Java Functional Interfaces: java.util.function and When to Write Your Own | Effective Java L44

Lecture 44 of the Effective Java Masterclass by Engineer X · Part 6: Lambdas and Streams. In this lecture • A real boxing bug: UnaryOperator‹Integer› results compared with == fail above the Integer cache; IntUnaryOperator doesn't • Lambdas change API design: take a policy as a function object instead of a Template Method subclass hook • Choosing the parameter type: a hook that needs its owner and an argument is a BiPredicate, not a custom interface • The six basic interfaces and how the other 37 follow (primitive versions, primitive results, two-argument forms, BooleanSupplier) • When to write your own: no fit (three arguments, checked exceptions), or a name, a contract and default methods like Comparator • Always @FunctionalInterface, and the compile error when a second abstract method sneaks in • Don't overload methods on different functional interfaces: the ambiguity and the cast it forces • Modern Java: still exactly 43 interfaces on Java 21, counted by a program Chapters 0:00 The fee that broke equality 1:14 Lambdas change API design 2:40 Forty-three interfaces, six to remember 3:48 When to write your own 5:10 Always @FunctionalInterface 5:44 Overloads that need a cast 6:34 Modern Java: still 43 7:10 In the interview 7:49 Recap and quiz Follows the topics of Effective Java, 3rd edition, by Joshua Bloch (Item 44: Favor the use of standard functional interfaces). Every explanation, example and line of code in this lecture is original. Previous lecture: L043 Java Method References: Five Kinds, Two Receiver Traps, and When a Lambda Wins Next lecture: L045 Java Streams vs Loops: When Streams Help and When a Plain Loop Wins #Java #EffectiveJava #JavaInterview #SeniorJavaDeveloper #JavaBestPractices #EngineerX #JavaFunctionalInterfaces #JavaLambdas #JavaAPIDesign