JAVA8特点

Lambda表达式

允许把函数作为一个方法的参数（函数作为参数传递到方法中）

表达式实例

有参数有返回值

    interface LambdaDemo {
        int operation(int a, int b);
    }

    @Test
    void test() {
        LambdaDemo demo1 = (a, b) -> a - b;
        int operation1 = demo1.operation(2, 1);
        System.out.println(operation1);

        LambdaDemo demo2 = (a, b) -> a + b;
        int operation2 = demo2.operation(2, 1);
        System.out.println(operation2);
    }

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有参数无返回值

    interface LambdaDemo {
        void operation(int a, int b);
    }

    @Test
    void test() {
        LambdaDemo demo = (a, b) -> System.out.println(a + b);
        demo.operation(2, 1);
        
    }

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方法引用

可以直接引用已有Java类或对象（实例）的方法或构造器。
方法参考描述使用“::”符号
@FunctionalInterface,主要用于检查不符合函数式接口定义

@FunctionalInterface
public interface Supplier<T> {
    T get();
}
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public class Consumer {

    public static Consumer create(final Supplier<Consumer> supplier) {
        Consumer consumer = supplier.get();
        System.out.println("createConsumer " + consumer);
        return consumer;
    }

    public static void buyFood(final Consumer con) {
        System.out.println("buyFood " + con.toString());
    }

    public void buyDrink() {
        System.out.println("buyDrink " + this.toString());
    }
}

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引用

    @Test
    void test() {
        // 构造器引用：它的语法是Class::new，或者更一般的Class< T >::new实例如下：
        Consumer consumer = Consumer.create(Consumer::new);
        List<Consumer> list = Arrays.asList(consumer);

        //静态方法引用：它的语法是Class::static_method，实例如下：
        list.forEach(Consumer::buyFood);

        //特定类的任意对象的方法引用：它的语法是Class::method实例如下：
        list.forEach(Consumer::buyDrink);
    }
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函数式接口

函数式接口就是一个有且仅有一个抽象方法，但是可以有多个非抽象方法的接口。
新增加的函数接口：java.util.function

Predicate示例

    @Test
    void test() {
        demo1(10, n -> n % 2 == 0);

        Predicate<Integer> predicate = n -> n > 10;
        boolean demo2 = predicate.test(2);
        System.out.println(demo2);
    }

    public static void demo1(Integer n, Predicate<Integer> predicate) {
        if (predicate.test(n)) {
            System.out.println(n + " ");
        }
    }
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默认方法

默认方法就是接口可以有实现方法，而且不需要实现类去实现其方法。
用 default 关键字
例如Predicate中的默认方法，and、negate、or

 @FunctionalInterface
public interface Predicate<T> {

    boolean test(T t);

    default Predicate<T> and(Predicate<? super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) && other.test(t);
    }

    default Predicate<T> negate() {
        return (t) -> !test(t);
    }

    default Predicate<T> or(Predicate<? super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) || other.test(t);
    }

    static <T> Predicate<T> isEqual(Object targetRef) {
        return (null == targetRef)
                ? Objects::isNull
                : object -> targetRef.equals(object);
    }
}

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Stream

声明的方式处理数据。

forEach: 遍历

        List<Integer> list = Arrays.asList(7, 2, 3, 7, 7, 4, 4, 3, 5);
        list.forEach(System.out::println);
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map: 映射每个元素对应的结果

List<Integer> list = Arrays.asList(7, 2, 3, 7, 7, 4, 4, 3, 5);
List<Integer> newList = list.stream().map(i -> i + i).collect(Collectors.toList());
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filter: 过滤，只要不等于7的

List<Integer> list = Arrays.asList(7, 2, 3, 7, 7, 4, 4, 3, 5);
List<Integer> newList = list.stream().filter(item -> item != 7).collect(Collectors.toList());
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limit： 限制大小

        List<Integer> list = Arrays.asList(7, 2, 3, 7, 7, 4, 4, 3, 5);
        List<Integer> newList = list.stream().limit(2).collect(Collectors.toList());
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sorted： 排序

        List<Integer> list = Arrays.asList(7, 2, 3, 7, 7, 4, 4, 3, 5);
        List<Integer> newList = list.stream().sorted().collect(Collectors.toList());
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统计

        List<Integer> list = Arrays.asList(7, 2, 3, 7, 7, 4, 4, 3, 5);
        IntSummaryStatistics statistics = list.stream().mapToInt(item -> item).summaryStatistics();
        System.out.println(statistics.getMax());
        System.out.println(statistics.getMin());
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Optional 类

用来解决空指针异常。

Nashorn, JavaScript 引擎

嵌入式 JavaScript 引擎

使用 ScriptEngineManager, JavaScript 代码可以在 Java 中执行

    @Test
    void test() {
        ScriptEngineManager scriptEngineManager = new ScriptEngineManager();
        ScriptEngine nashorn = scriptEngineManager.getEngineByName("nashorn");

        String name = "hello word!";

        try {
            nashorn.eval("print('" + name + "')");
        } catch (ScriptException e) {
            System.out.println("执行脚本错误: " + e.getMessage());
        }
    }
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日期时间: java.time

java.time包涵盖了所有处理日期，时间，日期/时间，时区，时刻（instants），过程（during）与时钟（clock）的操作

本地时间

//获得本地年月日时分秒
LocalDateTime currentTime = LocalDateTime.now();
//获得时间的年月日
LocalDate date = currentTime.toLocalDate();
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时区

    @Test
    void test() {
        //根据巴黎时区获取当前时间
        ZoneId parisZone = ZoneId.of("Europe/Paris");
        System.out.println("ZoneId: " + parisZone);
        System.out.println(LocalDateTime.now(parisZone));
        
        //根据本地时区获取当前时间
        ZoneId currentZone = ZoneId.systemDefault();
        System.out.println("CurrentZone: " + currentZone);
        System.out.println(LocalDateTime.now(currentZone));

    }
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时间枚举ChronoUnit

        //Get the current date
        LocalDate today = LocalDate.now();
        System.out.println("Current date: " + today);
        //add 1 week to the current date
        LocalDate nextWeek = today.plus(1, ChronoUnit.WEEKS);
        System.out.println("Next week: " + nextWeek);
        //add 1 month to the current date
        LocalDate nextMonth = today.plus(1, ChronoUnit.MONTHS);
        System.out.println("Next month: " + nextMonth);
        //add 1 year to the current date
        LocalDate nextYear = today.plus(1, ChronoUnit.YEARS);
        System.out.println("Next year: " + nextYear);
        //add 10 years to the current date
        LocalDate nextDecade = today.plus(1, ChronoUnit.DECADES);
        System.out.println("Date after ten year: " + nextDecade);
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日期数学计算 TemporalAdjuster

        LocalDate localDate = LocalDate.now();
        System.out.println("Current date: " + localDate);

        //获得下个星期二的日期
        LocalDate nextTuesday = localDate.with(TemporalAdjusters.next(DayOfWeek.TUESDAY));
        System.out.println("Next Tuesday on : " + nextTuesday);
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Base64

    @Test
    void test() {
        try {

            // 使用基本编码
            String base64encodedString = Base64.getEncoder().encodeToString("hello word".getBytes("utf-8"));
            System.out.println("Base64 编码字符串 (基本) :" + base64encodedString);
            // 解码
            byte[] base64decodedBytes = Base64.getDecoder().decode(base64encodedString);
            System.out.println("原始字符串: " + new String(base64decodedBytes, "utf-8"));

            base64encodedString = Base64.getUrlEncoder().encodeToString("hello word".getBytes("utf-8"));
            System.out.println("Base64 编码字符串 (URL) :" + base64encodedString);
            System.out.println("原始字符串: " + new String(Base64.getUrlDecoder().decode(base64encodedString), "utf-8"));

            StringBuilder stringBuilder = new StringBuilder();

            for (int i = 0; i < 10; ++i) {
                stringBuilder.append(UUID.randomUUID().toString());
            }

            byte[] mimeBytes = stringBuilder.toString().getBytes("utf-8");
            String mimeEncodedString = Base64.getMimeEncoder().encodeToString(mimeBytes);
            System.out.println("原始字符串: " + stringBuilder);
            System.out.println("Base64 编码字符串 (MIME) :" + mimeEncodedString);
            System.out.println("原始字符串: " + new String(Base64.getMimeDecoder().decode(mimeEncodedString), "utf-8"));
        } catch (UnsupportedEncodingException e) {
            System.out.println("Error :" + e.getMessage());
        }

    }
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