JDK 8 new features

  1. Lambda Expressions

    1. A lambda expression is an anonymous function. A function that doesn’t have a name and doesn’t belong to any class;
    2. Syntax: (parameter_list) -> {function_body} 如果只有1个参数,可以省去括号, 如果函数体只有一句,可以省略大括号;
    3. 一个方法method(函数function)包含4部分: 方法名, 参数列表, 方法体, 返回值类型. 一个 lambda 表达式: 1)没有名字; 2) 有参数列表; 3) 有函数体; 4) 没有返回值类型, 但是函数可以返回值, JVM 动态侦测类型;
    4. Java 8 之前, 很多地方我们使用匿名内部类, 8及之后, 都可以使用 lambda 表达式来替换;
    5. 通常使用 lambda 表达式的地方 -> 函数式接口(只有一个抽象的方法), 比如 Callable, Runnable, 各种 Listener;

  2. Method References 方法引用
    它通常是 lambda 表达式的一种缩写形式 str -> System.out.println(str) -> System.out::println
    4种类型的方法引用:

    1. Method reference to an instance method of an object – object::instanceMethod
    2. Method reference to a static method of a class – Class::staticMethod
    3. Method reference to an instance method of an arbitrary object of a particular type – Class::instanceMethod
    4. Method reference to a constructor – Class::new

  3. Functional Interfaces 函数式接口

    1. An interface with only single abstract method is called functional interface(Single Abstract Method interfaces);
    2. JDK 预先定义的函数式接口 链接;
    3. 可选的 Annotation: @FunctionalInterface 编译时能侦测是不是符合函数式接口定义;
    4. 定义的时候, 除了这个唯一的抽象的方法, 可以有任何多个非抽象的实例方法或静态方法;
    5. Java 8 之前通常使用匿名内部类来实现这种接口, Java 8 及以后可以使用 lambda 表达式;

  4. 接口增加 default method and static method

    1. Java 8 之前接口只能有抽象方法, 所有的方法默认都是 public & abstract;
    2. Java 8 使接口可以有 default 方法和 static 方法;
    3. default method and static method 可以是已存在的接口添加新的功能而不影响原有逻辑;
    4. static method 类似 default method, 只是实现者不能 override 这些方法;

    5. default method 值方法签名前加 default 关键字;

      default void myDefaultMethod(){ ... }
    6. 抽象类可以有构造函数, interface 不行. 接口侧重定义规范, 抽象类侧重实现整体, 细节留空;
    7. 一个类实现多个接口中如果有相同的 default 方法, 编译会报错, 需要在实现类中解决冲突;

  5. Stream API (java.util.stream)

    1. using streams we can perform various aggregate operations on the data returned from collections, arrays, Input/Output operations;

    2. Stream 可顺序也可以通过并行执行(Parallel execution)的方式(理论上,实际不一定)加快执行速度;

      1. stream.sequential()
      2. stream.parallel()
    3. 步骤: 1) 一次创建 Stream; 2) 0或多个中间操作; 3) 一次终止操作;
    4. 例子: Arrays.asList("line0", "line1").stream().filter(str->str.length()<6).count()

    5. 常见的操作:
      Intermediate operations:

      1. filter - Exclude all elements that don't match a Predicate.
      2. map - Perform a one-to-one transformation of elements using a Function.
      3. flatMap - Transform each element into zero or more elements by way of another Stream.
      4. peek - Perform some action on each element as it is encountered. Primarily useful for debugging.
      5. distinct - Exclude all duplicate elements according to their .equals behavior. This is a stateful operation.
      6. sorted - Ensure that stream elements in subsequent operations are encountered according to the order imposed by a Comparator. This is a stateful operation.
      7. limit - Ensure that subsequent operations only see up to a maximum number of elements. This is a stateful, short-circuiting operation.
      8. skip - Ensure that subsequent operations do not see the first n elements. This is a stateful operation.

      Terminal operations:

      1. forEach - Perform some action for each element in the stream.
      2. toArray - Dump the elements in the stream to an array.
      3. reduce - Combine the stream elements into one using a BinaryOperator.
      4. collect - Dump the elements in the stream into some container, such as a Collection or Map.
      5. min - Find the minimum element of the stream according to a Comparator.
      6. max - Find the maximum element of the stream according to a Comparator.
      7. count - Find the number of elements in the stream.
      8. anyMatch - Find out whether at least one of the elements in the stream matches a Predicate. This is a short-circuiting operation.
      9. allMatch - Find out whether every element in the stream matches a Predicate. This is a short-circuiting operation.
      10. noneMatch - Find out whether zero elements in the stream match a Predicate. This is a short-circuiting operation.
      11. findFirst - Find the first element in the stream. This is a short-circuiting operation.
      12. findAny - Find any element in the stream, which may be cheaper than findFirst for some streams. This is a short-circuiting operation.
    6. stream 可以 infinite;
    7. stream 处理可以短路(Short-circuiting), 不在执行剩下的, 对于无限 stream 最有用;

  6. Optional

    1. 改变编程习惯, 避免 NullPointerException - 原来直接用, 现在用各种方法;
    2. A container object which may or may not contain a non-null value;
    3. 创建: 1) Optional.empty(), 2) Optional.of(T value), 3) Optional.ofNullable(T value);
    4. 判断: isPresent();
    5. 获得: get(), 若null -> NoSuchElementException;

    6. 获得带 fallback 机制:

      1. 若空返回 other: orElse(T other)
      2. 若空执行函数接口返回值: orElseGet(Supplier<? extends T> other)
      3. 若空抛出特定异常: orElseThrow(Supplier<? extends X> exceptionSupplier) throws X

    7. 其它

      1. 非空并且满足predicate filter(Predicate<? super T> predicate);
      2. map(Function<? super T, ? extends U> mapper)
      3. flatMap(Function<? super T, Optional<U>> mapper)
  7. StringJoiner 类似 guava 的 Joiner, 前缀, 后缀, 分隔符.
  8. Arrays.parallelSort -> 多线程排序加速

  9. java.util.function 一些常见的函数

    1. Function<T, R> - take a T as input, return an R as ouput
    2. Predicate - take a T as input, return a boolean as output
    3. Consumer - take a T as input, perform some action and don't return anything
    4. Supplier - with nothing as input, return a T
    5. BinaryOperator - take two T's as input, return one T as output, useful for "reduce" operations
  10. java.time 包

refer:

  1. https://beginnersbook.com/2017/10/java-8-features-with-examples/
  2. https://www.techempower.com/blog/2013/03/26/everything-about-java-8/
  3. https://www.javatpoint.com/java-8-features
  4. https://www.oracle.com/technetwork/java/javase/8-whats-new-2157071.html

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