// 赋值
var func1 = function func1() {  }
// 函数作为参数
function func2(fn) {
    fn()
}   
// 函数作为返回值
function func3() {
    return function() {}
}

// 是纯函数
function sum(x,y){
    return x + y
}
// 输出不确定，不是纯函数
function random(x){
    return Math.random() * x
}
// 有副作用，不是纯函数
function setFontSize(el,fontsize){
    el.style.fontsize = fontsize ;
}
// 输出不确定、有副作用，不是纯函数
let count = 0;
function addCount(x){
    count+=x;
    return count;
}

@FunctionalInterface
public interface Functional {
    void method();
}

@FunctionalInterface
public interface FunctionalDefaultMethods {
    void method();
    default void defaultMethod() {            
    }        
}

private interface Defaulable {
    // Interfaces now allow default methods, the implementer may or 
    // may not implement (override) them.
    default String notRequired() { 
        return "Default implementation"; 
    }        
}
private static class DefaultableImpl implements Defaulable {
}
private static class OverridableImpl implements Defaulable {
    @Override
    public String notRequired() {
        return "Overridden implementation";
    }
}

private interface DefaulableFactory {
    // Interfaces now allow static methods
    static Defaulable create( Supplier< Defaulable > supplier ) {
        return supplier.get();
    }
}

public static void main( String[] args ) {
    Defaulable defaulable = DefaulableFactory.create( DefaultableImpl::new );
    System.out.println( defaulable.notRequired() );
    defaulable = DefaulableFactory.create( OverridableImpl::new );
    System.out.println( defaulable.notRequired() );
}

Default implementation
Overridden implementation

左侧：Lambda表达式的参数列表 -> 右侧：Lambda表达式中要执行的功能

(parameters) -> expression
或
(parameters) ->{ statements; }

// 1. 不需要参数,返回值为 5  
() -> 5  
// 2. 接收一个参数(数字类型),返回其2倍的值  
x -> 2 * x  
// 3. 接受2个参数(数字),并返回他们的差值  
(x, y) -> x – y  
// 4. 接收2个int型整数,返回他们的和  
(int x, int y) -> x + y  
// 5. 接受一个 string 对象,并在控制台打印,不返回任何值(看起来像是返回void)  
(String s) -> System.out.print(s)

public static void main(String[] args) {
        List<String> list = Arrays.asList("t", "m", "l");
        for (String s : list) {
            System.out.println(s);
        }
    }

Arrays.asList( "t", "m", "l" ).forEach(e -> System.out.println( e ) );

default void forEach(Consumer<? super T> action) {
        Objects.requireNonNull(action);
        for (T t : this) {
            action.accept(t);
        }
    }

package java.util.function;
import java.util.Objects;
/**
 * 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); };
    }
}

Arrays.asList( "t", "m", "l" ).forEach( ( String e ) -> System.out.println( e ) );

public static void main(String[] args) {
        Arrays.asList("t", "m", "l").forEach((String e) ->
                {
                    String a = e + "帅";
                    System.out.println(a);
                }
        );
    }

public class Demo01Inner {
    public static void main(String[] args) {
        //使用匿名内部类的方式实现多线程。
        new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(Thread.currentThread().getName() + "执行了");
            }
        }).start();
        //使用Lambda表达式实现多线程
        new Thread(() -> System.out.println(Thread.currentThread().getName() + "执行了")).start();
    }
}

public Thread(Runnable target) {
        init(null, target, "Thread-" + nextThreadNum(), 0);
    }

package java.lang;
/**
 * The <code>Runnable</code> interface should be implemented by any
 * class whose instances are intended to be executed by a thread. The
 * class must define a method of no arguments called <code>run</code>.
 * <p>
 * This interface is designed to provide a common protocol for objects that
 * wish to execute code while they are active. For example,
 * <code>Runnable</code> is implemented by class <code>Thread</code>.
 * Being active simply means that a thread has been started and has not
 * yet been stopped.
 * <p>
 * In addition, <code>Runnable</code> provides the means for a class to be
 * active while not subclassing <code>Thread</code>. A class that implements
 * <code>Runnable</code> can run without subclassing <code>Thread</code>
 * by instantiating a <code>Thread</code> instance and passing itself in
 * as the target.  In most cases, the <code>Runnable</code> interface should
 * be used if you are only planning to override the <code>run()</code>
 * method and no other <code>Thread</code> methods.
 * This is important because classes should not be subclassed
 * unless the programmer intends on modifying or enhancing the fundamental
 * behavior of the class.
 *
 * @author  Arthur van Hoff
 * @see     java.lang.Thread
 * @see     java.util.concurrent.Callable
 * @since   JDK1.0
 */
@FunctionalInterface
public interface Runnable {
    /**
     * When an object implementing interface <code>Runnable</code> is used
     * to create a thread, starting the thread causes the object's
     * <code>run</code> method to be called in that separately executing
     * thread.
     * <p>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     * @see     java.lang.Thread#run()
     */
    public abstract void run();
}

public class Person {
    // ...
    LocalDate birthday;
    public int getAge() {
        // ...
    }
    public LocalDate getBirthday() {
        return birthday;
    }   
    public static int compareByAge(Person a, Person b) {
        return a.birthday.compareTo(b.birthday);
    }
    // ...
}

Person[] rosterAsArray = new Person[]{
                new Person("003", LocalDate.of(2021,9,1)),
                new Person("001", LocalDate.of(2021,2,1)),
                new Person("002", LocalDate.of(2021,3,1)),
                new Person("004", LocalDate.of(2021,12,1))};
class PersonAgeComparator implements Comparator<Person> {
    public int compare(Person a, Person b) {
        return a.getBirthday().compareTo(b.getBirthday());
    }
}
Arrays.sort(rosterAsArray, new PersonAgeComparator());

static <T> void sort(T[] a, Comparator<? super T> c)

public static <T> void sort(T[] a, Comparator<? super T> c) {
        if (c == null) {
            sort(a);
        } else {
            if (LegacyMergeSort.userRequested)
                legacyMergeSort(a, c);
            else
                TimSort.sort(a, 0, a.length, c, null, 0, 0);
        }
    }

@FunctionalInterface
public interface Comparator<T> {
    int compare(T o1, T o2);
    boolean equals(Object obj);//equals是Object中的方法，这种对Object类的方法的重新声明会让方法不再是抽象的
    ...
    }

Arrays.sort(rosterAsArray,
    (Person a, Person b) -> {
        return a.getBirthday().compareTo(b.getBirthday());
    }
);

Arrays.sort(rosterAsArray,
    (a, b) -> Person.compareByAge(a, b)
);