Pattern Design Ultimate Design Pattern 5

Question 1

The State pattern allows an object to alter its behavior when its internal state changes. How is this typically implemented?

Accepted Answer

Delegating behavior to a State object that changes at runtime

Answer

The State pattern replaces conditional logic with state objects; the context delegates behavior to the current state object, which can be replaced.

Question 2

In the Observer pattern, what is the risk of having observers that maintain strong references to the subject?

Accepted Answer

Memory leaks if observers are not properly deregistered

Answer

If observers hold strong references to the subject and are never removed, they prevent garbage collection, causing memory leaks — especially in long-lived subjects.

Question 3

Which anti-pattern does an overused Singleton often lead to?

Accepted Answer

The God Object anti-pattern and hidden global state that makes testing difficult

Answer

Singletons introduce hidden global state and tight coupling, making unit testing and parallel execution harder.

Question 4

When would you choose the Prototype pattern over a regular constructor call?

Accepted Answer

When object creation is costly and you can clone a preconfigured instance cheaply

Answer

Prototype is ideal when constructing an object from scratch is expensive — cloning a ready-made prototype is faster and simpler.

Question 5

In a multi-threaded environment, which technique makes a Singleton implementation thread-safe without synchronizing every access?

Accepted Answer

Eager initialization (creating the instance at class load time)

Answer

Eager initialization creates the Singleton when the class is loaded, leveraging the JVM's class-loading mechanism which is inherently thread-safe.

Pattern Design Practice Test

Pattern Design Practice Test

Pattern Design Ultimate Design Pattern 5