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Oracle號稱今年一定發布jdk8, 即使有bug, jdk8無疑最大的寵兒就是lambda表達式了，還是直接上代碼，看看lambda表達式怎么了。

• 環境：

????? ? openjdk8, eclipse4.3.1(支持jdk8編譯)

• 實例代碼：

// Demo1 public class RunnableTest {public static void main(String[] args) {Runnable runner = () -> System.out.println("I am lambda runner.");runner.run();} } // Demo2 import javax.swing.JButton;public class ListenerTest {public static void main(String[] args) {JButton btn = new JButton();btn.addActionListener(e -> System.out.println("Event Source is: " + e.getSource()));} } // Demo3 //Student.java public class Student {private String name;private float score;public Student(String name, float score) {this.name = name;this.score = score;}public String getName() {return name;}public void setName(String name) {this.name = name;}public float getScore() {return score;}public void setScore(float score) {this.score = score;}@Overridepublic String toString() {return "Student [name=" + name + ", score=" + score + "]";} }// ComparatorTest.java import java.util.ArrayList; import java.util.Collections; import java.util.List; import java.util.Random;public class ComparatorTest {public static void main(String[] args) {List<Student> students = new ArrayList<>();Random r = new Random();for (int i=0; i<5; i++){students.add(new Student("stu"+i, r.nextFloat() * 100));}Collections.sort(students, (Student s1, Student s2) -> (int)(s1.getScore() - s2.getScore()));for (Student s: students){System.out.println(s);}} } 看了這3個例子，就會覺得lambda很偏向于函數式編程(如Lisp這種語言)，這種語言很簡潔，但是易讀性不是那么好，沒辦法，又想簡潔又想易讀式不太容易的，以前java有匿名類，我們通常就實現這個匿名類里的一個接口，這叫Function Interface，現在我們就可以不用寫這個匿名類了，就直接實現接口就行了，就像上面的Demo一樣。

下面說一下lambda在Collection上的工作：

• Function接口:

public interface Function<T, R> {R apply(T t);... } Function接受一種類型T輸入，以另一類型R輸出, 看實例： import java.util.function.Function;public class Person {private String firstName;private String lastName;private int age;public Person(String firstName, String lastName){this.firstName = firstName;this.lastName = lastName;}public Person(String firstName, String lastName, int age) {this.firstName = firstName;this.lastName = lastName;this.age = age;}public String getFirstName() {return firstName;}public void setFirstName(String firstName) {this.firstName = firstName;}public String getLastName() {return lastName;}public void setLastName(String lastName) {this.lastName = lastName;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}/*** 這里就用Function實現可訂制的信息輸出* Function: 將輸入類型轉換為輸出類型，這里即將Person輸入，以String輸出*/public String printCustom(Function<Person, String> f){return f.apply(this);} } 測試用例： import java.util.function.Function;/*** Function 測試*/ public class FunctionTest {public static void main(String[] args) {Person person = new Person("Hello", "Function");// 用lambda表達式作為參數傳遞System.out.println(person.printCustom(p -> "FirstName : " + p.getFirstName()));// 用lambda表達式構建Function對象Function<Person, String> westernStyle = p -> {return "LastName: " + p.getLastName() + ", FirstName: " + p.getFirstName();};Function<Person, String> eastStyle = p -> {return "FirstName: " + p.getFirstName() + ", LastName: " + p.getLastName();};System.out.println(person.printCustom(westernStyle));System.out.println(person.printCustom(eastStyle));} }

輸出結果：

• 集合的迭代：

import java.util.ArrayList; import java.util.List;/*** 通過lambda迭代集合*/ public class LoopTest {public static void main(String[] args) {List<Person> ps = new ArrayList<>();for (int i=0; i<5; i++){ps.add(new Person("XX"+i, "OO"+i));}ps.forEach(p -> System.out.println(p.getFirstName()));//ps.forEach(Person::printEast);System.out.println("-------------------------");ps.forEach(p -> {System.out.println(p.printCustom(r->r.getLastName()+" " + r.getFirstName()));});} }

• Predicate接口，它接受一種輸入類型，并通過test接口方法返回boolean值：

public interface Predicate<T> {boolean test(T t);... ｝

Predicate可以有效實現集合過濾操作：

測試用例：

/*** Predicate實現集合過濾*/ public class PredicateTest {public static void main(String[] args) {List<Person> ps = new ArrayList<>();Random r = new Random();for (int i=0; i<10; i++){ps.add(new Person("firstName"+i, "lastName"+i, r.nextInt(20) + 1));}Predicate<Person> children = p -> p.getAge() < 12;Predicate<Person> teenagers = p -> p.getAge() >= 12 && p.getAge() < 18;Predicate<Person> adults = p -> p.getAge() >= 18;for (Person p : ps){System.out.print(p.getFirstName() + "'s age is " + p.getAge());if (children.test(p)){System.out.println(", is a child.");} else if (teenagers.test(p)){System.out.println(", is a teenager.");} else if(adults.test(p)){System.out.println(", is an adult.");}}} }

輸出結果：

• 2個綜合些的例子：

import java.util.ArrayList; import java.util.List; import java.util.Random; import java.util.function.Predicate; import java.util.stream.Collectors;public class CollectTest {public static void main(String[] args) {List<Person> ps = new ArrayList<>();Random r = new Random();for (int i=0; i<10;i++){ ps.add(new Person("firstname"+i, "lastname"+i, r.nextInt(30) + 1));}// 過濾條件>18歲Predicate<Person> adults = p -> p.getAge() > 18;// 通過collect獲取過濾后的集合List<Person> filteredPersons = ps.stream().filter(adults).collect(Collectors.toList());filteredPersons.forEach(p -> System.out.println(p.getFirstName() + " "+ p.getLastName()+"'s age: " + p.getAge()));} } import java.util.ArrayList; import java.util.List; import java.util.OptionalDouble; import java.util.OptionalInt; import java.util.Random; import java.util.function.Predicate;/*** 通過map來實現求最大值，平均值*/ public class MapTest {public static void main(String[] args) {List<Person> ps = new ArrayList<>();Random r = new Random();for (int i=0; i<10;i++){ ps.add(new Person("firstname"+i, "lastname"+i, r.nextInt(30) + 1));}// 過濾條件>18歲Predicate<Person> adults = p -> p.getAge() > 18;//最大年齡OptionalInt maxAge = ps.parallelStream().filter(adults).mapToInt(p -> p.getAge()).max();System.out.println("最大年齡：" + maxAge.getAsInt());// 求18歲以上的平均值OptionalDouble average = ps.parallelStream().filter(adults).mapToDouble(p -> p.getAge()).average();System.out.println("平均年齡：" + average.getAsDouble());} }

• 收工。

轉載于:https://my.oschina.net/indestiny/blog/196347

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