你真的了解Lambda表达式么？

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lambda表达式实战

从例子引出lambda

传递Runnable创建Thread

  • java8之前
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Thread thread=new Thread(new Runnable() {
    @Override
    public void run() {
        // do something
    }
});
  • java 8 之后
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new Thread(()->{});

上边的例子比较简单，但是有两个疑问。什么是Lambda表达式？怎么使用lambda表达式？

什么是Lambda表达式？

从上述例子入手，首先我们知道Lambda一般代表的是一个匿名对象；其次我们点击“->”,IDE会帮助我们进入到符合Lambda规范的函数接口。我们来观察下这个符合规范的类的变化。

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// 省略注释
package java.lang;
public interface Runnable {
   public abstract void run();
}
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// 省略注释
package java.lang;
@FunctionalInterface
public interface Runnable {
   public abstract void run();
}

我们发现java8后Runnable接口新增了一个注解@FunctionalInterface。下边我们一起来看下这个注解是什么。

FunctionalInterface
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/**
 * An informative annotation type used to indicate that an interface
 * type declaration is intended to be a <i>functional interface</i> as
 * defined by the Java Language Specification.
 *
 * Conceptually, a functional interface has exactly one abstract
 * method.  Since {@linkplain java.lang.reflect.Method#isDefault()
 * default methods} have an implementation, they are not abstract.  If
 * an interface declares an abstract method overriding one of the
 * public methods of {@code java.lang.Object}, that also does
 * <em>not</em> count toward the interface's abstract method count
 * since any implementation of the interface will have an
 * implementation from {@code java.lang.Object} or elsewhere.
 *
 * <p>Note that instances of functional interfaces can be created with
 * lambda expressions, method references, or constructor references.
 *
 * <p>If a type is annotated with this annotation type, compilers are
 * required to generate an error message unless:
 *
 * <ul>
 * <li> The type is an interface type and not an annotation type, enum, or class.
 * <li> The annotated type satisfies the requirements of a functional interface.
 * </ul>
 *
 * <p>However, the compiler will treat any interface meeting the
 * definition of a functional interface as a functional interface
 * regardless of whether or not a {@code FunctionalInterface}
 * annotation is present on the interface declaration.
 *
 * @jls 4.3.2. The Class Object
 * @jls 9.8 Functional Interfaces
 * @jls 9.4.3 Interface Method Body
 * @since 1.8
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface FunctionalInterface {}
  • 上边文档的核心意思是：@FunctionInterface注解是为了表明这个类是一个函数式接口。
  • 函数式接口有这样的特点：只有一个抽象方法。java8提供了default方法，以及超类Object中的方法(toString,Equals),这些方法不计算抽象方法数量的统计中。
  • 使用上：函数式接口可以配合lambda表达式、方法引用、构造引用使用。
  • 如果类上标记了这个注解，编译器会在编译期进行检查
  • 最后，即使我们没有标注这个注解，编译器也会将它看待成一个函数式接口

好了，从上边我们知道了lambda的特点，接下来我们来聊下怎么使用?

如何使用Lambda

首先，我们去官网查阅Java8新特性，找到Lambda表达式的说明。我们从这个文档的**“Syntax of Lambda Expressions”**部分入手，大概可以得到如下的结论。

Lambda的组成

Lambda主要由下边几部分组成;参数列表，连接符，主体。

  • 参数列表

    • 圆括号内部，参数以“,”分割开来。如(String a,Object b)。
    • 此外，参数的类型和括号，有些时候是可以省略

  • 箭头记号

    • 通过“->”这种特殊符号形式，连接前后。

  • 主体

    • 可以由单个表达式，或者语句块组成。

    • 单个表达式，如”System.out.println(“xxx”)”

    • 语句块

      • 示例1
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      {
      	System.out.println("xxx");
      }
      • 示例2
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      {
      	// do something return some result
      	return 100;
      }
Lambda的完整用法示例
无返回值的lambda的用例

目的，将具体业务实现交给调用者处理。

  • 定义一个无返回值，符合FunctionInterface规范的接口对象
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interface Print<String>{
    void printName(String string);
}
使用示例1

我这里的业务逻辑是根据输入参数，执行日志打印操作。实际业务场景下，可能对应的是发送邮件或者MQ这样的具体操作。

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public class LambdaDemo {
    public static void main(String[] args) {
        PrintSomeThing(name->System.out.println(name),"Hello baigt");
    }
    public static void PrintSomeThing(Print<String> str,String name) {
        str.printName(name);
    }
}
使用示例1 的延伸使用
  • 定义 一个使用类
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class Doctor{
    String name;
    String interest;
    public Doctor(String name, String interest) {
        this.name = name;
        this.interest = interest;
    }
    public void printName(Print<String> str) {
            str.printName(name);
    }
}
  • 具体使用
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Doctor doctor=new Doctor("baigt","java and javascript");
doctor.printName(name->System.out.println(name));
有返回值的lambda的用例

目的，将具体业务实现交给调用者处理，并将结果返回。

  • 定义一个有返回值，符合FunctionInterface规范的接口对象
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interface GetSomething<String>{
    String getThing();
}
  • 定义一个使用者
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class Doctor{
    String name;
    String interest;
    public Doctor(String name, String interest) {
        this.name = name;
        this.interest = interest;
    }

    public String getInterest(GetSomething<String> get) {
        return get.getThing()+","+name;
    }
}
  • 使用示例

我这里的业务逻辑是根据输入参数(隐式interest)，计算出一个结果返回出来，并对这个结果执行打印操作。

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Doctor doctor=new Doctor("baigt","java and javascript");
System.out.println(doctor.getInterest(() -> "Hi"));

到此处，我们已经大概明白lambda表达式的基本用法。但是还会有两个疑问？

  • 上边例子我们自定义了几个函数式接口，那么还有其他常用的函数式接口？
  • 函数式接口不仅可以通过lambda表达式使用，还可以通过方法引用和构造引用来使用。那么这种引用又是怎么回事？

常用函数接口

我们选中@FunctionInterface注解类，通过Ide的Find Usages功能，会发现在java.util.function包下java8新增了很多类。这里挑几个基础的(其他的基本是功能上的增强或变种)来说。大致上有这么几种。

  • Consumer
  • Supplier
  • Predicate
  • Function

下边会做一个简单的说明和使用。可能不会细致的去讲每一个Api。旨在让大家快速熟悉使用java8 lambda。

Consumer
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/**
 * Represents an operation that accepts a single input argument and returns no
 * result. Unlike most other functional interfaces, {@code Consumer} is expected
 * to operate via side-effects.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #accept(Object)}.
 *
 * @param <T> the type of the input to the operation
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Consumer<T> {

    /**
     * Performs this operation on the given argument.
     *
     * @param t the input argument
     */
    void accept(T t);

    /**
     * Returns a composed {@code Consumer} that performs, in sequence, this
     * operation followed by the {@code after} operation. If performing either
     * operation throws an exception, it is relayed to the caller of the
     * composed operation.  If performing this operation throws an exception,
     * the {@code after} operation will not be performed.
     *
     * @param after the operation to perform after this operation
     * @return a composed {@code Consumer} that performs in sequence this
     * operation followed by the {@code after} operation
     * @throws NullPointerException if {@code after} is null
     */
    default Consumer<T> andThen(Consumer<? super T> after) {
        Objects.requireNonNull(after);
        return (T t) -> { accept(t); after.accept(t); };
    }
}

首先此接口只有一个抽象方法accept，该方法接收一个入参，不返回结果。

定义使用类
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public static void doConsumer(Consumer consumer,String input) {
    consumer.accept(input);
}
  • 使用示例1

接收 “something input”输入，并执行打印操作

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Consumer consumer = input -> System.out.println(input);
doConsumer(consumer,"something input");
  • 使用示例2

将两个Consumer操作串连起来，andThen的后执行。

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Consumer consumer = input -> System.out.println(input);
doConsumer(consumer.andThen(input2->{
            System.out.println("input2");
        }),"something input");
Supplier
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/**
 * Represents a supplier of results.
 *
 * <p>There is no requirement that a new or distinct result be returned each
 * time the supplier is invoked.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #get()}.
 *
 * @param <T> the type of results supplied by this supplier
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Supplier<T> {

    /**
     * Gets a result.
     *
     * @return a result
     */
    T get();
}

首先此接口只有一个抽象方法get，该方法不接收参数，返回一个T类型的结果。

定义使用类
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public static <T> T doSupplier(Supplier<T> supplier) {
	return supplier.get();
}
  • 使用示例1

不传入参数，生成一个指定类型为String或Integer的对象

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System.out.println(doSupplier(() -> "baigt"));
System.out.println(doSupplier(() -> {return Integer.valueOf("10");}));
Predicate
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import java.util.Objects;

/**
 * Represents a predicate (boolean-valued function) of one argument.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #test(Object)}.
 *
 * @param <T> the type of the input to the predicate
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Predicate<T> {

    /**
     * Evaluates this predicate on the given argument.
     *
     * @param t the input argument
     * @return {@code true} if the input argument matches the predicate,
     * otherwise {@code false}
     */
    boolean test(T t);

    /**
     * Returns a composed predicate that represents a short-circuiting logical
     * AND of this predicate and another.  When evaluating the composed
     * predicate, if this predicate is {@code false}, then the {@code other}
     * predicate is not evaluated.
     *
     * <p>Any exceptions thrown during evaluation of either predicate are relayed
     * to the caller; if evaluation of this predicate throws an exception, the
     * {@code other} predicate will not be evaluated.
     *
     * @param other a predicate that will be logically-ANDed with this
     *              predicate
     * @return a composed predicate that represents the short-circuiting logical
     * AND of this predicate and the {@code other} predicate
     * @throws NullPointerException if other is null
     */
    default Predicate<T> and(Predicate<? super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) && other.test(t);
    }
}

首先此接口只有一个抽象方法test，该方法接受一个T类型的对象，返回一个boolean类型的结果。

定义使用类
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public static boolean doPredicate(Predicate<String> predicate,String string) {
    return predicate.test(string);
}
  • 使用示例1

根据条件，判断输入对象是否符合过滤规则。

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System.out.println(doPredicate(input -> input.length() > 5, "12345"));
System.out.println(doPredicate(((Predicate<String>) (input -> input.length() > 5))
        .and(input -> input.equalsIgnoreCase("12345")), "12345"));
Function
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import java.util.Objects;

/**
 * Represents a function that accepts one argument and produces a result.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #apply(Object)}.
 *
 * @param <T> the type of the input to the function
 * @param <R> the type of the result of the function
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Function<T, R> {

    /**
     * Applies this function to the given argument.
     *
     * @param t the function argument
     * @return the function result
     */
    R apply(T t);

    /**
     * Returns a composed function that first applies the {@code before}
     * function to its input, and then applies this function to the result.
     * If evaluation of either function throws an exception, it is relayed to
     * the caller of the composed function.
     *
     * @param <V> the type of input to the {@code before} function, and to the
     *           composed function
     * @param before the function to apply before this function is applied
     * @return a composed function that first applies the {@code before}
     * function and then applies this function
     * @throws NullPointerException if before is null
     *
     * @see #andThen(Function)
     */
    default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
        Objects.requireNonNull(before);
        return (V v) -> apply(before.apply(v));
    }

    /**
     * Returns a composed function that first applies this function to
     * its input, and then applies the {@code after} function to the result.
     * If evaluation of either function throws an exception, it is relayed to
     * the caller of the composed function.
     *
     * @param <V> the type of output of the {@code after} function, and of the
     *           composed function
     * @param after the function to apply after this function is applied
     * @return a composed function that first applies this function and then
     * applies the {@code after} function
     * @throws NullPointerException if after is null
     *
     * @see #compose(Function)
     */
    default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) {
        Objects.requireNonNull(after);
        return (T t) -> after.apply(apply(t));
    }

    /**
     * Returns a function that always returns its input argument.
     *
     * @param <T> the type of the input and output objects to the function
     * @return a function that always returns its input argument
     */
    static <T> Function<T, T> identity() {
        return t -> t;
    }
}

首先此接口只有一个抽象方法apply，该方法接收一个T类型对象，返回一个R类型的结果。

定义使用类
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public static Integer doFunction(Function<String,Integer> function,String input) {
    return function.apply(input);
}
  • 使用示例1

接收一个String类型的入参，返回Integer类型的结果。示例中没做具体异常判断。

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System.out.println(doFunction(input -> input.length(), "baigt"));
// 上述结果为 5
 System.out.println(doFunction(((Function<String, Integer>) (input -> input.length())).compose(input -> String.valueOf(input.length() * 3)), "baigt"));
// 上述结果为 2
 System.out.println(doFunction(((Function<String, Integer>) (input -> {
     System.out.println("notcompose:"+input);
     return Integer.valueOf(input)+1;
 })).compose(input -> {
     System.out.println("compose:"+input);
     return String.valueOf(Integer.valueOf(input)*3);
 }), "22"));
 // 上述结果为 67

compose是先执行的部分，上述例子中，是根据输入参数进行进一步的加工，再作为输入参数传递给具体调用者。

引用

前边提到了方法引用和构造引用两种，其实构造引用是一种特殊方法引用。具体参照官方文档说明中“Kinds of Method References”部分。

种类 用例
类名::静态方法 String::valueOf
实例对象::实例方法 doctor1::getInterest
类名::实例方法 String::toUpperCase
类名::new (构造引用) String::new
静态引用
  • 使用类
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public static String doStaticReference(Function<Integer,String> function, Integer input) {
    return function.apply(input);
}
  • 示例
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doStaticReference(String::valueOf,123456);
实例对象引用实例方法
  • 使用类
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class Doctor{
   String name;
   String interest;
   public Doctor(String name, String interest) {
       this.name = name;
       this.interest = interest;
   }
   public String getStringInstance(){
       return new String(name);
   }
}
  • 示例
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Doctor doctor1=new Doctor("baigt007","java");
Supplier<String> instance = doctor1::getInterest;
类引用实例方法
  • 使用类
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public static String doMethodReference(Function<String,String> function, String input) {
    return function.apply(input);
}
  • 示例
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doMethodReference(String::toUpperCase,"baigt");O
构造引用
  • 示例
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Supplier<String> stringInstance = String::new;

原文地址：https://my.oschina.net/lt0314/blog/3144851