java-core-examples/copying/README.md

copying

Companion code for the ankurm.com post "Deep Copy vs Shallow Copy in Java: clone(), Copy Constructors, Serialization and Records." Module in java-core-examples, the Java-core series. There is no docs/ folder: the deeper material lives in the post itself, inside its expandable sections.

Versions this was built and tested against

Component	Version	Notes
JDK	25.0.5+8 (Temurin, LTS)	Records, List.copyOf, Arrays.copyOf.
JUnit Jupiter	5.11.0	Same as the sibling modules.
Maven	3.9.11

Quickstart

export JAVA_HOME=/path/to/jdk-25
mvn compile
scripts/run.sh CloneDemo          # one demo
scripts/broken.sh                 # the three deliberate compile errors
scripts/run-all.sh                # regenerate every file in output/

What's in here

File	What it shows
ShallowVsDeepDemo.java	Assignment vs shallow copy vs deep copy on one Person graph.
CloneDemo.java	clone() skips the constructor, is shallow, needs Cloneable, and a parent that uses new breaks subclasses. Arrays and ArrayList.clone() are shallow too.
src/broken/FinalFieldClone.java	Does not compile: a final field cannot be deep-copied inside clone(). Not part of the Maven build.
src/broken/ForgotToOverride.java	Does not compile: implements Cloneable without a public clone() override leaves clone() protected.
src/broken/WithExpression.java	Does not compile on JDK 25: the with expression of JEP 468 (derived record creation) is not part of the language yet.
CopyConstructorDemo.java	Copy constructors with final fields, subtype slicing, and a virtual copy() that fixes it.
SerializationCopyDemo.java	Deep copy via ObjectOutputStream: transient loss, shared references and cycles preserved, NotSerializableException.
RecordCopyDemo.java	Records are only shallowly immutable: List.copyOf in the compact constructor, withers, a mutable Date component.
DefensiveCopyDemo.java	Leaks through constructor and getter; unmodifiableList (view) vs List.copyOf (snapshot).
CollectionCopyDemo.java	Every standard list, map and array copy is shallow; the deep versions.
CopyOfIdentityDemo.java	When List.copyOf returns the same instance and when it copies; null elements.
GraphCopyDemo.java	Shared nodes and cycles: a recursive copy constructor duplicates the shared node and overflows the stack on a cycle; an identity-map copy and serialization both handle them.
SerializationRulesDemo.java	Which constructors run during a serialization copy (classes vs records) and an ObjectInputFilter allow-list.
RecordArrayDemo.java	An array record component leaks through the constructor and the accessor; the generated equals() compares arrays by identity.
DefensiveCloneDemo.java	Why a defensive copy must not call clone() on a non-final parameter type (a Date subclass hands back itself).
CopyCostDemo.java	Indicative cost of copy constructor vs clone() vs serialization. Timing: results drift per run.
CopyingTest.java, CopyingExtrasTest.java	Pin every claim above as an assertion (17 + 7 = 24 tests).
output/01-17	Captured runs of everything above. 10-cost.txt is the only non-deterministic file.

License

MIT - see LICENSE.