java8 lambda表达式的基本语法

java8 lambda表达式: 可以很清晰的定义或展示一个匿名的函数

一、lambda的语法

  parameter list 部分             lambd body部分

   (o1, o2)       ->   o1.getColor().compareTo(o2.getColor());

lambda语法：
　　? （parameters） -> expression

　　? （parameters） -> { statments; }
例子：
　　() -> {};  
　　() -> { return "Hello"; }
　　() -> { return "Hello World"; }
　　(Integer i) -> return "Alex" + i invalid;
　　(String s) -> { return "Hello Alex"; }

public class lambdaExpression {
    public static void main(String[] args) {
        Comparator byColor = new Comparator() {
            @Override
            public int compare(Apple o1, Apple o2) {
                return o1.getColor().compareTo(o2.getColor());
            }
        };
        //以前比较的写法
        List list = Collections.emptyList();
        list.sort(byColor);

        //Lambda表达式： 一个以上的参数，必须有括号
        Comparator byColor2 = ((o1, o2) -> {
            return o1.getColor().compareTo(o2.getColor());
        });

　　　　 或

　　　　Comparator byColo3 = (o1, o2) -> o1.getColor().compareTo(o2.getColor());

    }
}

　　其他语法

Function ： 给一个值，传出来另一个值

public interface Function{
     /**
      * 将此函数应用于给定参数
      * @param t
      * @return
      */
    R apply(T t);
}

Function f = (a) -> a.getColor().equals("green");
Function flambda = s -> s.length();

　　示例：Function

public class lambdaExpression {

    private static String testFunction(Apple apple, Function fun){
       //方法中只有一个参数
       return fun.apply(apple);
    }

    public static void main(String[] args) {

        Apple apple = new Apple("green", 120);
        // Function类中，apply方法只有一个参数，括号可加可不加
        String s = testFunction(apple, a -> a.toString());
        System.out.println(s);
    }
}

　　示例： BiFunction  

   类中的方法 ：  R apply(T t, U u);  // 根据输入的参数T和U , 得到输出参数 R

public class lambdaExpression {

    private static Apple testBiFunction(String c, double w, BiFunction fun){
       return fun.apply(c, w);
    }

    public static void main(String[] args) {
　　　　　//BiFunction类中， apply方法有两个参数，lambda表达式之针对这些特定的函数
        Apple apple = testBiFunction("yellow", 123, (c, w) -> new Apple(c, w));
        System.out.println(apple);
    }
}

Predicate :

public interface Predicate{
   boolean test(T t);
}

示例：

Predicate p = (Apple a) -> a.getColor().equals("green");

public class lambdaExpression {

    private static List filter(List source, Predicate predicate){
        List result = new ArrayList<>();
        for (Apple apple : source) {
            if (predicate.test(apple)){
                result.add(apple);
            }
        }
        return result;
    }

    public static void main(String[] args) {
        List list = Arrays.asList(new Apple("green", 120), new Apple("red", 150));
　　　　 //Predicate类中， 方法只有一个参数，下面的lambda表达式中 “（）”可加可不加
        List appleList = filter(list, (apple) -> apple.getColor().equals("green"));
        System.out.println(appleList);
    }
}

BiPredicate: test方法有2个参数：

public class lambdaExpression {

    private static List filterByBiPredicate(List source, BiPredicate predicate){
        List result = new ArrayList<>();
        for (Apple apple : source) {
            if (predicate.test(apple.getColor(), apple.getWeight())){
                result.add(apple);
            }
        }
        return result;
    }

    public static void main(String[] args) {
        List list = Arrays.asList(new Apple("green", 120), new Apple("red", 150));
　　　　　//Predicate类中，方法有2个参数一定要用小括号包裹
        List appleList = filterByBiPredicate(list, (s, w) -> s.equals("green") && w > 100);
        System.out.println(appleList);
    }
}

Supplier: 方法： T get(); 

Supplier supplier = Apple::new;

 　示例： 

public static void main(String[] args) {
    //方法推导
    Supplier s = String::new;
    System.out.println(s.get().getClass());
}

Consumer : 

/**
 * Performs this operation on the given argument.
 * params : t -- the input argument
 */
void accept(T t);

 　　示例：Consumer中有一个参数

public class lambdaExpression {
　　 //Consumer类中，方法是一个参数的情况
    private static void simpleTestConsumer(List source, Consumer consumer){
        List result = new ArrayList<>();
        for (Apple apple : source) {
            consumer.accept(apple);
        }
    }

    public static void main(String[] args) {
        List list = Arrays.asList(new Apple("green", 120), new Apple("red", 150));
　　　　 //Consumner中的accept方法有一个参数
        simpleTestConsumer(list, a -> System.out.println(a));
    }
}

　　示例： Consumer中是两个参数的情况：

public class lambdaExpression {
　　//BiConsumer类中， 方法有两个参数的情况
    private static void simpleTestBiConsumer(String c, List source, BiConsumer consumer){
        List result = new ArrayList<>();
        for (Apple apple : source) {
            consumer.accept(apple, c);
        }
    }

    public static void main(String[] args) {
        List list = Arrays.asList(new Apple("green", 120), new Apple("red", 150));
　　　　 //lambda表达式，根据方法中的参数个数来
        simpleTestBiConsumer("XXX-", list, (a, s) -> System.out.println(s+ "color:" + a.getColor()+", weight:"+a.getWeight()));
    }
}

二、如何使用lambda

public class lambdaExpression {
    public static void main(String[] args) {
        Runnable r1 = () -> System.out.println("Hello");

        Runnable r2 = new Runnable() {
            @Override
            public void run() {
                System.out.println("HEllo");
            }
        };

　　　　 //这三种调用情况是一样的
        process(r1);
        process(r2);
        process(() -> System.out.println("Hello"));
    }

    private static void process(Runnable r){
        r.run();
    }
}

三、execute around pattern

四、函数式接口(Functional interfaces)

 函数式接口：接口上面都有一个注解@FunctionalInterface

Function ： 给一个值，传出来另一个值

public interface Function{
     /**
      * 将此函数应用于给定参数
      * @param t
      * @return
      */
    R apply(T t);
}

Function f = (a) -> a.getColor().equals("green");
Function flambda = s -> s.length();

　　示例：Function

public class lambdaExpression {

    private static String testFunction(Apple apple, Function fun){
       //方法中只有一个参数
       return fun.apply(apple);
    }

    public static void main(String[] args) {

        Apple apple = new Apple("green", 120);
        // Function类中，apply方法只有一个参数，括号可加可不加
        String s = testFunction(apple, a -> a.toString());
        System.out.println(s);
    }
}

　　示例： BiFunction  

   类中的方法 ：  R apply(T t, U u);  // 根据输入的参数T和U , 得到输出参数 R

public class lambdaExpression {

    private static Apple testBiFunction(String c, double w, BiFunction fun){
       return fun.apply(c, w);
    }

    public static void main(String[] args) {
　　　　　//BiFunction类中， apply方法有两个参数，lambda表达式之针对这些特定的函数
        Apple apple = testBiFunction("yellow", 123, (c, w) -> new Apple(c, w));
        System.out.println(apple);
    }
}

Predicate :

public interface Predicate{
   boolean test(T t);
}

示例：

Predicate p = (Apple a) -> a.getColor().equals("green");

public class lambdaExpression {

    private static List filter(List source, Predicate predicate){
        List result = new ArrayList<>();
        for (Apple apple : source) {
            if (predicate.test(apple)){
                result.add(apple);
            }
        }
        return result;
    }

    public static void main(String[] args) {
        List list = Arrays.asList(new Apple("green", 120), new Apple("red", 150));
　　　　 //Predicate类中， 方法只有一个参数，下面的lambda表达式中 “（）”可加可不加
        List appleList = filter(list, (apple) -> apple.getColor().equals("green"));
        System.out.println(appleList);
    }
}

BiPredicate: test方法有2个参数：

public class lambdaExpression {

    private static List filterByBiPredicate(List source, BiPredicate predicate){
        List result = new ArrayList<>();
        for (Apple apple : source) {
            if (predicate.test(apple.getColor(), apple.getWeight())){
                result.add(apple);
            }
        }
        return result;
    }

    public static void main(String[] args) {
        List list = Arrays.asList(new Apple("green", 120), new Apple("red", 150));
　　　　　//Predicate类中，方法有2个参数一定要用小括号包裹
        List appleList = filterByBiPredicate(list, (s, w) -> s.equals("green") && w > 100);
        System.out.println(appleList);
    }
}

Supplier: 方法： T get(); 

Supplier supplier = Apple::new;

 　示例： 

public static void main(String[] args) {
    //方法推导
    Supplier s = String::new;
    System.out.println(s.get().getClass());
}

Consumer : 

/**
 * Performs this operation on the given argument.
 * params : t -- the input argument
 */
void accept(T t);

 　　示例：Consumer中有一个参数

public class lambdaExpression {
　　 //Consumer类中，方法是一个参数的情况
    private static void simpleTestConsumer(List source, Consumer consumer){
        List result = new ArrayList<>();
        for (Apple apple : source) {
            consumer.accept(apple);
        }
    }

    public static void main(String[] args) {
        List list = Arrays.asList(new Apple("green", 120), new Apple("red", 150));
　　　　 //Consumner中的accept方法有一个参数
        simpleTestConsumer(list, a -> System.out.println(a));
    }
}

　　示例： Consumer中是两个参数的情况：

public class lambdaExpression {
　　//BiConsumer类中， 方法有两个参数的情况
    private static void simpleTestBiConsumer(String c, List source, BiConsumer consumer){
        List result = new ArrayList<>();
        for (Apple apple : source) {
            consumer.accept(apple, c);
        }
    }

    public static void main(String[] args) {
        List list = Arrays.asList(new Apple("green", 120), new Apple("red", 150));
　　　　 //lambda表达式，根据方法中的参数个数来
        simpleTestBiConsumer("XXX-", list, (a, s) -> System.out.println(s+ "color:" + a.getColor()+", weight:"+a.getWeight()));
    }
}

五、方法推导(Method references)

 方法推导的适用条件：

类的静态方法：

Function  --->  R apply(T t);

Function fun = Integer::parseInt;
Integer result = fun.apply("123");
System.out.println(result);

public class MethodReference {


    private static  void userConsumer(Consumer consumer, T t){
        consumer.accept(t);
        consumer.accept(t);
    }

    public static void main(String[] args) {
        //以前lambda的方式
        Consumer consumer = (s) -> System.out.println(s);
        userConsumer(consumer, "Hello Alex");

        userConsumer(s -> System.out.println(s), "Hello1234");
        //方法推导
        userConsumer(System.out::println, "Hello wangwu099");
        System.out.println("@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@");
        //
        List list = Arrays.asList(new Apple("green", 110), new Apple("abc", 150), new Apple("red", 170));
        System.out.println(list);

        list.sort((a1, a2) ->{ return a1.getColor().compareTo(a2.getColor()); });
        System.out.println(list);

　　　　 //方法推导
　　　　 list.stream().foreach(System.out::println);

    }
}

六、类型推导(Type inference)

 

七、组合(Composing lambdas)

实体类

public class Apple {

    private String color;
    private double weight;

    public Apple(String color, double weight) {
        this.color = color;
        this.weight = weight;
    }

    public double getWeight() {
        return weight;
    }

    public void setHeigh(double weight) {
        this.weight = weight;
    }

    public String getColor() {
        return color;
    }

    public void setColor(String color) {
        this.color = color;
    }

    @Override
    public String toString() {
        return "Apple{" +
                "color='" + color + '\'' +
                ", weight=" + weight +
                '}';
    }
}

　　利用接口

public class AppleFilter {

    public interface FilterApple{
        boolean filter(Apple apple);
    }

    public static List findApple(List apples, FilterApple filterApple){
        List list = new ArrayList<>();
        for (Apple apple : apples) {
            if (filterApple.filter(apple)){
                list.add(apple);
            }
        }
        return list;
    }

    public static class GreenAnd150WeightFilter implements FilterApple{
        @Override
        public boolean filter(Apple apple) {
            return (apple.getColor().equals("green")) && (apple.getWeight() >= 160);
        }
    }

    public static List findGreenApple(List apples){
        List list = new ArrayList<>();
        for (Apple apple : apples) {
            if ("green".equals(apple.getColor())){
                list.add(apple);
            }
        }
        return list;
    }

    public static List findApple(List apples, String color){
        List list = new ArrayList<>();
        for (Apple apple : apples) {
            if (color.equals(apple.getColor())){
                list.add(apple);
            }
        }
        return list;
    }

    public static void main(String[] args) {
        List list = Arrays.asList(new Apple("green", 150), new Apple("yellow", 120), new Apple("green", 170));
//        List appleList = findGreenApple(list);
//        assert appleList.size() == 2;

//        List greenApples = findApple(list, "green");
//        System.out.println(greenApples);

        List apple = findApple(list, new GreenAnd150WeightFilter());
        System.out.println(apple);

        List yellowApples = findApple(list, new FilterApple() {
            @Override
            public boolean filter(Apple apple) {
                return "yellow".equals(apple.getColor());
            }
        });
        System.out.println(yellowApples);

    }
}

　　匿名内部类； 结果为： 5

public class MeaningOfThis {

    public final int value = 4;

    public void doIt() {
        int value = 6;
        Runnable r = new Runnable() {

            public final int value = 5;

            @Override
            public void run() {
                int value = 10;
                System.out.println(this.value);
            }
        };
        r.run();
    }

    public static void main(String[] args) {
        MeaningOfThis m = new MeaningOfThis();
        m.doIt();
    }
}

　　lamda表达式

public class AppleFilter {

    public interface FilterApple{
        boolean filter(Apple apple);
    }

    public static List findApple(List apples, FilterApple filterApple){
        List list = new ArrayList<>();
        for (Apple apple : apples) {
            if (filterApple.filter(apple)){
                list.add(apple);
            }
        }
        return list;
    }

    public static void main(String[] args) {
        List list = Arrays.asList(new Apple("green", 150), new Apple("yellow", 120), new Apple("green", 170));

        //lambda表达式
        List appleList = findApple(list, (Apple apple) -> {
            return apple.getColor().equals("green");
        });

        System.out.println(appleList);

    }

}