Creational patterns (Singleton, Factory, Builder) in Software Engineering - Step-by-Step Execution

Concept Flow - Creational patterns (Singleton, Factory, Builder)

Start

↓

Need object creation

↓

Singleton

↓

One instance

↓

Use object

↓

End

The flow starts when an object is needed. You pick one of the three creational patterns: Singleton for one instance, Factory for creating objects based on input, or Builder for step-by-step construction. Then you use the created object.

Execution Sample

Software Engineering

class Singleton:
    _instance = None
    def __new__(cls):
        if cls._instance is None:
            cls._instance = super().__new__(cls)
        return cls._instance

This code creates a Singleton class that ensures only one instance exists.

Analysis Table

Step	Action	Condition	Result	Instance ID
1	Call Singleton()	_instance is None	Create new instance	ID_1
2	Call Singleton() again	_instance is not None	Return existing instance	ID_1
3	Call Singleton() third time	_instance is not None	Return existing instance	ID_1
Exit	No more calls	-	All calls return same instance	ID_1

💡 Singleton pattern stops creating new instances after the first one.

State Tracker

Variable	Start	After 1	After 2	After 3	Final
_instance	None	ID_1	ID_1	ID_1	ID_1

Key Insights - 3 Insights

Why does Singleton return the same instance every time?

Because after the first creation (_instance is set), the condition in step 2 and 3 returns false for creating new, so it returns the stored instance (see execution_table rows 2 and 3).

How does Factory pattern decide which object to create?

Factory uses input or parameters to choose which subclass or object to create, unlike Singleton which always returns one instance (not shown in table but conceptually different).

Why use Builder instead of directly creating an object?

Builder allows step-by-step construction of complex objects, useful when many parts or configurations are needed, unlike Singleton or Factory which create objects in one step.

Visual Quiz - 3 Questions

Look at the execution_table, what is the instance ID returned at step 3?

A ID_1

B ID_2

C None

D New instance created

💡 Hint

Check the 'Instance ID' column at step 3 in the execution_table.

At which step does the Singleton pattern create a new instance?

A Step 3

B Step 2

C Step 1

D Exit

💡 Hint

Look at the 'Result' column to see when 'Create new instance' happens.

If the _instance variable started as ID_5 instead of None, what would happen at step 1?

A Create new instance with ID_1

B Return existing instance ID_5

C Error because _instance is not None

D Create multiple instances

💡 Hint

Refer to variable_tracker and execution_table step 1 condition checking _instance.

Concept Snapshot

Creational Patterns:
- Singleton: One instance only, reused everywhere.
- Factory: Creates objects based on input type.
- Builder: Builds complex objects step-by-step.
Use Singleton to save resources, Factory for flexible creation, Builder for complex setups.

Full Transcript

Creational patterns help create objects in software. Singleton ensures only one instance exists by checking if an instance already exists before creating a new one. Factory creates different objects based on input, allowing flexibility. Builder constructs complex objects stepwise, useful when many parts or configurations are needed. The execution table shows Singleton creating one instance at step 1 and returning the same instance later. Variables track the instance state. Key moments clarify why Singleton returns the same instance and differences with Factory and Builder. Quizzes test understanding of instance creation and reuse.