[转]Java8 lambda表达式及新特新

分享自：Vincent

package info.liuwenjun.test;

import org.junit.Test;

import java.util.*;
import java.util.function.*;
import java.util.stream.Collectors;

/**
 * Created by Vincent on 2016-04-12.
 */
public class TestJava8 {

    private void print(Object obj) {
        System.out.println(obj);
    }

    @Test
    public void sort() {
        List<String> names = Arrays.asList("peter", "anna", "mike", "xenia");

        Collections.sort(names, new Comparator<String>() {
            @Override
            public int compare(String a, String b) {
                return a.compareTo(b);
            }
        });

        Collections.sort(names, (String a, String b) -> {
            return a.compareTo(b);
        });

        Collections.sort(names, (String a, String b) -> a.compareTo(b));

        Collections.sort(names, (a, b) -> a.compareTo(b));

        print(names);

        new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("inner class");
            }
        }).start();

        new Thread(() -> System.out.println("lambda")).start();
    }

    @Test
    public void typeDeduction() {
        Function<Integer, String> intToStr = a -> a.toString();

        Function<Double, String> doubleToStr = a -> a.toString();

        // doubleToStr = intToStr;

        // () -> print("typeDeduction");
        ((Action) () -> print("typeDeduction")).action();
    }

    @FunctionalInterface
    private interface Action {
        void action();
    }

    @Test
    public void functionalInterface() {
        Function<Integer, Integer> plusOne = i -> i + 1;
        print(plusOne.apply(1));

        Predicate<Character> emoticon = o->'皿'<-o;
        print(emoticon.test('皿'));

        Consumer<Integer> printInt = i -> print(i);
        printInt.accept(1);

        Supplier<Long> getCurrentTime = () -> System.currentTimeMillis();
        print(getCurrentTime.get());

        Action sayHello = () -> print("hello");
        sayHello.action();
    }

    @Test
    public void testThis() {
        Action innerClass = new Action() {
            @Override
            public void action() {
                this.print("testThis");
            }

            private void print(Object obj) {
                System.out.println("Inner: " + obj);
            }
        };
        innerClass.action();

        Action lambda = () -> this.print("testThis");
        lambda.action();
    }

    @Test
    public void testFinal() {
        final int finalVar = 0;
        int effectivelyFinalVar = 0;
        // effectivelyFinalVar = 1;

        new Action() {
            @Override
            public void action() {
                print(finalVar);
                print(effectivelyFinalVar);
            }
        }.action();
    }

    private List<String> stringCollection = new ArrayList<String>() {
        {
            add("ddd2");
            add("aaa2");
            add("bbb1");
            add("aaa1");
            add("bbb3");
            add("ccc");
            add("bbb2");
            add("ddd1");
        }
    };

    @Test
    public void filter() {
        stringCollection
                .stream()
                .filter(s -> s.startsWith("a"))
                .forEach(s -> print(s));
    }

    @Test
    public void sorted() {
        stringCollection
                .stream()
                .sorted((a, b) -> b.compareTo(a))
                .filter(s -> s.startsWith("a"))
                .forEach(s -> print(s));
    }

    @Test
    public void map() {
        stringCollection
                .stream()
                .map(s -> s.toUpperCase())
                .sorted(Comparator.comparing(s -> s.length()))
                .forEach(s -> print(s));
    }

    @Test
    public void flatMap() {
        stringCollection
                .stream()
                .filter(s -> s.startsWith("a"))
                .flatMap(s -> Arrays.stream(s.split("")))
                .forEach(c -> print(c));
    }

    @Test
    public void match() {
        print(stringCollection.stream().anyMatch(s -> s.startsWith("a")));

        print(stringCollection.stream().allMatch(s -> s.startsWith("a")));

        print(stringCollection.stream().noneMatch(s -> s.startsWith("z")));
    }

    @Test
    public void count() {
        print(stringCollection.stream().filter(s -> s.startsWith("b")).count());
    }

    @Test
    public void reduce() {
        Optional<Integer> optional = stringCollection
                .stream()
                .map(s -> s.length())
                .reduce((a, b) -> a + b);
        print(optional.get());

        int sum = stringCollection.get(0).length();
        for (int i = 1; i < stringCollection.size(); i++) {
            sum = sum + stringCollection.get(i).length();
        }
        print(sum);
    }

    @Test
    public void intStream() {
        IntSummaryStatistics statistics = stringCollection
                .stream()
                .mapToInt(s -> s.length())
                .summaryStatistics();
        print(statistics.getAverage());
        print(statistics.getSum());
        print(statistics.getMax());
        print(statistics.getMin());
        print(statistics.getCount());
    }

    @Test
    public void complex() {
        stringCollection
                .stream()
                .sorted()
                .filter(s -> s.startsWith("a") && s.endsWith("1"))
                .map(s -> s.length())
                .filter(i -> i == 4)
                .forEach(i -> print(i));
    }

    @Test
    public void toList() {
        List<Integer> len = stringCollection.stream().map(s -> s.length()).collect(Collectors.toList());
        print(len);
    }

    @Test
    public void toMap() {
        Map<String, Integer> len = stringCollection.stream().collect(Collectors.toMap(s -> s, s -> s.length()));
        print(len);
    }

    @Test
    public void groupingBy() {
        Map<Integer, List<String>> groups = stringCollection
                .stream()
                .collect(Collectors.groupingBy(s -> s.length()));
        print(groups);
    }

    @Test
    public void complexCollect() {
        Map<Integer, List<String>> groups = stringCollection
                .stream()
                .collect(Collectors.groupingBy(s -> s.length(),
                        Collectors.mapping(s -> s.toUpperCase(), Collectors.toList())));
        print(groups);
    }

    @Test
    public void removeIf() {
        List<String> copy = new ArrayList<>(stringCollection);

        stringCollection.removeIf(s -> s.startsWith("a"));
        print(stringCollection);

        Iterator<String> iter = copy.iterator();
        while (iter.hasNext()) {
            if(iter.next().startsWith("a")) {
                iter.remove();
            }
        }
        print(copy);
    }

    @Test
    public void iterateMap() {
        Map<String, Integer> map = stringCollection.stream().collect(Collectors.toMap(s -> s, s -> s.length()));

        map.forEach((key, value) -> print(key + ": " + value));

        print("---------");

        for(Map.Entry entry : map.entrySet()) {
            print(entry.getKey() + ": " + entry.getValue());
        }
    }

    @Test
    public void mapStream() {
        Map<String, Integer> map = stringCollection.stream().collect(Collectors.toMap(s -> s, s -> s.length()));

        map.entrySet()
                .stream()
                .filter(e -> e.getValue() == 4)
                .forEach(e -> print(e.getKey() + ": " + e.getValue()));
    }
}