jdk8新特性-亮瞎眼的lambda表达式
jdk8之前，尤其是在写GUI程序的事件监听的时候，各种的匿名内部类，大把大把拖沓的代码，程序毫无美感可言！既然java中一切皆为对象，那么，就类似于某些动态语言一样，函数也可以当成是对象啊！代码块也可以当成是对象啊！随着函数式编程的概念越来越深入人心，java中CODE=OBJECT的这一天终于到来了！如果你认为lambda表达式仅仅是为了从语法上简化匿名内部类，那就太小看jdk8的lambda了！
下面我们就来看下lambda表达式是如何亮瞎你的眼的！
lambda的定义
Funda-men-tally, a lambda expression is just a shorter way of writing an implementation of a method for later execution.
（1）lambda是方法的实现
（2）lambda是延迟执行的
首先看一个用匿名内部类的例子：
public class Test1{
public static void main(String args[]){
Runnable r = new Runnable(){
public void run(){
System.out.println("hello,lambda!");
}
};
r.run();
}
}public class Test2{
public static void main(String args[]){
Runnable r = ()->System.out.println("hello,lambda");
r.run();
}
}这他妈的得省多少代码啊！
有木有很兴奋啊！
下面还有更刺激的！
lambda是如何做到的呢？看一下反编译之后的字节码：
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=1, locals=2, args_size=1
0: invokedynamic #2, 0 // InvokeDynamic #0:run:()Ljava/lang/Runnable;
5: astore_1
6: aload_1
7: invokeinterface #3, 1 // InterfaceMethod java/lang/Runnable.run:()V
12: return
LineNumberTable:
line 3: 0
line 4: 6
line 5: 12
}
可以看出来lambda并不是语法糖，它不是像匿名内部类那样生成那种带有$的匿名类。
简单的说，这里只是定义了一个方法调用点，具体调用那个方法要到运行时才能决定，这就是前面所说的：延迟执行。
具体的细节请google：invokedynamic。
为了配合lambda，jdk8引入了一个新的定义叫做：函数式接口（Functional interfaces）
函数式接口：
it is an interface that requires exactly one method to be implemented in order to satisfy the requirements of the interface.
（1）是一个接口
（2）只有一个待实现的方法
因为jdk8开始，接口可以有default方法，所以，函数式接口也是可以有default方法的，但是，只能有一个未实现的方法。
与此对应，新引入了一个注解： @FunctionalInterface
这个注解只是起文档的作用，说明这个接口是函数式接口，编译器并不会使用这个注解来决定一个接口是不是函数式接口。
不管加不加@FunctionalInterface这个注解，下面的接口都是函数式接口：
interface Something {
public String doit(Integer i);
}
lambda的语法
A lambda in Java essentially consists of three parts: a parenthesized set of parameters, an arrow, and then a body,
which can either be a single expression or a block of Java code.
lambda包含3个部分：
（1）括弧包起来的参数
（2）一个箭头
（3）方法体，可以是单个语句，也可以是语句块
参数可以写类型，也可以不写，jvm很智能的，它能自己推算出来
public class Test3{
public static void main(String... args) {
Comparator c = (String lhs, String rhs) -> lhs.compareTo(rhs);
int result = c.compare("Hello", "World");
System.out.println(result);
}
}
public class Test4{
public static void main(String... args) {
Comparator c =(lhs, rhs) ->{
System.out.println("I am comparing " +lhs + " to " + rhs);
return lhs.compareTo(rhs);
};
int result = c.compare("Hello", "World");
System.out.println(result);
}
} 一个很有意思的事情：
之前我们说Object是一切类的父类，然而在加入了lambda以后，这种大一统的局面将不复存在：
public class Test5{
public static void main(String args[]){
Object r = ()->System.out.println("hello,lambda");
}
}Test5.java:3: error: incompatible types: Object is not a functional interface
Object r = ()->System.out.println("hello,lambda");
^
1 error
很显然，编译器会检查变量的引用类型里面是否真的是一个函数式接口。那么如何让这段代码通过编译呢？
只需要加一个强制类型转换就可以了：
public class Test6{
public static void main(String args[]){
Object r = (Runnable)()->System.out.println("hello,lambda");
}
}我们知道，在匿名内部类中：
class Hello {
public Runnable r = new Runnable() {
public void run() {
System.out.println(this);
System.out.println(toString());
}
};
public String toString() {
return "Hello's custom toString()";
}
}
public class InnerClassExamples {
public static void main(String... args) {
Hello h = new Hello();
h.r.run();
}
}
想要引用Hello类需要Hello.this这样：
class Hello {
public Runnable r = new Runnable() {
public void run() {
System.out.println(Hello.this);
System.out.println(Hello.this.toString());
}
};
}下面我们就来看一下伟大的lambda是什么样子的：
class Hello{
public Runnable r = () -> {
System.out.println(this);
System.out.println(toString());
};
public String toString() {
return "Hello's custom toString()";
}
}
public class Test7{
public static void main(String args[]){
Hello h = new Hello();
h.r.run();
}
}
Hello's custom toString()
Hello's custom toString()
System.out.println(this);这里的this指的是Hello，而非lambda表达式！
但是，如果我们想在lambda表达式中返回lambda本身该怎么做呢？
变量捕获
匿名内部类只能引用作用域外面的final的变量，在lambda中对这个限制做了削弱，只需要是“等价final”就可以，没必要用final关键字来标识。
public class Test8{
public static void main(String args[]){
String message = "Howdy, world!";//不需要是final的
Runnable r = () -> System.out.println(message);//这里也能访问
r.run();
}
}public class Test9{
public static void main(String args[]){
String message = "Howdy, world!";
Runnable r = () -> System.out.println(message);
r.run();
message = "change it";
}
}Runnable r = () -> System.out.println(message);
^
1 error
如果上面的内容看上去平淡无奇的话，真正的大杀器出现了：方法引用
我们有一个这样的类：
class Person {
public String firstName;
public String lastName;
public int age;
}class Person{
public String firstName;
public String lastName;
public int age;
public Person(String firstName, String lastName, int age){
this.firstName = firstName;
this.lastName = lastName;
this.age = age;
}
public String toString(){
return firstName+","+lastName+","+age;
}
}
public class Test10{
public static void main(String args[]){
Person people[] = new Person[]{
new Person("Ted", "Neward", 41),
new Person("Charlotte", "Neward", 41),
new Person("Michael", "Neward", 19),
new Person("Matthew", "Neward", 13)
};
//sort by firstName
Arrays.sort(people, (lhs,rhs)->lhs.firstName.compareTo(rhs.firstName));
for(Person p : people){
System.out.println(p);
}
}
}
class Person{
public String firstName;
public String lastName;
public int age;
public Person(String firstName, String lastName, int age){
this.firstName = firstName;
this.lastName = lastName;
this.age = age;
}
public String toString(){
return firstName+","+lastName+","+age;
}
public final static Comparator compareFirstName =
(lhs, rhs) -> lhs.firstName.compareTo(rhs.firstName);
public final static Comparator compareLastName =
(lhs, rhs) -> lhs.lastName.compareTo(rhs.lastName);
public final static Comparator compareAge =
(lhs, rhs) -> lhs.age - rhs.age;
}
public class Test11{
public static void main(String args[]){
Person people[] = new Person[]{
new Person("Ted", "Neward", 41),
new Person("Charlotte", "Neward", 41),
new Person("Michael", "Neward", 19),
new Person("Matthew", "Neward", 13)
};
Arrays.sort(people, Person.compareFirstName);//这里直接引用lambda
for(Person p : people){
System.out.println(p);
}
}
}
然后引用这个变量！所以，还有这么一种调用方法：
class Person{
public String firstName;
public String lastName;
public int age;
public Person(String firstName, String lastName, int age){
this.firstName = firstName;
this.lastName = lastName;
this.age = age;
}
public String toString(){
return firstName+","+lastName+","+age;
}
public static int compareFirstName(Person lhs, Person rhs){
return lhs.firstName.compareTo(rhs.firstName);
}
public static int compareLastName(Person lhs, Person rhs){
return lhs.lastName.compareTo(rhs.lastName);
}
}
public class Test12{
public static void main(String args[]){
Person people[] = new Person[]{
new Person("Ted", "Neward", 41),
new Person("Charlotte", "Neward", 41),
new Person("Michael", "Neward", 19),
new Person("Matthew", "Neward", 13)
};
Arrays.sort(people, Person::compareFirstName);
for(Person p : people){
System.out.println(p);
}
}
}
如果是static方法使用：类名::方法名
如果是instance方法：instance::方法名
但是，上面的代码还是不是很美观，因为Person只是一个数据对象，它不应该的对外提供compareFirstName或者是compareLastName这样的方法，
我们需要的仅仅是根据某个字段排序而已！很幸运的是jdk的api帮我们做了这件事：
import java.util.*;
class Person{
public String firstName;
public String lastName;
public int age;
public Person(String firstName, String lastName, int age){
this.firstName = firstName;
this.lastName = lastName;
this.age = age;
}
public String getFirstName(){
return this.firstName;
}
public String getLastName(){
return this.lastName;
}
public String toString(){
return firstName+","+lastName+","+age;
}
}
public class Test13{
public static void main(String args[]){
Person people[] = new Person[]{
new Person("Ted", "Neward", 41),
new Person("Charlotte", "Neward", 41),
new Person("Michael", "Neward", 19),
new Person("Matthew", "Neward", 13)
};
Arrays.sort(people, Comparator.comparing(Person::getFirstName));
for(Person p : people){
System.out.println(p);
}
}
}
但是注意这里的Person::getFirstName，很显然getFirstName()并不是static的，这是jdk做了封装的缘故！
这样做就非常完美了！
假如我们的排序算法改为：先按照lastName，然后按照age排序呢？
public class Test15{
public static void main(String args[]){
Person people[] = new Person[]{
new Person("Ted", "Neward", 10),
new Person("Charlotte", "Neward", 41),
new Person("Michael", "Naward", 19),
new Person("Matthew", "Nmward", 13)
};
Collections.sort(Arrays.asList(people), (lhs, rhs)->{
if(lhs.getLastName().equals(rhs.getLastName())){
return lhs.getAge()-rhs.getAge();
}else{
return lhs.getLastName().compareTo(rhs.getLastName());
}
});
for(Person p : people){
System.out.println(p);
}
}
}
public class Test16{
public static void main(String args[]){
Person people[] = new Person[]{
new Person("Ted", "Neward", 10),
new Person("Charlotte", "Neward", 41),
new Person("Michael", "Naward", 19),
new Person("Matthew", "Nmward", 13)
};
Collections.sort(Arrays.asList(people),Comparator.comparing(Person::getLastName).thenComparing(Person::getAge));
for(Person p : people){
System.out.println(p);
}
}
}
还有更多的诸如：andThen()这样的方法，可以查api。
虚方法扩展
因为接口可以有default方法，所以很多类库都重写了，加入了一些default的方法，比如：
interface Iterator
boolean hasNext();
T next();
void remove();
void skip(int i) default {
for (; i > 0 && hasNext(); i--) next();
}
}
skip(i)就是一个default方法，这样所有的Iterator的子类都具有了一个叫skip的方法！
但是，大家对default方法的争议还是比较大的，比如：
interface I1 {
public default void print(){
System.out.println("I1");
}
public void hello();
}
interface I2{
public default void print(){
System.out.println("I2");
}
public void world();
}
class Impl implements I1,I2{
public void hello(){
}
public void world(){
}
}
Stream：
之前的文章已经有介绍，下面只据一些使用的例子：
过滤age>12的元素：
people
.stream()
.filter(it -> it.getAge() >= 21) ; people.stream()
.filter((it) -> it.getAge() >= 21)
.forEach((it) ->
System.out.println("Have a beer, " + it.getFirstName())); Predicate drinkingAge = (it) -> it.getAge() >= 21;
Predicate brown = (it) -> it.getLastName().equals("Brown");
people.stream()
.filter(drinkingAge.and(brown))
.forEach((it) ->System.out.println("Have a beer, " + it.getFirstName()));
IntStream ages =
people.stream()
.mapToInt((it) -> it.getAge());
//sum：
int sum = people.stream()
.mapToInt(Person::getAge)
.sum(); public class Test17{
public static void main(String args[]){
List values = Arrays.asList(1,2,3,4,5);
int sum = values.stream().reduce(0, (l,r)->l+r);
System.out.println(sum);
}
}
二者运算的结果作为下一次运算的左操作数，依次循环。
最后看一个好玩的例子：
class Person{
public String firstName;
public String lastName;
public int age;
public Person(String firstName, String lastName, int age){
this.firstName = firstName;
this.lastName = lastName;
this.age = age;
}
public String getFirstName(){
return this.firstName;
}
public String getLastName(){
return this.lastName;
}
public int getAge(){
return this.age;
}
public String toString(){
return firstName+","+lastName+","+age;
}
public String toJson(){
return "{"+
"firstName:\""+firstName+"\","+
"lastName:\""+lastName+"\","+
"age:"+age +
"}";
}
}
public class Test18{
public static void main(String args[]){
Person people[] = new Person[]{
new Person("Ted", "Neward", 10),
new Person("Charlotte", "Neward", 41),
new Person("Michael", "Naward", 19),
new Person("Matthew", "Nmward", 13)
};
String json = Arrays.asList(people).stream().map(Person::toJson).reduce("[",(l,r)->l + (l.equals("[")?"":",") + r)+"]";
System.out.println(json);
}
}
[{firstName:"Ted",lastName:"Neward",age:10},{firstName:"Charlotte",lastName:"Neward",age:41},{firstName:"Michael",lastName:"Naward",age:19},{firstName:"Matthew",lastName:"Nmward",age:13}]
还可以这样：
public class Test19{
public static void main(String args[]){
Person people[] = new Person[]{
new Person("Ted", "Neward", 10),
new Person("Charlotte", "Neward", 41),
new Person("Michael", "Naward", 19),
new Person("Matthew", "Nmward", 13)
};
String joined = Arrays.asList(people).stream().map(Person::toJson).collect(Collectors.joining(", "));
System.out.println("[" + joined + "]");
}
}更进一步：
public class Test20{
public static void main(String args[]){
Person people[] = new Person[]{
new Person("Ted", "Neward", 10),
new Person("Charlotte", "Neward", 41),
new Person("Michael", "Naward", 19),
new Person("Matthew", "Nmward", 13)
};
String json = Arrays.asList(people).stream().map(Person::toJson).collect(Collectors.joining(", ", "[", "]"));
System.out.println(json);
}
}
如果只能用一个字来形容，那就是perfect！
http://www.oracle.com/technetwork/articles/java/architect-lambdas-part1-2080972.html
http://www.oracle.com/technetwork/articles/java/architect-lambdas-part2-2081439.html