Demonstration of mutability of records & defensive copying to address it

RecordMutabilityTest.java

import static org.junit.jupiter.api.Assertions.*;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Date;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.Arguments;
import org.junit.jupiter.params.provider.MethodSource;

public class RecordMutabilityTest {
	
	public static interface RecordWithList {
		List<Date> values();
	}
	
	public static record NaiveRecord(String name, List<Date> values) implements RecordWithList {}
	
	public static record BetterRecord(String name, List<Date> values) implements RecordWithList {
		public BetterRecord {
			values = Collections.unmodifiableList(values);
		}
	};
		
	public static record EvenBetterRecord(String name, List<Date> values) implements RecordWithList {
		public EvenBetterRecord { 
			values = Collections.unmodifiableList(new ArrayList<>(values));
		}
	};
	
	public static record ImmutableRecord(String name, List<Date> values) implements RecordWithList {
		public ImmutableRecord {
			values = values.stream()
					.map(Date::clone)
					.map(Date.class::cast)
					.collect(Collectors.toUnmodifiableList());
		}
	}
	
	public static record ImmutableAlternateRecord(String name, List<Date> values) implements RecordWithList {
		public ImmutableAlternateRecord {
			values = values.stream()
					.map(Date::clone)
					.map(Date.class::cast)
					.collect(Collectors.toList());
		}
		
		@Override
		public List<Date> values() {
			return new ArrayList<>(values);
		}
	}
	
	@SuppressWarnings("deprecation")
	public static void main(String[] args) {
		
		var naive = new NaiveRecord("naive", new ArrayList<>());
		System.out.println(naive);
		//prints NaiveRecord[name=naive, values=[]]
		naive.values().add(new Date(99, 01, 01));
		System.out.println(naive);
		// prints NaiveRecord[name=naive, values=[Mon Feb 01 00:00:00 CET 1999]]
		
		var original = new ArrayList<Date>();
		original.add(new Date(99, 01, 01));
		
		// --- naive record
		
		naive = new NaiveRecord("naive", original);
		System.out.println("%s \t\t hash=%s".formatted(naive, naive.hashCode()));
		
		naive.values().add(new Date(100, 01, 01));
		System.out.println("%s \t\t hash=%s".formatted(naive, naive.hashCode()));
		System.out.println("-----");
		
		// --- better record, return unmodifiable list
		
		original = new ArrayList<Date>();
		original.add(new Date(99, 01, 01));
		var better = new BetterRecord("better", original);
		System.out.println("%s \t\t hash=%s".formatted(better, better.hashCode()));
		// BetterRecord[name=better, values=[Mon Feb 01 00:00:00 CET 1999]] 		 hash=-1516124796
		original.add(new Date(102, 01, 01));
		System.out.println("%s \t hash=%s".formatted(better, better.hashCode()));
		// BetterRecord[name=better, values=[Mon Feb 01 00:00:00 CET 1999, Fri Feb 01 00:00:00 CET 2002]] 	 hash=1357225607
		System.out.println("-----");
		
		// --- even better record, also defensively clone the list
		
		original = new ArrayList<Date>();
		var someDay  = new Date(100, 01, 01);
		original.add(someDay);
		var evenBetter = new EvenBetterRecord("even better", original);
		System.out.println("%s \t hash=%s".formatted(evenBetter, evenBetter.hashCode()));
		// EvenBetterRecord[name=even better, values=[Tue Feb 01 00:00:00 CET 2000]] 	 hash=-1168472954
		
		someDay.setYear(99);
		System.out.println("%s \t\t hash=%s".formatted(evenBetter, evenBetter.hashCode()));
		// EvenBetterRecord[name=even better, values=[Mon Feb 01 00:00:00 CET 1999]] 		 hash=1655265406
		System.out.println("-----");
		
		// -- immutable record, all tricks fail
		
		original = new ArrayList<Date>();
		someDay  = new Date(100, 01, 01);
		original.add(someDay);
		var immutable = new ImmutableRecord("immutable", original);
		System.out.println("%s \t hash=%s".formatted(immutable, immutable.hashCode()));
		
		
		try {
			immutable.values().add(new Date(104, 01, 01));
		} catch (UnsupportedOperationException e) {
			e.printStackTrace();
		}
		System.out.println("%s \t\t hash=%s".formatted(immutable, immutable.hashCode()));
		
		someDay.setYear(105);
		System.out.println("%s \t\t hash=%s".formatted(immutable, immutable.hashCode()));
	}
	
	@SuppressWarnings("deprecation")
	@ParameterizedTest(name = "{0}")
	@MethodSource("recordTestProvider")
	void canNotChangeThroughGetter(RecordWithList record, List<Date> list, Date date) {
		var string = record.toString();
		var hash = record.hashCode();
		
		try {
			record.values().add(new Date(100, 01, 01));
		} catch (UnsupportedOperationException e) {}
		
		assertEquals(string, record.toString());
		assertEquals(hash, record.hashCode());
	}
	
	@SuppressWarnings("deprecation")
	@ParameterizedTest(name = "{0}")
	@MethodSource("recordTestProvider")
	void canNotChangeThroughOriginalList(RecordWithList record, List<Date> list, Date date) {
		var string = record.toString();
		var hash = record.hashCode();
		
		list.add(new Date(101, 01, 01));
		
		assertEquals(string, record.toString());
		assertEquals(hash, record.hashCode());
	}
	
	@SuppressWarnings("deprecation")
	@ParameterizedTest(name = "{0}")
	@MethodSource("recordTestProvider")
	void canNotChangeThroughOriginalObject(RecordWithList record, List<Date> list, Date date) throws UnsupportedOperationException {
		var string = record.toString();
		var hash = record.hashCode();
		
		date.setYear(102);
		
		assertEquals(string, record.toString());
		assertEquals(hash, record.hashCode());
	}
	
	
	
	@SuppressWarnings({ "deprecation"})
	private static Stream<Arguments> recordTestProvider() {
		var list_naive = new ArrayList<Date>();
		var date_naive = new Date(99, 01, 01);
		list_naive.add(date_naive);
		var record_naive = new NaiveRecord("naive", list_naive);
		
		var list_better = new ArrayList<Date>();
		var date_better = new Date(99, 01, 01);
		list_better.add(date_better);
		var record_better = new BetterRecord("better", list_better);
		
		var list_even_better = new ArrayList<Date>();
		var date_even_better = new Date(99, 01, 01);
		list_even_better.add(date_even_better);
		var record_even_better = new EvenBetterRecord("even better", list_even_better);
		
		var list_immutable = new ArrayList<Date>();
		var date_immutable = new Date(99, 01, 01);
		list_immutable.add(date_immutable);
		var record_immutable = new ImmutableRecord("immutable", list_immutable);
		
		return Stream.of(
				Arguments.of(record_naive, list_naive, date_naive),
				Arguments.of(record_better, list_better, date_better),
				Arguments.of(record_even_better, list_even_better, date_even_better),
				Arguments.of(record_immutable, list_immutable, date_immutable)
		);
	}
}