1.基础语法

1.1 "->" 操作符（lambda操作符）

"->" 操作符将整个lambda表达式拆分成两部分

左侧：表达式的参数列表（接口中方法的参数列表）
右侧：接口中方法的实现

ps：如果接口中有多个方法，就不好使了

1.2 demo

1.2.1 无返回值，无参数

()->xxxx;

        Runnable r1 = new Runnable(){
            @Override
            public void run() {
                System.out.println("r1");
            }
        };

        Runnable r2 = ()-> System.out.println("r2");

1.2.2 无返回值，一个参数

(x) -> System.out.println(x);

//x是形参
        Consumer<String> consumer = (x) -> System.out.println(x);
        consumer.accept("xxxxxxxxxxx");

还能简写成：

        Consumer<String> consumer = System.out::println;
        consumer.accept("xxxxxxxxxxx");

1.2.3 有返回值，多个参数，lambda体中有多条语句

(x,y)->{
System.out.println("xxx");
return Integer.compare(x,y);
};

        Comparator<Integer> com = (x,y)->{
            System.out.println("xxx");
            return Integer.compare(x,y);
        };
        int i = com.compare(2, 1);
        System.out.println(i);

如果只有一条语句，return和{}都可以省略不写

        Comparator<Integer> com = (x,y) -> Integer.compare(x,y);
        int i = com.compare(2, 1);
        System.out.println(i);

2. lambda表达式需要函数式接口的支持

只有一个抽象方法的接口就叫函数式接口

可以使用@FunctionalInterface修饰接口，此时写多个抽象方法会报错

3. java8 内置四大核心函数式接口

3.1 Consumer

@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); };
    }
}

3.2 Supplier

@FunctionalInterface
public interface Supplier<T> {

    /**
     * Gets a result.
     *
     * @return a result
     */
    T get();
}

3.3 Function

@FunctionalInterface
public interface Function<T, R> {

    /**
     * Applies this function to the given argument.
     *
     * @param t the function argument
     * @return the function result
     */
    R apply(T t);

3.4 Predicate

@FunctionalInterface
public interface Predicate<T> {

    /**
     * Evaluates this predicate on the given argument.
     *
     * @param t the input argument
     * @return {@code true} if the input argument matches the predicate,
     * otherwise {@code false}
     */
    boolean test(T t);

3.5 尝试

public class Main {

    public static void main(String[] args) {
        happy(1.0,(m)->{
            System.out.println(m+1);
        });
    }

    private static void happy(double money,Consumer<Double> consumer){
        consumer.accept(money);
    }
}

4. 方法引用

若lambda体中的内容有方法已经实现了，就可以使用方法引用(意思是lambda体中就调用了一个方法，没有别的语句)
这个方法的参数和返回值需要和接口中的方法的参数和返回值一致。

三种语法格式：

• 对象::实例方法名
• 类::静态方法名
• 类::实例方法名

        Consumer<String> con = (x)-> System.out.println(x);
        
        //等价于
        Consumer<String> con2 = System.out::println;

        Comparator<Integer> comparator = (x,y)->Integer.compare(x,y);

        //等价于
        Comparator<Integer> comparator2 = Integer::compare;

        BiPredicate<String,String> biPredicate = (x,y)->x.equals(y);
        
        //等价于
        //调用时传进来的两个参数，需要满足第一个参数x是String类型，第二个参数是equals的参数类型
        BiPredicate<String,String> biPredicate2 = String::equals;

5. 构造器引用

格式：
classname::new

        Supplier<Person> p = ()->new Person();
        
        //等价于
        Supplier<Person> p2 = ()->Person::new;