3分钟看完Java 8——史上最强Java 8新特性总结之第一篇函数式编程基础

行为参数化

1. 理解函数式编程要先理解行为参数化。

2. 行为参数化：一个方法接受多个不同的行为作为参数，并在内部使用它们，完成不同行为的能力。

3. 行为参数化优点：可让代码更好地适应不断变化的需求，减轻未来的工作量。

4. 实现方式

a) Java 8以前：通过接口实现类或接口匿名类实现。

b) Java 8及以后：通过Lambda表达式实现。

5. 举例

a) 通过接口实现类实现行为参数化

i. Apple.java（后续举例将多次使用到该类）

 public class Apple {

     private Integer weight;

     private String color;

     public Apple(Integer weight, String color) {

         this.weight = weight;

         this.color = color;

     }

     public Integer getWeight() {

         return weight;

     }

     public void setWeight(Integer weight) {

         this.weight = weight;

     }

     public String getColor() {

         return color;

     }

     public void setColor(String color) {

         this.color = color;

     }

     @Override

     public String toString() {

         return "weight=" + weight + " color=" + color;

     }

 }

ii. ApplePredicate.java

 public interface ApplePredicate {

     boolean test(Apple apple);

 }

iii. AppleHeavyWeightPredicate.java

 public class AppleHeavyWeightPredicate implements ApplePredicate {

     public boolean test(Apple apple) {

         return apple.getWeight() > 150;

     }

 }

iv. AppleGreenColorPredicate.java

 public class AppleGreenColorPredicate implements ApplePredicate {

     public boolean test(Apple apple) {

         return "green".equals(apple.getColor());

     }

 }

v. Test.java

 import java.util.ArrayList;

 import java.util.Arrays;

 import java.util.List;

 public class Test {

     public static List<Apple> filterApples(List<Apple> inventory, ApplePredicate p) {

         List<Apple> result = new ArrayList<>();

         for (Apple apple : inventory) {

             if (p.test(apple)) {

                 result.add(apple);

             }

         }

         return result;

     }

     public static void printApples(List<Apple> inventory) {

         for (Apple apple : inventory) {

             System.out.println(apple);

         }

     }

     public static void main(String[] args) {

         List<Apple> inventory = Arrays.asList(

                 new Apple(100, "red"),

                 new Apple(110, "red"),

                 new Apple(190, "red"),

                 new Apple(170, "red"),

                 new Apple(100, "green"),

                 new Apple(120, "green"),

                 new Apple(160, "green"),

                 new Apple(180, "green")

         );

         List<Apple> newInventory1 = filterApples(inventory, new AppleHeavyWeightPredicate());

         printApples(newInventory1);

         System.out.println("-----");

         List<Apple> newInventory2 = filterApples(inventory, new AppleGreenColorPredicate());

         printApples(newInventory2);

     }

 }

b) 通过接口匿名类实现行为参数化

i. ApplePredicate.java

 public interface ApplePredicate {

     boolean test(Apple apple);

 }

ii. Test.java

 import java.util.ArrayList;

 import java.util.Arrays;

 import java.util.List;

 public class Test {

     public static List<Apple> filterApples(List<Apple> inventory, ApplePredicate p) {

         List<Apple> result = new ArrayList<>();

         for (Apple apple : inventory) {

             if (p.test(apple)) {

                 result.add(apple);

             }

         }

         return result;

     }

     public static void printApples(List<Apple> inventory) {

         for (Apple apple : inventory) {

             System.out.println(apple);

         }

     }

     public static void main(String[] args) {

         List<Apple> inventory = Arrays.asList(

                 new Apple(100, "red"),

                 new Apple(110, "red"),

                 new Apple(190, "red"),

                 new Apple(170, "red"),

                 new Apple(100, "green"),

                 new Apple(120, "green"),

                 new Apple(160, "green"),

                 new Apple(180, "green")

         );

         List<Apple> newInventory1 = filterApples(inventory, new ApplePredicate() {

             @Override

             public boolean test(Apple apple) {

                 return apple.getWeight() > 150;

             }

         });

         printApples(newInventory1);

         System.out.println("-----");

         List<Apple> newInventory2 = filterApples(inventory, new ApplePredicate() {

             @Override

             public boolean test(Apple apple) {

                 return "green".equals(apple.getColor());

             }

         });

         printApples(newInventory2);

     }

 }

Lambda表达式

概况

1. Lambda表达式：可把Lambda表达式看作只有一个方法的接口匿名类，即没有声明名称的方法，也可以作为参数传递给另一个方法。

2. Lambda表达式特点

a) 匿名：不像普通的方法那样有一个明确的名称。

b) 函数：不像方法那样属于某个特定的类。但和方法一样，Lambda有参数列表、函数主体、返回类型，还可能有可以抛出的异常列表。

c) 传递：可以作为参数传递给方法或存储在变量中。

d) 简洁：无需像匿名类那样写很多模板代码。

3. Lambda表达式语法

a) 语法格式1

(parameters) -> expression

b) 语法格式2

(parameters) -> { statements; }

c) 举例

场景	Lambda表达式
布尔表达式	(List<String> list) -> list.isEmpty()
创建对象	() -> new Apple(10, "red")
消费一个对象	(Apple a) -> { System.out.println(a.getWeight()); }
从一个对象中选择/抽取	(String s) -> s.length()
组合两个值	(int a, int b) -> a * b
比较两个对象	(Apple a1, Apple a2) -> a1.getWeight().compareTo(a2.getWeight())

4.Lambda表达式使用条件：只能在函数式接口上使用。

5.函数式接口（Functional Interface）：只定义一个抽象方法的接口（注意不包括默认方法）。

6.举例：通过Lambda表达式实现行为参数化

a) ApplePredicate.java

 public interface ApplePredicate {

     boolean test(Apple apple);

 }

b) Test.java

 import java.util.ArrayList;

 import java.util.Arrays;

 import java.util.List;

 public class Test {

     public static List<Apple> filterApples(List<Apple> inventory, ApplePredicate p) {

         List<Apple> result = new ArrayList<>();

         for (Apple apple : inventory) {

             if (p.test(apple)) {

                 result.add(apple);

             }

         }

         return result;

     }

     public static void printApples(List<Apple> inventory) {

         for (Apple apple : inventory) {

             System.out.println(apple);

         }

     }

     public static void main(String[] args) {

         List<Apple> inventory = Arrays.asList(

                 new Apple(100, "red"),

                 new Apple(110, "red"),

                 new Apple(190, "red"),

                 new Apple(170, "red"),

                 new Apple(100, "green"),

                 new Apple(120, "green"),

                 new Apple(160, "green"),

                 new Apple(180, "green")

         );

         List<Apple> newInventory1 = filterApples(inventory, (Apple apple) -> apple.getWeight() > 150);

         printApples(newInventory1);

         System.out.println("-----");

         List<Apple> newInventory2 = filterApples(inventory, (apple) -> "green".equals(apple.getColor()));

         printApples(newInventory2);

     }

 }

函数式接口

1. Java 8自带的函数式接口都在java.util.function包下。

2. 异常：Java 8自带函数式接口都不允许抛出Checked Exception。如果需要Lambda表达式来抛出异常，要么定义一个自己的函数式接口，并声明Checked Exception，要么把Lambda包在一个try/catch块中。

3. 装箱操作（Boxing）：为了避免装箱操作带来的开销问题，不应使用Predicate<T>或Function<T, R>等通用函数式接口，而应使用IntPredicate、IntToLongFunction等原始类型特化接口。

4. Java 8常用函数式接口

函数式接口	函数描述符	原始类型特化
Predicate<T>	T->boolean	IntPredicate LongPredicate DoublePredicate
Consumer<T>	T->void	IntConsumer LongConsumer DoubleConsumer
Function<T,R>	T->R	IntFunction<R> IntToDoubleFunction IntToLongFunction LongFunction<R> LongToDoubleFunction LongToIntFunction DoubleFunction<R> ToIntFunction<T> ToDoubleFunction<T> ToLongFunction<T>
Supplier<T>	()->T	BooleanSupplier IntSupplier LongSupplier DoubleSupplier
UnaryOperator<T>	T->T	IntUnaryOperator LongUnaryOperator DoubleUnaryOperator
BinaryOperator<T>	(T,T)->T	IntBinaryOperator LongBinaryOperator DoubleBinaryOperator
BiPredicate<L, R>	(L, R) -> boolean
BiConsumer<T, U>	(T, U) -> void	ObjIntConsumer<T> ObjLongConsumer<T> ObjDoubleConsumer<T>
BiFunction<T, U, R>	(T, U) -> R	ToIntBiFunction<T, U> ToLongBiFunction<T, U> ToDoubleBiFunction<T, U>

5. 举例

a) Lambda表达式与函数式接口对应

场景	Lambda表达式	对应的函数式接口
布尔表达式	(List<String> list) -> list.isEmpty()	Predicate<List<String>>
创建对象	() -> new Apple(10, "red")	Supplier<Apple>
消费一个对象	(Apple a) -> { System.out.println(a.getWeight()); }	Consumer<Apple>
从一个对象中选择/抽取	(String s) -> s.length()	Function<String, Integer>或 ToIntFunction<String>
组合两个值	(int a, int b) -> a * b	IntBinaryOperator
比较两个值	(Apple a1, Apple a2) -> a1.getWeight().compareTo(a2.getWeight())	Comparator<Apple> 或 BiFunction<Apple, Apple, Integer> 或 ToIntBiFunction<Apple, Apple>

b) Predicate<T>

 import java.util.ArrayList;

 import java.util.Arrays;

 import java.util.List;

 import java.util.function.Predicate;

 public class Test {

     public static <T> List<T> filter(List<T> list, Predicate<T> p) {

         List<T> results = new ArrayList<>();

         for (T s : list) {

             if (p.test(s)) {

                 results.add(s);

             }

         }

         return results;

     }

     public static void main(String[] args) {

         List<String> listOfStrings = Arrays.asList("", "A", "B", "", "C");

         Predicate<String> nonEmptyStringPredicate = (String s) -> !s.isEmpty();

         List<String> nonEmpty = filter(listOfStrings, nonEmptyStringPredicate);

         for (String string : nonEmpty) {

             System.out.println(string);

         }

     }

 }

c) Consumer<T>

 import java.util.Arrays;

 import java.util.List;

 import java.util.function.Consumer;

 public class Test {

     public static <T> void forEach(List<T> list, Consumer<T> c){

         for(T i: list){

             c.accept(i);

         }

     }

     public static void main(String[] args) {

         List<Integer> listOfNumbers = Arrays.asList(1, 2, 3, 4, 5);

         forEach(listOfNumbers, (Integer i) -> System.out.println(i));

     }

 }

d) Function<T, R>

 import java.util.ArrayList;

 import java.util.Arrays;

 import java.util.List;

 import java.util.function.Function;

 public class Test {

     public static <T, R> List<R> map(List<T> list, Function<T, R> f) {

         List<R> result = new ArrayList<>();

         for(T s: list){

             result.add(f.apply(s));

         }

         return result;

     }

     public static void main(String[] args) {

         List<String> listOfStrings = Arrays.asList("lambdas", "in", "action");

         List<Integer> l = map(listOfStrings, (String s) -> s.length());

         for (Integer i : l) {

             System.out.println(i);

         }

     }

 }

类型推断

1. 类型推断：同一个Lambda表达式就可赋予不同的函数式接口，只要它们的抽象方法签名能够兼容。

 Callable<Integer> c = () -> 42;

 PrivilegedAction<Integer> p = () -> 42;

 Comparator<Apple> c1 = (Apple a1, Apple a2) -> a1.getWeight().compareTo(a2.getWeight());

 ToIntBiFunction<Apple, Apple> c2 = (Apple a1, Apple a2) -> a1.getWeight().compareTo(a2.getWeight());

 BiFunction<Apple, Apple, Integer> c3 = (Apple a1, Apple a2) -> a1.getWeight().compareTo(a2.getWeight());

2. void兼容规则：如果一个Lambda的主体是一个语句表达式，它就和一个返回void的函数描述符兼容（当然需要参数列表也兼容）。例如，虽然List.add方法返回boolean，但Consumer<T>的T->void仍然兼容。

Consumer<String> b = s -> list.add(s);

3. Object类：Object不是函数式接口。下面的代码无法编译。

Object o = () -> {System.out.println("Tricky example"); };

4. Lambda表达式类型省略

a) 参数类型省略：省略和不省略都可能更易读。

 // 没有类型推断

 Comparator<Apple> c = (Apple a1, Apple a2) -> a1.getWeight().compareTo(a2.getWeight());

 // 有类型推断

 Comparator<Apple> c = (a1, a2) -> a1.getWeight().compareTo(a2.getWeight());

b) 参数括号省略：当Lambda仅有一个类型需要推断的参数时，参数名称两边的括号也可以省略。

List<Apple> greenApples = filter(inventory, a -> "green".equals(a.getColor()));

使用外层变量

1. 外层变量限制：Lambda表达式只能使用显式声明为final或实际上是final的外层变量。与匿名类类似，但匿名类更严格（只能使用显式声明为final的外层变量）。

2. 限制原因

a) Lambda表达式在访问外层变量时，实际上是在访问它的副本，而不是访问原始变量。

b) 函数式编程不鼓励使用外层变量（更容易并行）。

3. 举例

a) 可正常运行

 int number = 100;

 Runnable r = () -> System.out.println(number);

 new Thread(r).start();

b) 运行报错“local variables referenced from a lambda expression must be final or effectively final”

 int number = 100;

 Runnable r = () -> System.out.println(number);

 new Thread(r).start();

 number = 200;

方法引用

1. 方法引用：可以重复使用现有的方法定义，并像Lambda一样传递。

2. 方法引用优点：有时比Lambda表达式可读性更好。

3. 方法引用的种类

a) 指向静态方法的方法引用，例如Integer.parseInt()方法，写作Integer::parseInt。

b) 指向任意类型实例方法的方法引用，例如String.length()方法，写作String::length。

c) 指向现有对象的实例方法的方法引用，例如有一个局部变量apple有getWeight()实例方法，apple::getWeight。

d) 指向构造函数的方法引用，例如Date的构造方法，写作Date::new。

e) 针对构造函数、数组构造函数和父类调用（super-call）的一些特殊形式的方法引用。

4. 举例

a) Lambda表达式与方法引用对应

Lambda表达式	对应的方法引用
(Apple a) -> a.getWeight()	Apple::getWeight
() -> Thread.currentThread().dumpStack()	Thread.currentThread()::dumpStack
(str, i) -> str.substring(i)	String::substring
(String s) -> System.out.println(s)	System.out::println
() -> new Date()	Date::new

b) 指向现有对象的实例方法的方法引用

 import java.util.Arrays;

 import java.util.List;

 import static java.util.Comparator.comparing;

 public class Test {

     public static void printApples(List<Apple> inventory) {

         for (Apple apple : inventory) {

             System.out.println(apple);

         }

     }

     public static void main(String[] args) {

         List<Apple> inventory = Arrays.asList(

                 new Apple(100, "red"),

                 new Apple(110, "red"),

                 new Apple(190, "red"),

                 new Apple(170, "red"),

                 new Apple(100, "green"),

                 new Apple(120, "green"),

                 new Apple(160, "green"),

                 new Apple(180, "green")

         );

         // i.e. inventory.sort((Apple a1, Apple a2) -> a1.getWeight().compareTo(a2.getWeight()));

         // i.e. inventory.sort((a1, a2) -> a1.getWeight().compareTo(a2.getWeight()));

         inventory.sort(comparing(Apple::getWeight));

         printApples(inventory);

     }

 }

c) 指向构造函数的方法引用

 import java.util.Date;

 import java.util.function.Function;

 import java.util.function.Supplier;

 interface TriFunction<T, U, V, R>{

     R apply(T t, U u, V v);

 }

 public class Test {

     public static void main(String[] args) {

         Supplier<Date> s = Date::new; // i.e. () -> new Date()

         Date d1 = s.get();

         System.out.println(d1);

         Function<Long, Date> f = Date::new; // i.e. (Long l) -> new Date(l)

         Date d2 = f.apply(0L);

         System.out.println(d2);

         TriFunction<Integer, Integer, Integer, Date> tf = Date::new;

         Date d3 = tf.apply(2000, 1, 1);

         System.out.println(d3);

     }

 }

复合Lambda表达式

1. 复合Lambda表达式：把多个简单的Lambda表达式复合成复杂的表达式，比如使用and、or复合。

2. 举例

a) Comparator复合

 import java.util.Arrays;

 import java.util.List;

 import static java.util.Comparator.comparing;

 public class Test {

     public static void printApples(List<Apple> inventory) {

         for (Apple apple : inventory) {

             System.out.println(apple);

         }

     }

     public static void main(String[] args) {

         List<Apple> inventory = Arrays.asList(

                 new Apple(100, "red"),

                 new Apple(110, "red"),

                 new Apple(190, "red"),

                 new Apple(170, "red"),

                 new Apple(100, "green"),

                 new Apple(120, "green"),

                 new Apple(160, "green"),

                 new Apple(180, "green")

         );

         inventory.sort(

                 comparing(Apple::getWeight)

                 .reversed()

                 .thenComparing(Apple::getColor)

         );

         printApples(inventory);

     }

 }

b) Predicate复合

 import java.util.ArrayList;

 import java.util.Arrays;

 import java.util.List;

 import java.util.function.Predicate;

 public class Test {

     public static List<Apple> filterApples(List<Apple> inventory, Predicate<Apple> p) {

         List<Apple> result = new ArrayList<>();

         for (Apple apple : inventory) {

             if (p.test(apple)) {

                 result.add(apple);

             }

         }

         return result;

     }

     public static void printApples(List<Apple> inventory) {

         for (Apple apple : inventory) {

             System.out.println(apple);

         }

     }

     public static void main(String[] args) {

         List<Apple> inventory = Arrays.asList(

                 new Apple(100, "red"),

                 new Apple(110, "red"),

                 new Apple(190, "red"),

                 new Apple(170, "red"),

                 new Apple(100, "green"),

                 new Apple(120, "green"),

                 new Apple(160, "green"),

                 new Apple(180, "green")

         );

         Predicate<Apple> p = a -> "red".equals(a.getColor());

         p = p.negate()

                 .and(a -> a.getWeight() > 150)

                 .or(a -> a.getWeight() <= 110);

         List<Apple> newInventory = filterApples(inventory, p);

         printApples(newInventory);

     }

 }

c) 函数复合

 import java.util.function.Function;

 public class Test {

     public static void main(String[] args) {

         Function<Integer, Integer> f = x -> x + 1;

         Function<Integer, Integer> g = x -> x * 2;

         Function<Integer, Integer> h1 = f.andThen(g); // i.e. g(f(x))

         int result1 = h1.apply(1);

         System.out.println(result1); //

         Function<Integer, Integer> h2 = f.compose(g); // i.e. f(g(x))

         int result2 = h2.apply(1);

         System.out.println(result2); //

     }

 }

作者：netoxi
出处：http://www.cnblogs.com/netoxi
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