One of the structural patterns aims to reduce memory usage by sharing as much data as possible with similar objects.
It is particularly useful when dealing with a large number of similar objects, where creating a new instance for each object would be expensive in terms of memory consumption.
key concepts:
Intrinsic state: The state that is shared between multiple objects is independent of the context and remains the same across different objects.
Extrinsic state: The state that is unique to each object and is passed from the client. This state can vary and is not stored in the Flyweight object

Key participants:

Flyweight: Interface that the Flyweight object to receive the Extrinsic state and use it.
ConcreteFlyweight: Implements the Flyweight and stores the intrinsic state.
FlyweightFactory: Manages the Flyweight objects and ensures the sharing of interfaces, it returns an existing Flyweight if it already exists.

Client(like Main class): Maintains a reference to Flyweight and supplies Extrinsic state when it needs to interact with the Flyweight object.

Let's take an example of a Flyweight object of character
Suppose we have a text editor that needs to render a large amount of text. Each character can be represented as an object, but having a separate object for every character would waste a lot of memory. Instead, we can use Flyweights to share the character objects that represent each letter, and store the extrinsic state like the position or formatting outside

Flyweight

public interface Flyweight {
    public void display(int x, int y);//x, y are the extrinsic state of the Flyweight object
}

ConcreteFlyweight

public class CharacterFlyweight implements Flyweight {
    private char ch;
    public CharacterFlyweight(char c){
        this.ch  = c;
    }
    @Override
    public void display(int x ,int y){
        System.out.println("[drawing character: " this.ch " at co-ordinates:(" x "," y ")]");
    }

}

FlyweightFactory

public class FlyweightFactory {
    private static HashMap flyweights = new HashMap();
    public static Flyweight getFlyweight(char c){
        Flyweight flyweight = flyweights.getOrDefault(c,null);
        if(null==flyweight){
            flyweight = new CharacterFlyweight(c);
            flyweights.put(c,flyweight);
        }
        return flyweight;
    }
}

Main

public class Main {
    public static void main(String args[]){
        Flyweight flyweight1 = FlyweightFactory.getFlyweight('a');
        Flyweight flyweight2 = FlyweightFactory.getFlyweight('b');
        Flyweight flyweight3 = FlyweightFactory.getFlyweight('a');// will use the same object that is referenced by flyweight1

        flyweight1.display(1, 2);//'a' displayed at 1,2
        flyweight2.display(3, 4);//'b' displayed at 3,4
        flyweight3.display(5, 7); // 'a'(shared) displayed at 5,7
    }
}

Output:

[drawing character: a at co-ordinates:(1,2)]
[drawing character: b at co-ordinates:(3,4)]
[drawing character: a at co-ordinates:(5,7)]

Key Points

• Memory Efficiency: Reduces memory usage by sharing objects, especially when the intrinsic state is large or there are many objects.
• Performance Improvement: Reducing the number of objects created, it can improve the performance of the application when managing large numbers of objects.

Disadvantages
Complexity: The pattern can add complexity to the code, especially in managing the extrinsic and intrinsic states separately.
Overhead: If there are few objects to share, the Flyweight pattern might introduce unnecessary complexity without significant memory savings.