默认方法既保证了新特性的添加，又保证了老版本的兼容
//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方法详解比较重要的是行为，//action行为，而不是数据
/** * 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 类型详解名字的由来：消费，只消费，没有返回值
/** * 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="http://www.likecs.com/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。