PROGRAMMING

m9/ JAVA
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Last update:   10-10-2021

Strategy

Used when you need to dynamically change an object behavior at run time. This pattern separates behavior from super and subclasses. It allows you to eliminate code duplication.
 
/** 
 * Using just interfaces ...
 * we can't change the behavior at runtime
 */
class App {
    
    public static void main(String[] args) {
        
        Master m = new Master();
        m.name = "Socrate";
        m.speak();

        Pupil p = new Pupil();
        p.name = "Alcibiades";
        p.speak();

        // after that ...
        // we can't change speak() behavior at runtime
    }    
}

class Master implements Speak {
    public String name;
    public void speak() {
        System.out.println(name + ": I can raise questions ...");
    }
}

class Pupil implements Speak {
    public String name;
    public void speak() {
        System.out.println(name + ": I can answer!");
    }
}

interface Speak {
    public void speak();
}

// Socrate: I can raise questions ...
// Alcibiades: I can answer!
... 27 lines
 
With Strategy Pattern we use an instance variable (composition technique). It allows us to change the capabilities of objects at run time!
 
class Alcibiades {
    
    public static void main(String[] args) {
        
        Master m = new Master("Socrate");
        m.type = new AskCapable();
        m.speak();

        Pupil p = new Pupil("Alcibiades");
        p.type = new AskIncapable();
        p.speak();

        // now, we are able ...
        // to change the behavior type at runtime
        
        p.type = new AskCapable();
        p.speak();
    }    
}

class Master extends Human {
    public Master(String n) {
        name = n;
    }
}
class Pupil extends Human {
    public Pupil(String n) {
        name = n;
    }
}

/**
 * The parent abstract class ...
 * doesn't konw what the behavior type is.
 * It just knows that it is available to its subclasses
 */
abstract class Human {
    String name;
    Speak type;
    public void speak() {
        System.out.print(name + ": ");
        type.speak(); // Look Here
    }
}

class AskCapable implements Speak {
    public void speak() {
        System.out.println("I can raise questions ...");
    }
}
class AskIncapable implements Speak {
    public void speak() {
        System.out.println("I can answer!");
    }
}
interface Speak {
    public void speak();
}

// Socrate: I can raise questions ...
// Alcibiades: I can answer!
// Alcibiades: I can raise questions ...
... 47 lines
 
 
class TheRings {

    public static void main(String[] args) {

        Wizard w1 = new Wizard("Saruman");
        Wizard w2 = new Wizard("Gandalf");
        Hobbit h1 = new Hobbit("Frodo");
        
        w1.type = new Fly(); // Saruman can Fly
        w2.type = new Ride(); // Gandalf can Ride
        h1.type = new Run(); // Frodo can Run

        int level = 1;
        while (level <= 3) {

            System.out.println("\nLevel " + level);

            if (level == 2) {
                h1.type = new Disappear(); // Frodo can Disappear
            }

            if (level == 3) {
                w2.type = new Fly(); // Gandalf can Fly
                w1.type = new Run(); // Saruman must Run :)
            }

            w1.display(); w1.move(); 
            w2.display(); w2.move();
            h1.display(); h1.move();

            level = level + 1;
        }
    }
}


// Items

class Wizard extends Item {
    String name;
    public Wizard(String n) {
        name = n;
    }
    public void display() {
        System.out.print(name);
    }
}

class Hobbit extends Item {
    String name;
    public Hobbit(String n) {
        name = n;
    }
    public void display() {
        System.out.print(name);
    }
}

abstract class Item {
    Move type;
    public void move() {
        type.move();
    }
    abstract void display();
}


// Strategies

interface Move {
    public void move();
}

class Fly implements Move {
    public void move() {
        System.out.print(" (Fly) ");
    }
}

class Ride implements Move {
    public void move() {
        System.out.print(" (Ride) ");
    }
}

class Run implements Move {
    public void move() {
        System.out.print(" (Run) ");
    }
}

class Disappear implements Move {
    public void move() {
        System.out.print(" (Disappear) ");
    }
}

/*

Level 1
Saruman (Fly) Gandalf (Ride) Frodo (Run) 
Level 2
Saruman (Fly) Gandalf (Ride) Frodo (Disappear) 
Level 3
Saruman (Run) Gandalf (Fly) Frodo (Disappear)

*/
... 77 lines
 
Strategy v2.0 Alcibiades can't ask Strategy v2.2 Alcibiades is wiser now, he can ask
Questions    
Mediator

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