JAVA8学习——深入浅出Lambda表达式（学习过程）

lambda表达式：

我们为什么要用lambda表达式

• 在JAVA中，我们无法将函数作为参数传递给一个方法，也无法声明返回一个函数的方法。
• 在JavaScript中，函数参数是一个函数，返回值是另一个函数的情况下非常常见的，JavaScript是一门非常典型的函数式编程语言，面向对象的语言

//如，JS中的函数作为参数
a.execute(callback(event){
event...
})

Java匿名内部类实例

后面补充一个匿名内部类的代码实例

我这里Gradle的使用来构建项目

需要自行补充对Gradle的学习

Gradle完全可以使用Maven的所有能力
Maven基于XML的配置文件，Gradle是基于编程式配置.Gradle文件

自定义匿名内部类

public class SwingTest {
public static void main(String[] args) {
JFrame jFrame = new JFrame("my Frame");
JButton jButton = new JButton("My Button");
jButton.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent actionEvent) {
System.out.println("Button Pressed");
}
});
jFrame.add(jButton);
jFrame.pack();
jFrame.setVisible(true);
jFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
}

改造前：

jButton.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent actionEvent) {
System.out.println("Button Pressed");
}
});

改造后：

jButton.addActionListener(actionEvent -> System.out.println("Button Pressed"));

Lambda表达式的基本结构

（pram1,pram2,pram3）->{

}

函数式接口

概念后期补(接口文档源码，注解源码)
抽象方法，抽象接口
1个接口里面只有一个抽象方法，可以有几个具体的方法

/**
* 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 {}

关于函数式接口：
1.如何一个接口只有一个抽象方法，那么这个接口就是函数式接口
2.如果我们在某个接口上生命了FunctionalInterface注解，那么编译器就会按照函数式接口的定义来要求该注解
3.如果某个接口只有一个抽象方法，但我们没有给该接口生命FunctionalInterface接口，编译器也还会把该接口当做成一个函数是接口。（英文最后一段）

通过对实例对函数式接口深入理解

对
@FunctionalInterface
public interface MyInterface {
void test();
}

错
@FunctionalInterface
public interface MyInterface {
void test();

String tostring1();
}

对 (tostring为重写Object类的方法)
@FunctionalInterface
public interface MyInterface {
void test();

String toString();
}

升级扩展,使用lambda表达式

@FunctionalInterface
interface MyInterface {
void test();

String toString();
}

public class Test2{
public void myTest(MyInterface myInterface){
System.out.println("1");
myInterface.test();
System.out.println("2");
}

public static void main(String[] args) {
Test2 test2 = new Test2();
//1.默认调用接口里面的接口函数。默认调用MyTest接口里面的test方法。
//2.如果没有参数传入方法，那么可以直接使用（）来表达，如下所示
test2.myTest(()-> System.out.println("mytest"));

MyInterface myInterface = () -> {
System.out.println("hello");
};

System.out.println(myInterface.getClass()); //查看这个类
System.out.println(myInterface.getClass().getSuperclass());//查看类的父类
System.out.println(myInterface.getClass().getInterfaces()[0]);// 查看此类实现的接口
}
}

默认方法：接口里面，从1.8开始，也可以拥有方法实现了。

//如，Iterable 中的 forEach方法
public interface Iterable<T> {
default void forEach(Consumer<? super T> action) {
Objects.requireNonNull(action);
for (T t : this) {
action.accept(t);
}
}
}

ForEach方法详解

/**
* Performs the given action for each element of the {@code Iterable}
* until all elements have been processed or the action throws an
* exception.  Unless otherwise specified by the implementing class,
* actions are performed in the order of iteration (if an iteration order
* is specified).  Exceptions thrown by the action are relayed to the
* caller.
*
* @implSpec
* <p>The default implementation behaves as if:
* <pre>{@code
*     for (T t : this)
*         action.accept(t);
* }</pre>
*
* @param action The action to be performed for each element
* @throws NullPointerException if the specified action is null
* @since 1.8
*/
default void forEach(Consumer<? super T> action) {
Objects.requireNonNull(action);
for (T t : this) {
action.accept(t);
}
}

Consumer 类型详解

/**
* 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.//接口本身是带有副作用的，会对传入的唯一参数进行修改
*
* <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); };
}
}

Lambda表达式的作用

• Lambda表达式为JAVA添加了缺失的函数式编程特性，使我们能够将函数当做一等公民看待
• 在将函数作为一等公民的语言中，Lambda表达式的类型是函数，但是在JAVA语言中，lambda表达式是一个对象，他们必须依附于一类特别的对象类型——函数是接口（function interface）

迭代方式（三种）

外部迭代：（之前使用的迭代集合的方式，fori这种的）

List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8);
for (int i = 0; i < list.size(); i++) {
System.out.println(list.get(i));
}

内部迭代： ForEach（完全通过集合的本身，通过函数式接口拿出来使用Customer的Accept来完成内部迭代）

List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8);
list.forEach(i -> System.out.println(i));

第三种方式：方法引用（method reference）

List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8);
list.forEach(System.out::println);

2019年12月29日00:07:05 要睡觉了。笔记后面持续更新，代码会上传到GitHub，欢迎一起学习讨论。