[移动开发]【Java】Java函数式编程以及流的操作

大纲

函数式编程学习的重点

• Lambda表达式
• Java Stream API

Lambda表达式

• 基于匿名类
• 基于函数式接口
• JDK8自带的函数式接口
• 方法引用和构造器引用

Java Stream API

• 操作Collections
• 操作NIO2.0
• 线程解析

lambda表达式

实现的接口

public interface PriceDashboard {
    Price getPrice(String symbol);
}

一般内部类

内部类可以调用外部属性

@Slf4j
public class DemoPart1Test {
    private Map<String, Price> stockPrices = new HashMap<>();
    private int accessCount = 0;

    public void init() {
        stockPrices.put("APPL", Price.builder()
                .currPrice(new BigDecimal("130.06"))
                .build());
    }

    private class PriceDashboardNewImpl implements PriceDashboard {
        @Override
        public Price getPrice(String symbol) {
            log.info("log this inner class action");
            return stockPrices.get(symbol);
//            log.info("about to reference by this");
//            return DemoPartOneTest.this.stockPrices.get(symbol);
        }
    }

    public static void main(String[] args) {
        DemoPart1Test test = new DemoPart1Test();
        test.init();
        PriceDashboard priceDashboard = test.new PriceDashboardNewImpl();
        log.info(priceDashboard.getPrice("APPL").toString());
    }
}

局部内部类

定义在成员方法中，在代码块之外无法使用

@Slf4j
public class DemoPart2Test {
    private Map<String, Price> stockPrices = new HashMap<>();
    private int accessCount = 0;

    public void init() {
        stockPrices.put("APPL", Price.builder()
                .currPrice(new BigDecimal("130.06"))
                .build());
    }

    public PriceDashboard priceBoard() {
         class PriceDashboardInMethodImpl implements PriceDashboard {
            @Override
            public Price getPrice(String symbol) {
                log.info("log this in-method action");
                return stockPrices.get(symbol);
            }
        }
        return new PriceDashboardInMethodImpl();
    }

    public static void main(String[] args) {
        DemoPart2Test test = new DemoPart2Test();
        test.init();
        PriceDashboard priceDashboard = test.priceBoard();
        log.info(priceDashboard.getPrice("APPL").toString());
    }
}

匿名内部类

一般写法

函数式写法

public PriceDashboard anonPriceBoard() {
    return symbol -> {
        log.info("log this in-method anonymous action");
        return stockPrices.get(symbol);
    };
}

基于函数式接口的lambda表达式

函数式接口的使用只能存在一个抽象方法，存在多余的抽象方法则编译不通过。

@FunctionalInterface
public interface PriceCalculation {
    BigDecimal increase(BigDecimal curr, BigDecimal inc);

    static void staticMethod() {
        System.out.println("static method");
    }

    default void defaultMethod() {
        System.out.println("default method");
    }
}

静态方法的调用可以通过接口名称直接调用，而default方法只能通过实现类来调用。

public class PriceCalculationImpl implements PriceCalculation{
    @Override
    public BigDecimal increase(BigDecimal curr, BigDecimal inc) {
        return curr.add(inc);
    }

    @Override
    public void defaultMethod() {
        System.out.println("customized default");
    }

    public static void main(String[] args) {
        //static method
        PriceCalculation.staticMethod();
        //default method
        new PriceCalculationImpl().defaultMethod();
    }
}

举例：

// 接口一
@FunctionalInterface
public interface Demo0ParamInterf {
    String print();
}

// 接口二
@FunctionalInterface
public interface Demo1ParamInterf {
    void print(int a);
}

// 接口三
@FunctionalInterface
public interface Demo2ParamInterf {
    int print(int a, int b);
}

// 接口四
@FunctionalInterface
public interface Demo3ParamInterf {
    int print();

    //equals is Object method
    boolean equals(Object a);

    //Object.clone is not public, so not functional
//    Object clone();
}

表达式

public class DemoParamTest {
    public static void main(String[] args) {
        Demo0ParamInterf impl0 = () -> "0 param";

        Demo1ParamInterf impl1 = (a) -> System.out.println(a);

        Demo2ParamInterf impl2 = (a, b) -> {
            int c = a * b;
            int d = Math.floorMod(c, a - 1);
            return d;
        };
    }
}

JDK8中自带的函数式接口

Predicate判断

接口

/**
 * Represents a predicate (boolean-valued function) of one argument.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #test(Object)}.
 *
 * @param <T> the type of the input to the predicate
 *
 * @since 1.8
 */
@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);

    /**
     * Returns a composed predicate that represents a short-circuiting logical
     * AND of this predicate and another.  When evaluating the composed
     * predicate, if this predicate is {@code false}, then the {@code other}
     * predicate is not evaluated.
     *
     * <p>Any exceptions thrown during evaluation of either predicate are relayed
     * to the caller; if evaluation of this predicate throws an exception, the
     * {@code other} predicate will not be evaluated.
     *
     * @param other a predicate that will be logically-ANDed with this
     *              predicate
     * @return a composed predicate that represents the short-circuiting logical
     * AND of this predicate and the {@code other} predicate
     * @throws NullPointerException if other is null
     */
    default Predicate<T> and(Predicate<? super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) && other.test(t);
    }

    /**
     * Returns a predicate that represents the logical negation of this
     * predicate.
     *
     * @return a predicate that represents the logical negation of this
     * predicate
     */
    default Predicate<T> negate() {
        return (t) -> !test(t);
    }

    /**
     * Returns a composed predicate that represents a short-circuiting logical
     * OR of this predicate and another.  When evaluating the composed
     * predicate, if this predicate is {@code true}, then the {@code other}
     * predicate is not evaluated.
     *
     * <p>Any exceptions thrown during evaluation of either predicate are relayed
     * to the caller; if evaluation of this predicate throws an exception, the
     * {@code other} predicate will not be evaluated.
     *
     * @param other a predicate that will be logically-ORed with this
     *              predicate
     * @return a composed predicate that represents the short-circuiting logical
     * OR of this predicate and the {@code other} predicate
     * @throws NullPointerException if other is null
     */
    default Predicate<T> or(Predicate<? super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) || other.test(t);
    }

    /**
     * Returns a predicate that tests if two arguments are equal according
     * to {@link Objects#equals(Object, Object)}.
     *
     * @param <T> the type of arguments to the predicate
     * @param targetRef the object reference with which to compare for equality,
     *               which may be {@code null}
     * @return a predicate that tests if two arguments are equal according
     * to {@link Objects#equals(Object, Object)}
     */
    static <T> Predicate<T> isEqual(Object targetRef) {
        return (null == targetRef)
                ? Objects::isNull
                : object -> targetRef.equals(object);
    }

    /**
     * Returns a predicate that is the negation of the supplied predicate.
     * This is accomplished by returning result of the calling
     * {@code target.negate()}.
     *
     * @param <T>     the type of arguments to the specified predicate
     * @param target  predicate to negate
     *
     * @return a predicate that negates the results of the supplied
     *         predicate
     *
     * @throws NullPointerException if target is null
     *
     * @since 11
     */
    @SuppressWarnings("unchecked")
    static <T> Predicate<T> not(Predicate<? super T> target) {
        Objects.requireNonNull(target);
        return (Predicate<T>)target.negate();
    }
}

使用

@Slf4j
public class DemoPredicateTest {

    @Test
    public void SimplePredicate() {
        Predicate<Integer> isAdult = age -> (age > 18);
        log.info(String.valueOf(isAdult.test(20)));
        log.info(String.valueOf(isAdult.test(16)));
    }

    @Test
    public void AndPredicate() {
        Predicate<Integer> isAdult = new Predicate<Integer>() {
            @Override
            public boolean test(Integer age) {
                log.info("we are young once");
                return age > 18;
            }
        };
        Predicate<Integer> isRetired = age -> {
            log.info("yes, pension");
            return age > 70;
        };

        log.info(String.valueOf(isAdult.and(isRetired).test(25)));
//        log.info(String.valueOf(isRetired.and(isAdult).test(25)));
    }

    @Test
    public void OrPredicate() {
        Predicate<Integer> cannotRead = age -> (age < 4);
        Predicate<Integer> cannotCode = age -> (age > 99);
        log.info("Should quit coding at 35? {}",
                cannotRead.or(cannotCode).test(35));
    }

    @Test
    public void NegatePredicate(){
        Predicate<Integer> isAdult = age -> (age >18);
        log.info(String.valueOf(isAdult.negate().test(16)));
    }

    @Test
    public void compositePredicate(){
        Predicate<Integer> cannotRead = age -> (age < 4);
        Predicate<Integer> cannotCode = age -> (age > 99);

        //composite them together
        Predicate<Integer> compositePredicate = cannotRead
                .or(cannotCode)
                .negate();
        //now you can pass this composite predicate around in your program
        log.info("Should quit coding at 35? {}", compositePredicate.test(35));
    }

}

Consumer消费

接口

/**
 * 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.
 *
 * <p>This is a <a href="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); };
    }
}

使用

@Slf4j
public class DemoConsumerTest {

    @Test
    public void LogConsumer() {
        Consumer<Object> logConsumer = o ->  {
                log.info(o.toString());
        };
        logConsumer.accept("Print something");
        logConsumer.accept(System.currentTimeMillis());
    }

    @Test
    public void AndThen() {
        Consumer<List<String>> upperCaseConsumer = strings -> {
            for (int i = 0; i < strings.size(); i++) {
                strings.set(i, strings.get(i).toUpperCase());
            }
        };
        Consumer<List<String>> logConsumer = strings -> strings.forEach(str -> System.out.println(str));

        List<String> list = Lists.newArrayList("foo", "bar", "baz");
        upperCaseConsumer.andThen(logConsumer).accept(list);
    }
}

Supplier供应商

接口

/**
 * Represents a supplier of results.
 *
 * <p>There is no requirement that a new or distinct result be returned each
 * time the supplier is invoked.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #get()}.
 *
 * @param <T> the type of results supplied by this supplier
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Supplier<T> {

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

使用

@Slf4j
public class DemoSupplierTest {

    @Test
    public void RandomSupplier() {
        Supplier<Double> doubleSupplier = Math::random;

        Consumer<Double> logConsumer = num -> log.info(String.valueOf(num));
        logConsumer.accept(doubleSupplier.get());
    }
}

Function方法

接口

/**
 * Represents a function that accepts one argument and produces a result.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #apply(Object)}.
 *
 * @param <T> the type of the input to the function
 * @param <R> the type of the result of the function
 *
 * @since 1.8
 */
@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);

    /**
     * Returns a composed function that first applies the {@code before}
     * function to its input, and then applies this function to the result.
     * If evaluation of either function throws an exception, it is relayed to
     * the caller of the composed function.
     *
     * @param <V> the type of input to the {@code before} function, and to the
     *           composed function
     * @param before the function to apply before this function is applied
     * @return a composed function that first applies the {@code before}
     * function and then applies this function
     * @throws NullPointerException if before is null
     *
     * @see #andThen(Function)
     */
    default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
        Objects.requireNonNull(before);
        return (V v) -> apply(before.apply(v));
    }

    /**
     * Returns a composed function that first applies this function to
     * its input, and then applies the {@code after} function to the result.
     * If evaluation of either function throws an exception, it is relayed to
     * the caller of the composed function.
     *
     * @param <V> the type of output of the {@code after} function, and of the
     *           composed function
     * @param after the function to apply after this function is applied
     * @return a composed function that first applies this function and then
     * applies the {@code after} function
     * @throws NullPointerException if after is null
     *
     * @see #compose(Function)
     */
    default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) {
        Objects.requireNonNull(after);
        return (T t) -> after.apply(apply(t));
    }

    /**
     * Returns a function that always returns its input argument.
     *
     * @param <T> the type of the input and output objects to the function
     * @return a function that always returns its input argument
     */
    static <T> Function<T, T> identity() {
        return t -> t;
    }
}

使用

@Slf4j
public class DemoFunctionTest {
    @Test
    public void ArithmeticFunction() {
        Function<Integer, Integer> doubled = x -> x * 2;
        System.out.println(doubled.apply(3));

        Function<Integer, Integer> halved = x -> x / 2;
        System.out.println(halved.apply(10));

        Function<Integer, Integer> inched = x -> x + 2;
        System.out.println(inched.apply(20));
    }

    @Test
    public void TypeTransformationFunction() {
        Function<String, Integer> lengthFunction = str -> str.length();
        System.out.println(lengthFunction.apply("woyouhuile"));
    }

    @Test
    public void CompositeFunction() {
        Function<Integer, Integer> doubled = x -> x * 2;
        Function<Integer, Integer> inched = x -> x + 2;

        System.out.println(doubled.compose(inched).apply(10));
        System.out.println(doubled.andThen(inched).apply(10));
    }
    
    // 通过使用identity的方式，把方法本身作为一个对象传入了map中value中
    @Test
    public void IdentityFunction() {
        List<Stock> stocks = Lists.newArrayList(
                Stock.builder().name("APPLE").symbol("APPL").build(),
                Stock.builder().name("Amazon").symbol("AMZN").build(),
                Stock.builder().name("Starbucks").symbol("SBUX").build()
        );
        Map<String, Stock> map = stocks.stream()
                .collect(Collectors.toMap(Stock::getSymbol, Function.identity()));
        System.out.println(map);
    }
}

其他接口

在java.util包中存在其他函数式接口

bitfunction就是接收两个参数返回一个参数的接口

因为通用的函数式接口存在包装类的装箱与拆箱的操作，因此定义了许多不同的函数式接口进行使用，在数据量大的时候减少消耗。

方法引用和构造器引用

静态方法引用

使用类名::方法名的方式引用

public class DemoStaticMethodReferenceTest {
    @Test
    public void printStringTest() {
        Thread t = new Thread(() -> printString());
        Thread t1 = new Thread(DemoStaticMethodReferenceTest::printString);
        t.start();
    }

    public static void printString() {
        System.out.println("string");
    }

    //ClassName::staticMethodName
}

非静态方法引用

定义一个比较器

public class StockComparator implements Comparator<Stock> {
    @Override
    public int compare(Stock o1, Stock o2) {
        return o1.getSymbol().compareTo(o2.getSymbol());
    }
}

使用@Builder注解，可以使用build的方式创建类

@Data
@Builder
public class Stock {
    public Stock() {
    }

    public Stock(String symbol, String name) {
        this.symbol = symbol;
        this.name = name;
    }

    private String symbol;
    private String name;
}

使用

public class DemoInstanceMethodReferenceTest {
    @Test
    public void orderStockTest() {
        List<Stock> stocks = Lists.newArrayList(
                Stock.builder().name("APPLE").symbol("APPL").build(),
                Stock.builder().name("Amazon").symbol("AMZN").build(),
                Stock.builder().name("Starbucks").symbol("SBUX").build()
        );
        StockComparator stockComparator = new StockComparator();
        stocks.stream()
                .sorted((o1, o2) -> o1.getSymbol().compareTo(o2.getSymbol()))//
                .sorted(stockComparator::compare)
                .collect(Collectors.toList());
        stocks.stream()
//                .forEach(stock -> System.out.println(stock));
                .forEach(System.out::println);
    }
    //instanceReference::methodName

}

构造器引用

接口

@FunctionalInterface
public interface StockCreator {
    Stock getStock();
}

@FunctionalInterface
public interface StockCreator2 {
    Stock getStock(String symbol, String name);
}

根据参数的类型以及数量，引用对应的构造器。不对应时，无法使用构造器引用。

public class DemoConstructorReferenceTest {
    @Test
    public void stockConstructorTest() {
        StockCreator creator1 = Stock::new;
        StockCreator2 creator2 = Stock::new;
        Supplier<Stock> creator3 = Stock::new;
        Stock stock1 = creator1.getStock();
        Stock stock2 = creator2.getStock("APPL", "APPLE Inc");
        Stock stock3 = creator3.get();
    }

    //ClassName::new
}

lambda表达式中的异常处理

需要抛出异常的方法

public class ComplexComputer {
    public String encrypt(String input) throws GeneralSecurityException, InterruptedException {
        Thread.sleep(1000L);
        return "encoded";
    }
}

根据场景选择何时的异常处理逻辑可以使用@SneakyThrows或者try/catch在方法底层抛出异常，或者向上抛出，用exceptionhandle处理。

public class DemoThrowExceptionTest {

    public ComplexComputer complexComputer = new ComplexComputer();

    @Test
    public void forLoopTest() throws GeneralSecurityException, InterruptedException {
        List<String> items = Lists.newArrayList("foo", "bar", "baz");
        for (String item : items) {
            complexComputer.encrypt(item);
        }
    }

    @Test
    public void throwIOExceptionTest() {
        List<String> items = Lists.newArrayList("foo", "bar", "baz");
        items.forEach(new Consumer<String>() {
            @SneakyThrows
            @Override
            public void accept(String item) {
                    complexComputer.encrypt(item);
            }
        }
      );
    }
}

Currying in java

科利化是函数式编程一个重要的特性，意思是把接受多个参数的函数变为接受单一参数的函数。

科里方程如下

Named after Haskell Curry

function curry(f) {
    return function(a) {
        return function(b) {
            return f(a, b);
        };
    };
}

f(a,b) -> f(a)(b)

科利化的使用

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函数式改造之后

@Test
public void testSimpleStockModelCreation() {
    BiFunction<String, String, StockModelA> StockModelCreator = (symbol, name) -> new StockModelA(symbol, name);

    Function<String, Function<String, StockModelA>> CurryingStockModelInnerClassCreator =
            symbol -> name -> new StockModelA(symbol, name);

}

使用科利化构造器引用之后，传入对应的值

@Test
public void testMoreAttrStockModelCreation() {
    Function<String, Function<String, Function<String, Function<BigDecimal, Function<BigDecimal, StockModelB>>>>>
            curryingStockModelCreator =
            symbol -> name -> desc -> currPrice -> prevPrice -> new StockModelB(symbol, name, desc, currPrice, prevPrice);

    StockModelB demoStockModel = curryingStockModelCreator
            .apply("SYMBOL")
            .apply("NAME")
            .apply("DESCRIPTION")
            .apply(new BigDecimal("1.00"))
            .apply(new BigDecimal("0.99"));
}

部分属性不变时

@Test
public void testPartialModelCreation() {
    Function<String, Function<String, Function<String, Function<BigDecimal, Function<BigDecimal, StockModelB>>>>>
            curryingStockModelCreator =
            symbol -> name -> desc -> currPrice -> prevPrice -> new StockModelB(symbol, name, desc, currPrice, prevPrice);

    Function<BigDecimal, Function<BigDecimal, StockModelB>>
            partialStockModelCreator =
            currPrice -> prevPrice -> curryingStockModelCreator
                    .apply("SYMBOL")
                    .apply("NAME")
                    .apply("DESCRIPTION")
                    .apply(currPrice)
                    .apply(prevPrice);
    StockModelB fromPartialCreated = partialStockModelCreator
            .apply(new BigDecimal("1.00"))
            .apply(new BigDecimal("0.99"));
    System.out.println(fromPartialCreated);
}

Java Stream API

stream流是可以更方便的使用lambda表达式操作集合的数据结构，并且实现了并行计算等功能。只有在真正需要计算的时候才会执行，“延迟计算”。

stream-category

Stream
    - intermiate 中间操作，返回流但是不会计算
        - stateless 无状态操作，元素的处理不会受之前或之后元素的影响
        - stateful 有状态操作，只有在拿到了所有元素之后才能继续
    - terminal 终结操作，会进行计算
        - short-circuiting 短路操作，遇到符合的元素就可以得到结果
        - un-short-circuiting 非短路操作，只有所有元素都拿到才能计算结果

使用stream流操作Collections

创建stream流

方法一：Stream.generate

使用limit限制无限流生成的数据

@Test
public void streamGenerate() {
    Stream<Double> stream = Stream.generate(Math::random);
    stream.limit(5).forEach(System.out::println);
}

方法二：Stream.of

@Test
public void streamOf() {
    Stream<String> stream1 = Stream.of("foo");
    Stream<String> stream2 = Stream.of("foo", "bar", "baz");
    //stream2 and stream3 are equivalent
    Stream<String> stream3 = Arrays.stream(new String[]{"foo", "bar", "baz"});
}

方法三：streamFromCollections

因为map不是继承collection接口，所以要使用stream流遍历时需要entrySet()方法。

@Test
public void streamFromCollections() {
    List<Stock> stocks = Lists.newArrayList(
            Stock.builder().name("APPLE").symbol("APPL").build(),
            Stock.builder().name("Amazon").symbol("AMZN").build(),
            Stock.builder().name("Starbucks").symbol("SBUX").build()
    );
    Stream<Stock> stream1 = stocks.stream();
    Map map = new HashMap();
    map.entrySet().stream();
}

方法四：Stream.builder

@Test
public void streamBuilder() {
    Stream.Builder<Stock> builder = Stream.builder();
    builder.accept(Stock.builder().name("APPLE").symbol("APPL").build());
    builder.accept(Stock.builder().name("Amazon").symbol("AMZN").build());
    builder.accept(Stock.builder().name("Starbucks").symbol("SBUX").build());
    Stream<Stock> stockStream = builder.build();
}

操作stream

foreach

@Test
public void streamForEach() {
    List<Stock> stocks = Lists.newArrayList(
            Stock.builder().name("APPLE").symbol("APPL").build(),
            Stock.builder().name("Amazon").symbol("AMZN").build(),
            Stock.builder().name("Starbucks").symbol("SBUX").build()
    );
    stocks.stream().forEach(stock -> stock.setSymbol(stock.getSymbol().toLowerCase()));
    System.out.println(stocks);

    //directly on Iterable and Map
    stocks.forEach(stock -> stock.setSymbol(stock.getSymbol() + "Iterable"));
    System.out.println(stocks);
    Map<String, Object> map = new HashMap();
    map.forEach((k, v)->{});
}

map

map是stream流的中间操作，会返回一个流供下一步操作。

@Test
public void streamMap() {
    Stream<String> stream = Stream.of("foo", "bar", "baz");
    Stream<Stock> stockStream = stream.map(str -> Stock.builder().name(str).build());
    stockStream.forEach(System.out::println);
}

输出结果

Stock(symbol=null, name=foo)
Stock(symbol=null, name=bar)
Stock(symbol=null, name=baz)

collect

把处理好的stream流的元素收集起来，返回一个数据集合。

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Collector中定义了三个泛型，T是规约操作的输入类型，A是中间暂存所用的类型，R是规约操作输出的类型。

public interface Collector<T, A, R> {
    // 创建可变的结果容器
    Supplier<A> supplier();
    // 将数据元素合并到结果容器中
    BiConsumer<A, T> accumulator();
    // 将部分结果合并到一个结果容器中
    BinaryOperator<A> combiner();
    // 将结果类型转换
    Function<A, R> finisher();

    Set<Characteristics> characteristics();

    public static<T, R> Collector<T, R, R> of(Supplier<R> supplier,
                                              BiConsumer<R, T> accumulator,
                                              BinaryOperator<R> combiner,
                                              Characteristics... characteristics) {
        Objects.requireNonNull(supplier);
        Objects.requireNonNull(accumulator);
        Objects.requireNonNull(combiner);
        Objects.requireNonNull(characteristics);
        Set<Characteristics> cs = (characteristics.length == 0)
                                  ? Collectors.CH_ID
                                  : Collections.unmodifiableSet(EnumSet.of(Collector.Characteristics.IDENTITY_FINISH,
                                                                           characteristics));
        return new Collectors.CollectorImpl<>(supplier, accumulator, combiner, cs);
    }

    public static<T, A, R> Collector<T, A, R> of(Supplier<A> supplier,
                                                 BiConsumer<A, T> accumulator,
                                                 BinaryOperator<A> combiner,
                                                 Function<A, R> finisher,
                                                 Characteristics... characteristics) {
        Objects.requireNonNull(supplier);
        Objects.requireNonNull(accumulator);
        Objects.requireNonNull(combiner);
        Objects.requireNonNull(finisher);
        Objects.requireNonNull(characteristics);
        Set<Characteristics> cs = Collectors.CH_NOID;
        if (characteristics.length > 0) {
            cs = EnumSet.noneOf(Characteristics.class);
            Collections.addAll(cs, characteristics);
            cs = Collections.unmodifiableSet(cs);
        }
        return new Collectors.CollectorImpl<>(supplier, accumulator, combiner, finisher, cs);
    }

    enum Characteristics {

        CONCURRENT,

        UNORDERED,

        IDENTITY_FINISH
    }
}

使用

@Test
public void streamCollectToList() {
    Stream<String> stream = Stream.of("foo", "bar", "baz");
    Stream<Stock> stockStream = stream.map(str -> Stock.builder().name(str).build());
    List<Stock> stocks = stockStream.collect(Collectors.toList());
}

@Test
public void streamCollectJoining() {
    Stream<String> stream = Stream.of("foo", "bar", "baz");
    String result = stream.collect(Collectors.joining("|"));
    // 使用 | 将元素进行分割
    System.out.println(result);
}

@Test
public void streamCollectGroupBy() {
    List<Student> students = getStudentsData();
    //group student by class 按照班级名称对流中的元素进行分组
    Map<String, List<Student>> byClass =
            students.stream().collect(Collectors.groupingBy(Student::getClassName));
    System.out.println(byClass);
}

@Test
public void streamCollectGroupByWithCustomizedDownstream() {
    List<Student> students = getStudentsData();
    //sum student score by class
    Map<String, Integer> sumScoreByClass =
            students.stream().collect(
                    Collectors.groupingBy(
                            Student::getClassName,
                            Collectors.summingInt(Student::getScore)
                    )
            );
    System.out.println(sumScoreByClass);
    //get student with highest score for each class
    Map<String, Optional<Student>> highestScoreByClass = students.stream().collect(
            Collectors.groupingBy(
                    Student::getClassName,
                    Collectors.maxBy(Comparator.comparing(Student::getScore))
            )
    );
    System.out.println(highestScoreByClass);
}

filter

stream流的中间操作，filter中实现了一个Predicate接口。对传入的元素进行判断，断言成功则返回，不成功便丢弃。

@Test
public void streamFilter() {
    List<Student> students = getStudentsData();
    // 筛选成绩大于80的学生
    Stream<Student> scoreOver80Stream = students.stream().filter(student -> student.getScore() > 80);
    System.out.println(scoreOver80Stream.collect(Collectors.toList()));
}

findFirst

短路操作。

@Test
public void streamFindFirst() {
    List<Student> students = getStudentsData();
    Optional<Student> firstGuy = students
            .stream()
            .filter(student -> student.getScore() > 80)
            .findFirst();
    System.out.println(firstGuy);
}
// 不使用Optional
@Test
public void streamFindFirst() {
    List<Student> students = getStudentsData();
    Student firstGuy = students
            .stream()
            .filter(student -> student.getScore() > 80)
            .findFirst()
            .orElse(new Student());
    System.out.println(firstGuy);
}

peek

与foreach类似，都实现了consumer接口，但是属于中间操作，操作结束之后会把元素重新返回流中。

@Test
public void streamPeek() {
    List<Student> students = getStudentsData();
    students
            .stream()
            .peek(student -> student.setScore(student.getScore() + 10))
            .peek(System.out::println);
            // 在指定终结操作之前都不会打印，“延迟计算”
//              .collect(Collectors.toList());
}

flatmap

Function.identity()是返回元素本身

@Test
public void streamFlatMap() {
    String paragraph = "this is a demo paragraph to calculate char occurrences";
    String[] words = paragraph.split(" ");
    Map<String, Long> occurrence = Arrays.stream(words)
            .map(str -> str.split(""))
            .flatMap(strAarray -> Arrays.stream(strAarray))// 整合为一个流
            .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));// 按照元素自己进行计数分类
    System.out.println(occurrence);
}

stream流操作nio

// 文件读取
@Test
public void FilesLinesTest() {
    Path path = new File(getClass().getResource("/stream/nio_datasource_1.txt").getFile()).toPath();
    try (Stream<String> content = Files.lines(path, Charset.defaultCharset())) {
        List<String> orderedList = content.map(String::toUpperCase).sorted().collect(Collectors.toList());
        System.out.println(orderedList);
    } catch (IOException e) {
        e.printStackTrace();
    }
}
// 深度遍历文件夹
@Test
public void FilesWalkTest() {
    Path rootPath = new File(getClass().getResource("/stream/a").getFile()).toPath();
    try (Stream<Path> matched = Files.walk(rootPath, 3)) {
        List<Path> matchedPathList = matched.collect(Collectors.toList());
        System.out.println(matchedPathList);
    } catch (IOException e) {
        e.printStackTrace();
    }
}
// 查找文件
@Test
public void FilesFindTest() throws IOException {
    Path rootPath = new File(getClass().getResource("/stream/a").getFile()).toPath();
    try (Stream<Path> matched = Files.find(rootPath, 3, (path, attr) -> path.endsWith("bar.txt"))) {
        List<Path> matchedPathList = matched.collect(Collectors.toList());
        System.out.println(matchedPathList);
    } catch (IOException e) {
        e.printStackTrace();
    }
}

ParallelStream

// 使用AtomicInteger保证线程安全
public static AtomicInteger counter = new AtomicInteger();

@Test
public void simpleParallel() {
    List<Student> students = getLotsOfStudents();
    List<Student> studentList = students.parallelStream().peek(student -> counter.getAndIncrement()).collect(Collectors.toList());
//        System.out.println(studentList);
    System.out.println(students.size());
    System.out.println(counter);
}

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