目录
· 行为参数化
· 概况
· 函数式接口
· 类型推断
· 使用外层变量
· 方法引用
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
本文版权归作者和博客园共有，欢迎转载，未经同意须保留此段声明，且在文章页面明显位置给出原文连接。欢迎指正与交流。