通过“->”这种特殊符号形式，连接前后。

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

{
System.out.println("xxx");
}

• 示例2

{
// 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

interface Print<String>{
void printName(String string);
}

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。

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);
}
}

• 定义 一个使用类

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

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

interface GetSomething<String>{
String getThing();
}

• 定义一个使用者

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。

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

/**
* 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

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

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。

Consumer consumer = input -> System.out.println(input);
doConsumer(consumer.andThen(input2->{
System.out.println("input2");
}),"something input");

Supplier

/**
* 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

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

System.out.println(doSupplier(() -> "baigt"));
System.out.println(doSupplier(() -> {return Integer.valueOf("10");}));

Predicate

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

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。

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

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

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。

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。

静态引用

• 使用类

public static String doStaticReference(Function<Integer,String> function, Integer input) {
return function.apply(input);
}

• 示例

doStaticReference(String::valueOf,123456);

实例对象引用实例方法

• 使用类

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

Doctor doctor1=new Doctor("baigt007","java");
Supplier<String> instance = doctor1::getInterest;

类引用实例方法

• 使用类

public static String doMethodReference(Function<String,String> function, String input) {
return function.apply(input);
}

• 示例

doMethodReference(String::toUpperCase,"baigt");O

构造引用

• 示例

Supplier<String> stringInstance = String::new;

结语

上述是个人心得，不对之处，欢迎指正。

作者：baigt 交流群：244930845