What is Strategy Pattern?

Strategy Pattern is behavioral pattern that defines a family of algorithms, encapsulates each one by putting them into a separate class, and makes them interchangeable.

A family of algorithms refers to a set of algorithms that do the same but differently. Sorting algorithms are good example since they do the same things, SORTING! But process is not the same such as merge sort, bubble sort, quick sort.

When to use it?

Use Strategy pattern when you have many possible variation of algorithms.
For example, for a set of data, there's many possible searching algorithms such as linear search, binary search, jump search. Using Strategy pattern let us to change strategy dynamically with help of composition (you'll see in UML), that is, you can change searching algorithm for the set of data at runtime.

A good indication for applicability of strategy pattern is if we find different algorithms/behaviors in our methods which are selected with conditional statements like if-else or switch-case.

UML diagram

Client only see context class, using Strategy pattern encapsulates algorithms.
Context uses strategy object to run actual algorithm e.g. strategy.runAlgorithm().
We can define multiple concrete strategies that implements actual algorithm in different way.

Example UML diagram

Implementation in Java

Hero Class:

/*
Content in Strategy pattern
 */
public abstract class Hero {

    private WeaponStrategy weapon;

    public Hero() {
        this.weapon = new NoWeaponStrategy();
    }

    public abstract void greet();

    public void surrender() {
        System.out.println("You lose");
    }

    public WeaponStrategy getWeapon() {
        return weapon;
    }

    public void setWeapon(WeaponStrategy weapon) {
        this.weapon = weapon;
    }
}

Warrior Class:

/*
Concrete content in Strategy pattern
 */
public class Warrior extends Hero {

    @Override
    public void greet() {
        System.out.println("Die with honor!");
    }
}

WeaponStrategy Interface:

/*
Strategy in Strategy pattern
Interface for algorithms implementation
 */
public interface WeaponStrategy {

    void useWeapon();
}

NoWeaponStrategy Class:

/*
Concrete strategy
 */
public class NoWeaponStrategy implements WeaponStrategy {

    @Override
    public void useWeapon() {
        System.out.println("Can't attack, you need a weapon!");
    }
}

AxeStrategy Class:

/*
Concrete strategy
 */
public class AxeStrategy implements WeaponStrategy {

    @Override
    public void useWeapon() {
        System.out.println("Chopping with an axe!");
    }
}

Client Class:

/*
Client can use different strategies (or algorithms) dynamically
 */
public class Client {

    public static void main(String[] args) {
        Hero warrior = new Warrior();
        // warrior hasn't equipped a weapon
        warrior.getWeapon().useWeapon();
        // warrior equips an axe
        warrior.setWeapon(new AxeStrategy());
        warrior.getWeapon().useWeapon();
    }
}

Output:

Can't attack, you need a weapon!
Chopping with an axe!

Notice warrior has used NoWeaponStrategy at the beginning, then by calling setWeapon method, warrior changed his strategy to AxeStrategy. Thus, it's proven that Strategy pattern let us to change algorithm at runtime.

You can check all the design pattern implementations here.
GitHub Repository

P.S.
I'm new to write tech blog, if you have advice to improve my writing, or have any confusing point, please leave a comment!
Thank you for reading :)