Immutable Classes in Java: 5 Rules, the Real Cost, and Why Records Leak | Effective Java L18

Lecture 18 of the Effective Java Masterclass by Engineer X · Part 3: Classes and Interfaces. In this lecture • A record whose list changes after it was built, shown in a memory view • The five rules of an immutable class, including defensive copies in and out • The functional approach: methods return new values (plus, not add) and the BigDecimal.add trap • Final classes vs private constructors with static factories; why BigInteger and BigDecimal aren't final • Benefits: one state, thread safety without locks, free sharing, no copies, safe map keys, failure atomicity • The cost of one object per value, multi-step methods, and mutable companions like StringBuilder • When a class must be mutable: few states, final fields, complete constructors • Modern Java: records are only shallowly immutable; List.copyOf vs unmodifiableList vs Stream.toList • Interview: how do you make a class immutable, and is a record immutable? Chapters 0:00 An order that changed itself 1:02 What immutable means 2:08 An immutable class, written out 3:59 Why immutable wins 4:48 The cost of a new object 6:24 Modern Java: records are shallow 7:48 In the interview 8:28 Recap 8:58 Quiz Follows the topics of Effective Java, 3rd edition, by Joshua Bloch (Item 17: Minimize mutability). Every explanation, example and line of code in this lecture is original. Previous lecture: L017 Java Records vs Getters and Setters: What Public Fields Really Cost Next lecture: L019 Composition over Inheritance in Java: The Bug Java 21 Exposed #Java #EffectiveJava #JavaInterview #SeniorJavaDeveloper #JavaBestPractices #EngineerX #JavaImmutability #ImmutableClass #JavaRecords