Overview Of Java 8 Functional Interfaces

Overview of Java 8 Functional InterfacesDouglas C. edu/ schmidtProfessor of Computer ScienceInstitute for SoftwareIntegrated SystemsVanderbilt UniversityNashville, Tennessee, USA

Learning Objectives in this Lesson Recognize foundational functionalprogramming features in Java 8, e.g., Lambda expressions Method & constructor references Key functional erfacesSupplier2Consumer

Learning Objectives in this Lesson Recognize foundational functionalprogramming features in Java 8, e.g., Lambda expressions Method & constructor references Key functional interfaces3 8’s concurrency/parallelism frameworksThese features are the foundation for Java

Learning Objectives in this Lesson Recognize foundational functionalprogramming features in Java 8 Understand how these Java 8 featuresare applied in concise example programs4See ster/Java8

Overview of CommonFunctional Interfaces5

Overview of Common Functional Interfaces A functional interface contains only one abstract method6See ces

Overview of Common Functional Interfaces A functional interface is the type used for a parameter when a lambdaexpression or method reference is passed as an argument T void runTest(Function T, T fact, T n) {long startTime System.nanoTime();T result fact.apply(n));long stopTime (System.nanoTime() - startTime) / 1 000 000;.}runTest(ParallelStreamFactorial::factorial, n);runTest(SequentialStreamFactorial::factorial, n);.7See ster/Java8/ex16

Overview of Common Functional Interfaces A functional interface is the type used for a parameter when a lambdaexpression or method reference is passed as an argument T void runTest(Function T, T fact, T n) {long startTime System.nanoTime();T result fact.apply(n));long stopTime (System.nanoTime() - startTime) / 1 000 000;.}runTest(ParallelStreamFactorial::factorial, n);.static BigInteger factorial(BigInteger n) {return LongStream.rangeClosed(1, (BigInteger.ONE, BigInteger::multiply);}8

Overview of Common Functional Interfaces Java 8 defines many types offunctional interfaces9See on/package-summary.html

Overview of Common Functional Interfaces Java 8 defines many types offunctional interfaces This list is large due to theneed to support referencetypes & primitive types.10See dzone.com/articles/whats-wrong-java-8-part-ii

Overview of Common Functional Interfaces Java 8 defines many types offunctional interfaces This list is large due to theneed to support referencetypes & primitive cesWe focus on the most commontypes of functional interfacesSupplier11Consumer

Overview of Common Functional Interfaces Java 8 defines many types offunctional interfaces This list is large due to theneed to support referencetypes & primitive cesSupplierConsumer12 upcoming examples are “stateless”!All usages of functional interfaces in the

Overview of FunctionalInterfaces: Predicate13

Overview of Common Functional Interfaces: Predicate A Predicate performs a test that returns true or false, e.g., public interface Predicate T { boolean test(T t); }14See on/Predicate.html

Overview of Common Functional Interfaces: Predicate A Predicate performs a test that returns true or false, e.g., public interface Predicate T { boolean test(T t); }15See on/Predicate.html

Overview of Common Functional Interfaces: Predicate A Predicate performs a test that returns true or false, e.g., public interface Predicate T { boolean test(T t); }Map String, Integer iqMap new ConcurrentHashMap String, Integer () { {put("Larry", 100); put("Curly", 90); put("Moe", 110);}};Create a map of “stooges” & their oveIf(entry - entry.getValue() 100);System.out.println(iqMap);16See ster/Java8/ex10

Overview of Common Functional Interfaces: Predicate A Predicate performs a test that returns true or false, e.g., public interface Predicate T { boolean test(T t); }Map String, Integer iqMap new ConcurrentHashMap String, Integer () { {put("Larry", 100); put("Curly", 90); put("Moe", 110);}};System.out.println(iqMap);This predicate lambda deletesentries with iq 100iqMap.entrySet().removeIf(entry - entry.getValue() 100);System.out.println(iqMap);17See tion.html#removeIf

Overview of Common Functional Interfaces: Predicate A Predicate performs a test that returns true or false, e.g., public interface Predicate T { boolean test(T t); }Map String, Integer iqMap new ConcurrentHashMap String, Integer () { {put("Larry", 100); put("Curly", 90); put("Moe", 110);}entry is short for (EntrySet String,};System.out.println(iqMap);Integer entry), which leverages the Java8 compiler’s type inference capabilitiesiqMap.entrySet().removeIf(entry - entry.getValue() 100);System.out.println(iqMap);18See ypeInference.html

Overview of Common Functional Interfaces: Predicate A Predicate performs a test that returns true or false, e.g., public interface Predicate T { boolean test(T t); }interface Collection E {.default boolean removeIf(Predicate ? super E filter) {.final Iterator E each iterator();while (each.hasNext()) {if (filter.test(each.next())) {each.remove();.19 uses the predicate passed to itHere’s how the removeIf() method

Overview of Common Functional Interfaces: Predicate A Predicate performs a test that returns true or false, e.g., public interface Predicate T { boolean test(T t); }interface Collection E {.default boolean removeIf(Predicate ? super E filter) {.final Iterator E each iterator();entry - while (each.hasNext()) {entry.getValue()if (filter.test(each.next())) { 100each.remove();.20the lambda expression passed to itThis predicate parameter is bound to

Overview of Common Functional Interfaces: Predicate A Predicate performs a test that returns true or false, e.g., public interface Predicate T { boolean test(T t); }interface Collection E {.default boolean removeIf(Predicate ? super E filter) {.final Iterator E each iterator();entry - while (each.hasNext()) {entry.getValue()if (filter.test(each.next())) { 100each.remove();.if (each.next().getValue() 100)The ‘entry’ in the lambda predicate 21is replaced by the parameter to test()

Overview of FunctionalInterfaces: Function22

Overview of Common Functional Interfaces: Function A Function applies a computation on 1 parameter & returns a result, e.g., public interface Function T, R { R apply(T t); }23See on/Function.html

Overview of Common Functional Interfaces: Function A Function applies a computation on 1 parameter & returns a result, e.g., public interface Function T, R { R apply(T t); }24See on/Function.html

Overview of Common Functional Interfaces: Function A Function applies a computation on 1 parameter & returns a result, e.g., public interface Function T, R { R apply(T t); }Map Integer, Integer primeCache new ConcurrentHashMap ();This map caches the resultsof prime # computations.Long smallestFactor primeCache.computeIfAbsent(primeCandidate, (key) - primeChecker(key));.Integer primeChecker(Integer primeCandidate) {. // Returns 0 if a number if prime or the smallest// factor if it’s not prime}25See ster/Java8/ex9

Overview of Common Functional Interfaces: Function A Function applies a computation on 1 parameter & returns a result, e.g., public interface Function T, R { R apply(T t); }Map Integer, Integer primeCache new ConcurrentHashMap ();If key isn’t already associated with avalue, compute the value using the givenmapping function & enter it into the map.Long smallestFactor primeCache.computeIfAbsent(primeCandidate, (key) - primeChecker(key));.Integer primeChecker(Integer primeCandidate) {. // Returns 0 if a number if prime or the smallest// factor if it’s not prime}See rent/ConcurrentHashMap.html#computeIfAbsent26

Overview of Common Functional Interfaces: Function A Function applies a computation on 1 parameter & returns a result, e.g., public interface Function T, R { R apply(T t); }Map Integer, Integer primeCache new ConcurrentHashMap ();This method provides atomic“check then act” semantics.Long smallestFactor primeCache.computeIfAbsent(primeCandidate, (key) - primeChecker(key));.Integer primeChecker(Integer primeCandidate) {. // Returns 0 if a number if prime or the smallest// factor if it’s not prime}27See dig.cs.illinois.edu/papers/checkThenAct.pdf

Overview of Common Functional Interfaces: Function A Function applies a computation on 1 parameter & returns a result, e.g., public interface Function T, R { R apply(T t); }Map Integer, Integer primeCache new ConcurrentHashMap ();A lambda expression that calls a function.Long smallestFactor primeCache.computeIfAbsent(primeCandidate, (key) - primeChecker(key));.Integer primeChecker(Integer primeCandidate) {. // Returns 0 if a number if prime or the smallest// factor if it’s not prime}28

Overview of Common Functional Interfaces: Function A Function applies a computation on 1 parameter & returns a result, e.g., public interface Function T, R { R apply(T t); }Map Integer, Integer primeCache new ConcurrentHashMap ();Could also be a passed as a method reference.Long smallestFactor primeCache.computeIfAbsent(primeCandidate, this::primeChecker);.Integer primeChecker(Integer primeCandidate) {. // Returns 0 if a number if prime or the smallest// factor if it’s not prime}29

Overview of Common Functional Interfaces: Function A Function applies a computation on 1 parameter & returns a result, e.g., public interface Function T, R { R apply(T t); }class ConcurrentHashMap K,V .public V computeIfAbsent(K key,Function ? super K, ? extends V mappingFunction) {.if ((f tabAt(tab, i (n - 1) & h)) null).if ((val mappingFunction.apply(key)) ! null)node new Node K,V (h, key, val, null);.30Here’s how the computeIfAbsent() methoduses the function passed to it

Overview of Common Functional Interfaces: Function A Function applies a computation on 1 parameter & returns a result, e.g., public interface Function T, R { R apply(T t); }class ConcurrentHashMap K,V .this::primeCheckerpublic V computeIfAbsent(K key,Function ? super K, ? extends V mappingFunction) {.if ((f tabAt(tab, i (n - 1) & h)) null).if ((val mappingFunction.apply(key)) ! null)node new Node K,V (h, key, val, null);.The function parameter is bound to 31this::primeChecker method reference

Overview of Common Functional Interfaces: Function A Function applies a computation on 1 parameter & returns a result, e.g., public interface Function T, R { R apply(T t); }class ConcurrentHashMap K,V .public V computeIfAbsent(K key,Function ? super K, ? extends V mappingFunction) {.if ((f tabAt(tab, i (n - 1) & h)) null).if ((val mappingFunction.apply(key)) ! null)node new Node K,V (h, key, val, null);.if ((val primeChecker(key)) ! null)The apply() method is replaced with32the primeChecker() lambda function

Overview of Common Functional Interfaces: Function This example also shows a Function, e.g., public interface Function T, R { R apply(T t); }List Thread threads new ArrayList (Arrays.asList(new Thread("Larry"),new Thread("Curly"),new Thread("Moe")));Create a list of threads namedafter the three ads.forEach(System.out::println);33See ster/Java8/ex5

Overview of Common Functional Interfaces: Function This example also shows a Function, e.g., public interface Function T, R { R apply(T t); }List Thread threads new ArrayList (Arrays.asList(new Thread("Larry"),new Thread("Curly"),new Thread("Moe")));A method reference to a Function used to sort threads by .forEach(System.out::println);34See -list

Overview of Common Functional Interfaces: Function This example also shows a Function, e.g., public interface Function T, R { R apply(T t); }List Thread threads new ArrayList (Arrays.asList(new Thread("Larry"),new Thread("Curly"),new Thread("Moe")));This method uses the Thread::getName method referenceto impose a total ordering on some collection of ads.forEach(System.out::println);35See ator.html#comparing