copying: deep vs shallow copy module (clone() traps, copy constructors, serialization copy, records, defensive copies, collection copies), 17 contract tests

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_017ic4b4X2xx7RrveVQU9R1N

copying/output/01-shallow-vs-deep.txt

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+== Identity before any change ==
+alias   == original                 : true
+shallow == original                 : false
+shallow.address == original.address : true
+shallow.tags    == original.tags    : true
+deep.address    == original.address : false
+deep.tags       == original.tags    : false
+
+== Now mutate the ORIGINAL (name, city, tags) ==
+original : Asha K | Pune | [java, spring, jpa]
+alias    : Asha K | Pune | [java, spring, jpa]
+shallow  : Asha | Pune | [java, spring, jpa]
+deep     : Asha | Mumbai | [java, spring]

copying/output/02-final-field-clone.txt

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+$ javac src/broken/FinalFieldClone.java
+src/broken/FinalFieldClone.java:17: error: cannot assign a value to final variable items
+            copy.items = new ArrayList<>(items);   // error: cannot assign a value to final variable
+                ^
+1 error

copying/output/03-clone.txt

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+== 1. clone() does not call a constructor, and it is shallow ==
+  Team constructor ran for 'platform'
+  b == a                 : false
+  b.members == a.members : true
+  after a.members.add("meera"), b.members = [asha, ravi, meera]
+
+== 2. The fix: copy the mutable field inside clone() ==
+  c.members = [asha, ravi, meera]
+  d.members = [asha, ravi]
+
+== 3. Calling super.clone() without implementing Cloneable ==
+  CloneNotSupportedException: com.ankurm.copying.CloneDemo$ForgotCloneable
+
+== 4. Array clone() is shallow for nested arrays ==
+  gridCopy[0] == grid[0] : true
+  grid     = [[99, 2], [3, 4]]
+  gridCopy = [[99, 2], [3, 4]]
+
+== 5. ArrayList.clone() is shallow too ==
+  original.get(0) = abc-changed
+  same element object: true
+
+== 6. A parent clone() that uses 'new' breaks every subclass ==
+  ClassCastException: class com.ankurm.copying.CloneDemo$Animal cannot be cast to class com.ankurm.copying.CloneDemo$Dog (com.ankurm.copying.CloneDemo$Animal and com.ankurm.copying.CloneDemo$Dog are in unnamed module of loader 'app')

copying/output/04-copy-constructor.txt

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+== 1. Copy constructor: new object, own list, final fields intact ==
+  rex  = Dog(Rex, [sit, roll]) bark=7
+  rex2 = Dog(Rex, [sit, roll]) bark=7
+  rex2.tricks == rex.tricks : false
+
+== 2. The static type picks the constructor, so a subtype is sliced off ==
+  new Animal(pet) = Animal(Rex, [sit, roll])
+
+== 3. A virtual copy() keeps the runtime type ==
+  pet.copy()      = Dog(Rex, [sit, roll]) bark=7

copying/output/05-serialization.txt

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+== 1. A full deep copy: nothing is shared ==
+  copy.dept == asha.dept     : false
+  copy.skills == asha.skills : false
+  asha.skills = [java, spring], asha.dept = Core
+  copy.skills = [java], copy.dept = Platform
+
+== 2. transient fields are lost ==
+  asha.sessionToken = secret-token
+  copy.sessionToken = null
+
+== 3. Sharing INSIDE the graph is preserved ==
+  original: same Dept object  : true
+  copy    : same Dept object  : true
+  copy dept is a new object   : true
+
+== 4. Cycles are handled ==
+  cyc.mentor.mentor == cyc : true
+  cyc != asha              : true
+
+== 5. A non-serializable field fails at run time ==
+  NotSerializableException: java.lang.Object

copying/output/06-records.txt

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+== 1. A record does not copy the list you give it ==
+  leaky.items()         = [pen, book, laptop]
+  after leaky.items().add("phone"), basket = [pen, book, laptop, phone]
+
+== 2. List.copyOf in the compact constructor ==
+  safe.items()          = [pen, book]
+  safe.items().add(...) -> UnsupportedOperationException
+
+== 3. 'Changing' a record means building a new one ==
+  safe   = SafeOrder[id=A-2, items=[pen, book]]
+  bigger = SafeOrder[id=A-2, items=[pen, book, charger]]
+
+== 4. A mutable Date component ==
+  LeakyEvent.when().getTime() after caller mutates d1 = 86400000
+  SafeEvent.when().getTime()  after both mutations    = 0

copying/output/07-defensive.txt

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+== 1. Leak through the constructor ==
+  leaky.size() after the caller clears its own list : 0
+
+== 2. Leak through the getter ==
+  leaky2.getNames() = [asha, ravi, intruder]
+
+== 3. The defensive version ==
+  safe.size() after the caller clears its list : 2
+  safe.getNames().add(...) -> UnsupportedOperationException
+
+== 4. unmodifiableList is a VIEW; List.copyOf is a SNAPSHOT ==
+  source   = [a, b, c]
+  view     = [a, b, c]
+  snapshot = [a, b]
+
+== 5. copyOf is shallow: the ELEMENTS are still shared ==
+  frozen.get(0) = x-mutated

copying/output/08-collections.txt

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+== 1. new ArrayList<>(list) and List.copyOf share the elements ==
+  boxes     = [Box(100), Box(2)]
+  viaCtor   = [Box(100), Box(2)]
+  viaCopyOf = [Box(100), Box(2)]
+  after boxes.add(Box(3)): viaCtor.size() = 2
+
+== 2. The deep version needs the element copy, so a stream or a loop ==
+  boxes = [Box(-1), Box(2), Box(3)]
+  deep  = [Box(100), Box(2), Box(3)]
+
+== 3. HashMap copy constructor: new map, same values ==
+  mCopy.get("k") = Box(55)
+
+== 4. 1-D primitive arrays: clone / copyOf are true copies ==
+  a = [99, 2, 3], clone = [1, 2, 3], copyOf = [1, 2, 3]
+
+== 5. 2-D arrays: the rows are shared ==
+  grid     = [[99, 2], [3, 4]]
+  shallow  = [[99, 2], [3, 4]]
+  deepGrid = [[1, 2], [3, 4]]
+
+== 6. Object arrays: copyOf copies references, not objects ==
+  arrCopy[0] = Box(77), same object: true

copying/output/09-tests.txt

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+[INFO] Tests run: 17, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.145 s -- in com.ankurm.copying.CopyingTest
+[INFO] Tests run: 17, Failures: 0, Errors: 0, Skipped: 0
+[INFO] BUILD SUCCESS

copying/output/10-cost.txt

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+Indicative only: plain nanoTime loop, best of 7 rounds, ns per deep copy.
+Absolute numbers drift per machine and run; read the ratios.
+
+copy constructor :      14 ns
+deep clone()     :      13 ns
+serialization    :    9765 ns
+serialization is about 698x the copy constructor

copying/output/11-copyof-identity.txt

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+copyOf(immutable list) is the same instance      : true
+copyOf(unmodifiableList view) is the same instance : false
+copyOf with a null element throws                : NullPointerException