design-patterns/interview

interview: 30 design pattern interview questions, each with a test

Companion module for the article Top 30 Design Patterns Interview Questions in Java (with Code). Every claim in the article is an assertion in a test here; each test also writes the facts it checked to output/.

Unlike the rest of this repository (plain javac), this module uses Maven because the tests need JUnit and Spring Framework.

JDK	25 (Temurin 25.0.5)
Spring Framework	7.0.10 (spring-context, spring-tx)
JUnit	6.1.3
AssertJ	3.27.7

Run

cd interview
./scripts/run-all.sh     # runs mvn test, regenerates every file in output/

Questions

Q	Topic	Source	Test	Output
1	Eager singleton	singleton/EagerSingleton.java	SingletonInterviewTest	01-singleton.txt
2	Lazy singleton is not thread-safe	singleton/NaiveLazySingleton.java	SingletonInterviewTest	01-singleton.txt
3	synchronized getInstance	singleton/SynchronizedSingleton.java	SingletonInterviewTest	01-singleton.txt
4	Double-checked locking needs volatile	singleton/DclSingleton.java, DclWithoutVolatile.java	SingletonInterviewTest	01-singleton.txt
5	Holder idiom	singleton/HolderSingleton.java	SingletonInterviewTest	01-singleton.txt
6	Enum singleton vs serialization and reflection	singleton/EnumSingleton.java, ClassicSingleton.java, HardenedSingleton.java	SingletonInterviewTest	01-singleton.txt
7	Spring singleton scope vs GoF Singleton	spring/ScopeDemo.java	SingletonInterviewTest	01-singleton.txt
8	Simple factory vs Factory Method	creational/SimpleFactory.java, Dialog.java	CreationalAndBehavioralTest	02-creational.txt
9	Factory Method vs Abstract Factory	creational/GuiFactory.java	CreationalAndBehavioralTest	02-creational.txt
10	Builder vs Factory	creational/HttpCall.java	CreationalAndBehavioralTest	02-creational.txt
11	Prototype: shallow vs deep copy	creational/Doc.java	CreationalAndBehavioralTest	02-creational.txt
12	Strategy vs State	behavioral/Order.java, Checkout.java	CreationalAndBehavioralTest	03-behavioral.txt
13	Strategy as a lambda	behavioral/Checkout.java	CreationalAndBehavioralTest	03-behavioral.txt
14	Observer: unsubscribing while notified	behavioral/EventSource.java	CreationalAndBehavioralTest	03-behavioral.txt
15	Template Method vs Strategy	behavioral/ReportJob.java	CreationalAndBehavioralTest	03-behavioral.txt
16	Chain of Responsibility	behavioral/Pipeline.java	CreationalAndBehavioralTest	03-behavioral.txt
17	Command with undo	behavioral/Editor.java	CreationalAndBehavioralTest	03-behavioral.txt
18	Visitor vs sealed switch	behavioral/Expr.java, src/broken/Q18Broken.java	CreationalAndBehavioralTest	03-behavioral.txt, 06-q18-compile-error.txt
19	Decorator vs Proxy	structural/Storage.java	StructuralTest	04-structural.txt
20	JDK dynamic proxy	structural/Greeter.java	StructuralTest	04-structural.txt
21	Adapter vs Facade	structural/Adapters.java	StructuralTest	04-structural.txt
22	Composite	structural/FileNode.java	StructuralTest	04-structural.txt
23	Flyweight	structural/GlyphFactory.java	StructuralTest	04-structural.txt
24	Factory in Spring: FactoryBean	spring/Beans.java	SpringPatternsTest	05-spring.txt
25	Proxy in Spring: JDK vs CGLIB	spring/Shared.java	SpringPatternsTest	05-spring.txt
26	Proxy traps: self-invocation, final methods	spring/Shared.java	SpringPatternsTest	05-spring.txt
27	Observer in Spring: events	spring/Beans.java	SpringPatternsTest	05-spring.txt
28	Template/callback: TransactionTemplate	(test only)	SpringPatternsTest	05-spring.txt
29	Strategy by injection	spring/Beans.java	SpringPatternsTest	05-spring.txt
30	Prototype inside a singleton	spring/Beans.java	SpringPatternsTest	05-spring.txt

What is NOT proven here

Q4: the missing-volatile failure is allowed by the Java Memory Model but is practically unobservable on x86, so the test asserts the modifier (volatile present in DclSingleton, absent in DclWithoutVolatile) rather than reproducing a torn read.

Q2 relies on a 20 ms sleep inside the constructor to make the race reliable; the exact number of extra instances varies per run, so the test only asserts "more than one".