【Java】Java8的Lambda入门记录

简化定义匿名实现类

匿名实现类的传统方式

创建一个线程，需要实现Runnable接口，并实现public void run()方法，用传统的方式是这样的：

    public static void main(String[] args) {
        new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("hello world");
            }
        }).start();
    }

无参数、单语句方法体

用lambda可以简化成这样：

Runnable r = () -> System.out.print("hello world");
new Thread(r).start();

所以也可以这样：

    public static void main(String[] args) {
        new Thread(() -> System.out.println("hello world")).start();
    }

所以，没参数、单语句的是这样的：

public class NoParameter {
    
    public static void main(String[] args) {
        MyInterface myInterface = () -> System.out.println("Hello World");
        myInterface.methodA();
    }
    
    public interface MyInterface {
        public void methodA();
    }

}

无参数、多语句方法体

当然实现方法中有多条语句的情况下，应该是这样的：

    public static void main(String[] args) {
        new Thread(() -> {
            System.out.println("1");
            System.out.println("2");
        }).start();
    }

多参数、多语句方法体

多参数、多语句的是这样的：

public class MoreParameterMoreStatement {

    public static void main(String[] args) {
        MyInterface myInterface = (x, y) -> {
            System.out.println("1st parameter : " + x);
            System.out.println("2nd parameter : " + y);
        };
        myInterface.methodA(1, 5);
    }

    public interface MyInterface {
        public void methodA(Integer x, Integer y);
    }

}

流，Stream

这里的流，并非Java IO的流，是简化处理Java Collection的流。

过滤条件

    public static void main(String[] args) {
        List<String> list = Arrays.asList(new String[] {"1", "2", "3"});
        
        Stream<String> stream = list.stream();
        Predicate<String> predicate = s -> s != null && s.equals("1"); // 断言，Predicate。入参对象，出参boolean，用于判别一个对象
        System.out.println("count -> " + stream.filter(predicate).count()); // 过滤并统计
    }

简写：

        /**
	 * 过滤
	 */
	@Test
	public void predicateTestx1() {
		List<String> list = Arrays.asList(new String[] {"1", "2", "3"});
		
		List<String> resultList = list.stream().filter(s -> s != null && s.equals("1")).collect(Collectors.toList());
		this.logger.info("resultList -> " + resultList);
	}

Predicate有一个抽象方法：boolean test(T t);，入参是对象，出参是布尔值。
filter方法会调用test方法：

    @Override
    public final Stream<P_OUT> filter(Predicate<? super P_OUT> predicate) {
        Objects.requireNonNull(predicate);
        return new StatelessOp<P_OUT, P_OUT>(this, StreamShape.REFERENCE,
                                     StreamOpFlag.NOT_SIZED) {
            @Override
            Sink<P_OUT> opWrapSink(int flags, Sink<P_OUT> sink) {
                return new Sink.ChainedReference<P_OUT, P_OUT>(sink) {
                    @Override
                    public void begin(long size) {
                        downstream.begin(-1);
                    }

                    @Override
                    public void accept(P_OUT u) {
                        if (predicate.test(u)) // 调用test()的逻辑
                            downstream.accept(u); // 加入下沉集合
                    }
                };
            }
        };
    }

转换为不同类型的集合

    public static void main(String[] args) {
        List<String> list = Arrays.asList(new String[] {"1", "2", "3"});
        
        Stream<String> stream = list.stream();
        Function<String, Integer> function = i -> Integer.valueOf(i); // Function<T, R>，转换成不同的类型
        List<Integer> resultList = stream.map(function).collect(Collectors.toList());
        
        System.out.println(resultList);
    }

简写：

        /**
	 * 转换类型
	 */
	@Test
	public void functionTestx1() {
		List<String> list = Arrays.asList(new String[] {"1", "2", "3"});
		
		List<Integer> resultList = list.stream().map(i -> Integer.valueOf(i)).collect(Collectors.toList());
		this.logger.info("resultList -> " + resultList);
	}

合并多个集合

    public static void main(String[] args) {
        List<String> list1 = Arrays.asList(new String[] {"1", "2", "3"});
        List<String> list2 = Arrays.asList(new String[] {"4", "5", "6"});
        
        Function<List<String>, Stream<String>> function = list -> list.stream(); // List<String>转换为Stream<String>
        List<String> allList = Stream.of(list1, list2).flatMap(function).collect(Collectors.toList()); // 合并多个集合
        System.out.println(allList);
    }

简写：

        /**
	 * 转换类型
	 */
	@Test
	public void mergeTestx1() {
		List<String> list1 = Arrays.asList(new String[] {"1", "2", "3"});
		List<String> list2 = Arrays.asList(new String[] {"4", "5", "6"});
		
		// 这个不是我想要的结果
		/*
		List<List<String>> resultList1 = Stream.of(list1, list2).collect(Collectors.toList());
		this.logger.info("resultList1 -> " + resultList1);
		*/
		
		List<String> resultList2 = Stream.of(list1, list2).flatMap(i -> i.stream()).collect(Collectors.toList());
		this.logger.info("resultList2 -> " + resultList2);
	}

获取集合最大、最小值

    public static void main(String[] args) {
        List<String> list = Arrays.asList(new String[] {"1", "2", "3"});
        
        String max = list.stream().max(String::compareTo).get(); // 使用compareTo
        String min = list.stream().min((x, y) -> x.compareTo(y)).get(); // 手动调用compareTo
        
        System.out.println("max -> " + max);
        System.out.println("min -> " + min);
    }

分解操作，Reduce

    /**
     * 计算1-10总和
     */
    public static void main(String[] args) throws Exception {
        List<Integer> list = Arrays.asList(new Integer[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }); // 声明集合

        BinaryOperator<Integer> binaryOperator = (x, y) -> {
            int temp = x + y;
            logger.info("temp sum -> {}", temp);
            return temp;
        };

        int sum = list.stream().reduce(binaryOperator).get();
        logger.info("sum -> {}", sum);
    }

日志：

2017-09-17 21:53:33.529 [main] INFO  c.n.exercise.stream.ReduceExercise - temp sum -> 3 
2017-09-17 21:53:33.535 [main] INFO  c.n.exercise.stream.ReduceExercise - temp sum -> 6 
2017-09-17 21:53:33.535 [main] INFO  c.n.exercise.stream.ReduceExercise - temp sum -> 10 
2017-09-17 21:53:33.535 [main] INFO  c.n.exercise.stream.ReduceExercise - temp sum -> 15 
2017-09-17 21:53:33.535 [main] INFO  c.n.exercise.stream.ReduceExercise - temp sum -> 21 
2017-09-17 21:53:33.535 [main] INFO  c.n.exercise.stream.ReduceExercise - temp sum -> 28 
2017-09-17 21:53:33.535 [main] INFO  c.n.exercise.stream.ReduceExercise - temp sum -> 36 
2017-09-17 21:53:33.535 [main] INFO  c.n.exercise.stream.ReduceExercise - temp sum -> 45 
2017-09-17 21:53:33.535 [main] INFO  c.n.exercise.stream.ReduceExercise - temp sum -> 55 
2017-09-17 21:53:33.536 [main] INFO  c.n.exercise.stream.ReduceExercise - sum -> 55 

并行操作

这里例子的关键在于parallel()设置了并行处理，具体对比与reduce的日志。

    /**
     * 计算1-10总和
     */
    public static void main(String[] args) throws Exception {
        List<Integer> list = Arrays.asList(new Integer[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }); // 声明集合

        BinaryOperator<Integer> binaryOperator = (x, y) -> {
            int temp = x + y;
            logger.info("temp sum -> {}", temp);
            return temp;
        };

        int sum = list.stream().parallel().reduce(binaryOperator).get();
        logger.info("sum -> {}", sum);
    }

日志：

2017-09-17 21:48:32.018 [ForkJoinPool.commonPool-worker-2] INFO  c.n.exercise.stream.ReduceExercise - temp sum -> 19 
2017-09-17 21:48:32.018 [ForkJoinPool.commonPool-worker-1] INFO  c.n.exercise.stream.ReduceExercise - temp sum -> 9 
2017-09-17 21:48:32.018 [main] INFO  c.n.exercise.stream.ReduceExercise - temp sum -> 13 
2017-09-17 21:48:32.018 [ForkJoinPool.commonPool-worker-3] INFO  c.n.exercise.stream.ReduceExercise - temp sum -> 3 
2017-09-17 21:48:32.023 [ForkJoinPool.commonPool-worker-2] INFO  c.n.exercise.stream.ReduceExercise - temp sum -> 27 
2017-09-17 21:48:32.023 [ForkJoinPool.commonPool-worker-1] INFO  c.n.exercise.stream.ReduceExercise - temp sum -> 12 
2017-09-17 21:48:32.023 [ForkJoinPool.commonPool-worker-2] INFO  c.n.exercise.stream.ReduceExercise - temp sum -> 40 
2017-09-17 21:48:32.023 [ForkJoinPool.commonPool-worker-1] INFO  c.n.exercise.stream.ReduceExercise - temp sum -> 15 
2017-09-17 21:48:32.023 [ForkJoinPool.commonPool-worker-1] INFO  c.n.exercise.stream.ReduceExercise - temp sum -> 55 
2017-09-17 21:48:32.024 [main] INFO  c.n.exercise.stream.ReduceExercise - sum -> 55 

两个集合的操作工具类：减去、交集、并集

import java.util.List;
import java.util.Set;
import java.util.stream.Collectors;
import java.util.stream.Stream;

import org.assertj.core.util.Lists;
import org.junit.Test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class CollectionUtils {
	
	private static Logger logger = LoggerFactory.getLogger(CollectionUtils.class);

	/**
	 * 计算在list1存在，而在list2不存在的记录的集合
	 */
	public static <T> List<T> subtract(List<T> list1, List<T> list2) {
		// 转换为Set
		Set<T> set = list2.stream().collect(Collectors.toSet());
		
		// 计算在list1存在，而在list2不存在的记录的集合
		List<T> resultList = list1.stream().filter(i -> !set.contains(i)).collect(Collectors.toList());
		return resultList;
	}
	
	/**
	 * 计算交集，在list1存在，并且在list2也存在的记录的集合
	 */
	public static <T> List<T> intersect(List<T> list1, List<T> list2) {
		// 转换为Set
		Set<T> set = list2.stream().collect(Collectors.toSet());
				
		// 计算在list1存在，而在list2不存在的记录的集合
		List<T> resultList = list1.stream().filter(i -> set.contains(i)).collect(Collectors.toList());
		return resultList;
	}
	
	/**
	 * 计算并集，在list1和list2的记录合并，并且去重的集合
	 */
	public static <T> List<T> union(List<T> list1, List<T> list2) {
		Set<T> set = Stream.of(list1, list2).flatMap(i -> i.stream()).collect(Collectors.toSet());
		List<T> resultList = set.stream().collect(Collectors.toList());
		
		return resultList;
	}
	
	@Test
	public void subtractTest() {
		List<Integer> list1 = Lists.newArrayList(1, 2, 3, 4, 5);
		List<Integer> list2 = Lists.newArrayList(1, 5);
		
		List<Integer> resultList = CollectionUtils.subtract(list1, list2);
		
		this.logger.info("list1 -> {}", list1);
		this.logger.info("resultList -> {}", resultList);
	}
	
	@Test
	public void intersectTest() {
		List<Integer> list1 = Lists.newArrayList(1, 2, 3, 4, 5);
		List<Integer> list2 = Lists.newArrayList(1, 5);
		
		List<Integer> resultList = CollectionUtils.intersect(list1, list2);
		
		this.logger.info("list1 -> {}", list1);
		this.logger.info("resultList -> {}", resultList);
	}
	
	@Test
	public void unionTest() {
		List<Integer> list1 = Lists.newArrayList(1, 2, 3, 4, 5);
		List<Integer> list2 = Lists.newArrayList(1, 5, 6);
		
		List<Integer> resultList = CollectionUtils.union(list1, list2);
		
		this.logger.info("list1 -> {}", list1);
		this.logger.info("resultList -> {}", resultList);
	}

}