Add interview module: 30 design pattern interview questions with tests (Java 25, Spring 7.0.10)

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_017ic4b7rnC7jppGGdHGESvy

interview/output/00-environment.txt

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+openjdk version "25.0.5" 2026-10-20 LTS
+tests: 31

interview/output/01-singleton.txt

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+Q1 eager: getInstance() == getInstance() -> true
+Q2 naive lazy, 8 threads: distinct instances=more than 1 constructor runs=8
+Q3 synchronized, 8 threads: distinct instances=1 constructor runs=1
+Q4 DCL with volatile, 8 threads: distinct instances=1 constructor runs=1
+Q4 DclSingleton.instance volatile? true
+Q4 DclWithoutVolatile.instance volatile? false
+Q5 holder before first getInstance(): constructor runs=0
+Q5 holder after two getInstance() calls: constructor runs=1 same=true
+Q6 serialisation round trip keeps the same instance? classic=false hardened(readResolve)=true enum=true
+Q6 reflection newInstance() on classic private constructor returns the singleton? false
+Q6 reflection on hardened constructor: InvocationTargetException caused by java.lang.IllegalStateException: already initialised
+Q6 reflection on enum constructor: java.lang.IllegalArgumentException: Cannot reflectively create enum objects
+Q7 Spring singleton scope: same bean twice from one context? true; same bean across two contexts? false

interview/output/02-creational.txt

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+Q8 simple factory: button("dark") -> [ dark button ]
+Q8 factory method: new DarkDialog().show() -> dialog with [ dark button ]
+Q9 LIGHT factory -> button=light checkbox=light
+Q9 DARK factory  -> button=dark checkbox=dark
+Q10 builder defaults: method=GET timeout=30; headers map mutable? false
+Q10 build() without url -> NullPointerException: url is required
+Q10 JDK HttpRequest.newBuilder(uri).build() default method=GET
+Q11 original tags after both copies were edited: [java, added-via-shallow]
+Q11 shallow.tags() == original.tags()? true; deep.tags() == original.tags()? false
+Q11 ArrayList.clone() shares elements: original.get(0) is now "a!"

interview/output/03-behavioral.txt

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+Q12 state: advance() x3 -> NEW -> PAID -> SHIPPED -> DELIVERED
+Q12 state: advance() on DELIVERED -> IllegalStateException: already delivered
+Q12 strategy: total(1000) with NONE=1000.0 then FESTIVAL=900.0
+Q13 sort by length (Comparator strategy): [fig, pear, banana]
+Q13 sort reversed alphabetical: [pear, fig, banana]
+Q13 inline lambda strategy: total(200)=150.0
+Q14 publishNaive, 2 listeners, first unsubscribes itself: delivered to [first] (no exception, second was skipped)
+Q14 publishNaive, 3 listeners, first unsubscribes itself -> ConcurrentModificationException
+Q14 publishSafe, two events, self-unsubscribing listener: [once, other, other]
+Q14 java.util.Observable @Deprecated(since="9")
+Q15 template method run(): [open, sales rows, close]
+Q15 strategy version runWith(): [open, ad-hoc rows, close]
+Q15 run() is final (subclasses cannot reorder the steps): true
+Q16 valid request   -> 200 ok via [auth, rate-limit, controller]
+Q16 no token        -> 401 via [auth]
+Q17 after two commands: "Hello, world"; undo -> "Hello"; undo -> ""
+Q18 2 + 3 * 4 via sealed switch=14 via Visitor=14

interview/output/04-structural.txt

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+Q19 LazyProxy: RealStorage instances before first read=0, after two reads=1
+Q19 ProtectionProxy: read("secret-1") -> SecurityException: denied: secret-1
+Q19 Decorators stacked Logging(Caching(Real)): read("k") twice -> log=[read k, read k]
+Q20 proxy.greet("Ankur") -> hello Ankur; intercepted=[greet]
+Q20 Proxy.isProxyClass(proxy.getClass())=true; proxy instanceof Greeter.Impl=false
+Q20 newProxyInstance with a class instead of an interface -> IllegalArgumentException (... is not an interface)
+Q21 adapter: 98.6F read through CelsiusSensor -> 37.0 C
+Q21 facade: watchMovie() -> [lights dimmed, projector on, surround sound on]
+Q21 JDK adapter InputStreamReader(InputStream) first char: h
+Q22 size() of a leaf=10, of sub dir=25, of root=35
+Q23 "hello world" has 11 characters but the pool holds 8 Glyph objects
+Q23 Integer.valueOf: 127==127 -> true, 128==128 -> false

interview/output/05-spring.txt

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+Q24 getBean("connection") -> Connection
+Q24 getBean("&connection") -> ConnectionFactoryBean
+Q24 ApplicationContext is itself a BeanFactory? true
+Q25 target with interface, defaults        -> jdkDynamicProxy=true cglib=false
+Q25 target with interface, proxyTargetClass -> jdkDynamicProxy=false cglib=true
+Q25 target with no interface                -> jdkDynamicProxy=false cglib=true
+Q25 JDK proxy instanceof GreeterImpl? false; CGLIB proxy instanceof GreeterImpl? true
+Q26 proxy.outer() returned 2; advice saw [outer] (inner() ran, but was never intercepted)
+Q26 proxy.inner() called from outside; advice saw [inner]
+Q26 final method on a CGLIB proxy: finalMethod() -> "name=null", advice saw []
+Q27 publishEvent(OrderPlaced): listener ran on the caller's thread? true
+Q28 TransactionTemplate success path: [begin, work, commit] returned done
+Q28 TransactionTemplate when the callback throws: [begin, work, rollback]
+Q29 Map<String, Payment> injected with keys [card, upi]
+Q29 pay("upi", 500) -> upi:500
+Q30 getBean(Ticket) twice from the context: same instance? false
+Q30 prototype injected into a singleton: same Ticket on every call? true
+Q30 prototype fetched via ObjectProvider: same Ticket on every call? false

interview/output/06-q18-compile-error.txt

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+Q18Broken.java:9: error: the switch expression does not cover all possible input values
+        return switch (e) {
+               ^
+1 error