattern is one of the Gang of Four design patterns published in their book: Elements of Reusable Object-Oriented Software. The intent of the strategy pattern is:

Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

We will develop a simple calculator where the strategies are basic arithmetic operations. We start with an interface that defines a single abstract method.

@FunctionalInterface
public interface OperationStrategy {
    T compute(T x, T y);
}

We now implement each arithmetic operation using a lambda expresion.

public enum Operation implements OperationStrategy {

    ADD((x, y) -> x + y),
    SUBTRACT((x, y) -> x - y),
    MULTIPLY((x, y) -> x * y),
    DIVIDE((x, y) -> x / y),
    MAX(Double::max);

    private OperationStrategy operationStrategy;

    Operation(final OperationStrategy operationStrategy) {
        this.operationStrategy = operationStrategy;
    }

    @Override
    public Double compute(Double x, Double y) {
        return operationStrategy.compute(x, y);
    }
}

A series of tests prove that it works.

@RunWith(MockitoJUnitRunner.class)
public class OperationStrategyTest {

    @Test
    public void shouldAddTwoNumbers() {
        assertThat(Operation.ADD.compute(5d, 5d)).isEqualTo(new Double(10));
    }

    @Test
    public void shouldSubtractTwoNumbers() {
        assertThat(Operation.SUBTRACT.compute(10d, 5d)).isEqualTo(new Double(5d));
    }

    @Test
    public void shouldMultiplyTwoNumbers() {
        assertThat(Operation.MULTIPLY.compute(5d, 5d)).isEqualTo(new Double(25));
    }

    @Test
    public void shouldDivideTwoNumbers() {
        assertThat(Operation.DIVIDE.compute(10d, 2d)).isEqualTo(new Double(5d));
    }

    @Test
    public void shouldDetermineMaximumOfTwoNumbers() {
        assertThat(Operation.MAX.compute(10d, 5d)).isEqualTo(new Double(10d));
    }
}

The use of lambdas as strategies reduces boilerplate code quite substantially, though it would not be correct to use lambdas if the strategy is complex and requires a lot of code. It would become cumbersome.

A git repository of this code is available here.