functional programming - Functional Interfaces

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Java 8 introduces java.util.function containing a set of interfaces which are target types for lambda expressions and method references. Each interface contains only one abstract method, called the functional method .

// functional/FunctionalAnnotation.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.

@FunctionalInterface
interface Functional {
  String goodbye(String arg);
}

interface FunctionalNoAnn {
  String goodbye(String arg);
}

/*
@FunctionalInterface  // [1]
interface NotFunctional {
  String goodbye(String arg);
  String hello(String arg);
}
Produces error message:
NotFunctional is not a functional interface
multiple non-overriding abstract methods
found in interface NotFunctional
*/

public class FunctionalAnnotation {
  public String goodbye(String arg) {
    return "Goodbye, " + arg;
  }

  public static void main(String[] args) {
    FunctionalAnnotation fa = new FunctionalAnnotation();
    Functional f = fa::goodbye;
    FunctionalNoAnn fna = fa::goodbye;
    // Functional fac = fa; // Incompatible
    Functional fl = a -> "Goodbye, " + a;
    FunctionalNoAnn fnal = a -> "Goodbye, " + a;
  }
}

The @FunctionalInterface annotation is optional; Java treats both Functional and FunctionalNoAnn as functional interfaces in main().

[1] ' error:

functional/FunctionalAnnotation.java:15: error: Unexpected @FunctionalInterface annotation
@FunctionalInterface
^
  NotFunctional is not a functional interface
    multiple non-overriding abstract methods found in interface NotFunctional
1 error

The solution is keeping one of method in interface NotFunctional.

Here’s an example that enumerates all the different Function variants, applied to lambda expressions:

// functional/FunctionVariants.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.

import java.util.function.*;

class Foo {}

class Bar {
  Foo f;

  Bar(Foo f) {
    this.f = f;
  }
}

class IBaz {
  int i;

  IBaz(int i) {
    this.i = i;
  }
}

class LBaz {
  long l;

  LBaz(long l) {
    this.l = l;
  }
}

class DBaz {
  double d;

  DBaz(double d) {
    this.d = d;
  }
}

public class FunctionVariants {
  static Function<Foo, Bar> f1 = f -> new Bar(f);
  static IntFunction<IBaz> f2 = i -> new IBaz(i);
  static LongFunction<LBaz> f3 = l -> new LBaz(l);
  static DoubleFunction<DBaz> f4 = d -> new DBaz(d);
  static ToIntFunction<IBaz> f5 = ib -> ib.i;
  static ToLongFunction<LBaz> f6 = lb -> lb.l;
  static ToDoubleFunction<DBaz> f7 = db -> db.d;
  static IntToLongFunction f8 = i -> i;
  static IntToDoubleFunction f9 = i -> i;
  static LongToIntFunction f10 = l -> (int) l;
  static LongToDoubleFunction f11 = l -> l;
  static DoubleToIntFunction f12 = d -> (int) d;
  static DoubleToLongFunction f13 = d -> (long) d;

  public static void main(String[] args) {
    System.out.println(f1 + ", " + f2 + ", " + f3 + ", " + f4 + ", " + f5);
    Bar b = f1.apply(new Foo());
    IBaz ib = f2.apply(11); // interface Function method
    LBaz lb = f3.apply(11);
    DBaz db = f4.apply(11);
    int i = f5.applyAsInt(ib); // interface ToIntFunction method
    long l = f6.applyAsLong(lb); // interface ToLongFunction method
    double d = f7.applyAsDouble(db);// interface ToDoubleFunction method
    System.out.println(i + ", " + l + ", " + d);
    l = f8.applyAsLong(12);
    d = f9.applyAsDouble(12);
    i = f10.applyAsInt(12);
    System.out.println(i + ", " + l + ", " + d);
    d = f11.applyAsDouble(12);
    i = f12.applyAsInt(13.0);
    l = f13.applyAsLong(13.0);
    System.out.println(i + ", " + l + ", " + d);
  }
}

My Output:

FunctionVariants$$Lambda$1/135721597@6e8cf4c6, FunctionVariants$$Lambda$2/455659002@12edcd21, FunctionVariants$$Lambda$3/2003749087@34c45dca, FunctionVariants$$Lambda$4/18319
32724@52cc8049, FunctionVariants$$Lambda$5/1149319664@5b6f7412
11, 11, 11.0
12, 12, 12.0
13, 13, 12.0

references:

1. On Java 8 - Bruce Eckel

2. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/functional/FunctionalAnnotation.java

3. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/functional/FunctionVariants.java

4. http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/util/function/Function.java

5. http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/util/function/ToIntFunction.java

6. http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/util/function/ToLongFunction.java

7. http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/util/function/ToDoubleFunction.java