Stream类的函数java

R collect(Collector super T, A, R> collector);

能够对数据集进行简单的分组统计。函数参数为接口Collector,其实如今final class Collectors的内部静态类CollectorImpl。Collector接口代码以下：web

public interface Collector {

Supplier supplier();

BiConsumer accumulator();

BinaryOperator combiner();

Function finisher();

Set characteristics();

public static Collector of(Supplier supplier,

BiConsumer accumulator,

BinaryOperator combiner,

Characteristics... characteristics) {

Objects.requireNonNull(supplier);

Objects.requireNonNull(accumulator);

Objects.requireNonNull(combiner);

Objects.requireNonNull(characteristics);

Set 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 Collector of(Supplier supplier,

BiConsumer accumulator,

BinaryOperator combiner,

Function finisher,

Characteristics... characteristics) {

Objects.requireNonNull(supplier);

Objects.requireNonNull(accumulator);

Objects.requireNonNull(combiner);

Objects.requireNonNull(finisher);

Objects.requireNonNull(characteristics);

Set 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

}

}

Collectors

Collectors中静态内部类实现了Collector接口，而且提供了诸如求最大值、最小值、平均值及求和等函数，能够搭配groupingBy(XXX)使用对数据集进行分组,相关函数及使用示例以下：app

1. Collectors.summingInt(XXX)

summingInt源码：

ide

/**

* 静态函数

* 返回一个Collector接口，其可用于产生输入元素‘int型和’的函数

*

*@param 输入元素类型

*@param mapper 函数，抽出须要汇总值得属性

*@return Collector ，用于产生'驱动属性'的和

*/

public static Collector

summingInt(ToIntFunction super T> mapper) {

return new Collectors.CollectorImpl<>(

() -> new int[1],//Supplier === T get();

(a, t) -> { a[0] += mapper.applyAsInt(t); },//BiConsumer === void accept(T t, U u)

(a, b) -> { a[0] += b[0]; return a; },// BinaryOperator === R apply(T t, U u)

a -> a[0], // Function === R apply(T t);

CH_NOID);// Set == int size();

}

参数ToIntFunction类源码svg

@FunctionalInterface

public interface ToIntFunction {

/**

* Applies this function to the given argument.

*

* @param value 函数参数

*/

int applyAsInt(T value);

}

返回值类所继承的接口Collector函数

/*

* 包含

* 1.返回值为函数类接口的抽象方法；

* 2.of方法返回其对象实例；

* 3.枚举类，包含

* CONCURRENT,UNORDERED,IDENTITY_FINISH三个元素

*/

public interface Collector {

Supplier supplier();

BiConsumer accumulator();

BinaryOperator combiner();

Function finisher();

Set characteristics();

/* 参数同其方法返回值类型

* 用给的三个函数类接口实例初始化Collector接口继承类Collectors.CollectorImpl实例，并返回

*/

public static Collector of(Supplier supplier,

BiConsumer accumulator,

BinaryOperator combiner,

Characteristics... characteristics) {

Objects.requireNonNull(supplier);//保证函数类接口不为null，不然抛运行时异常

Objects.requireNonNull(accumulator);//同上

Objects.requireNonNull(combiner);//同上

Objects.requireNonNull(characteristics);//同上

Set cs = (characteristics.length == 0)

? Collectors.CH_ID

: Collections.unmodifiableSet(EnumSet.of(Collector.Characteristics.IDENTITY_FINISH,

characteristics));

return new Collectors.CollectorImpl<>(supplier, accumulator, combiner, cs);

}

/*

* 用给的三个函数类接口实例初始化Collector接口继承类Collectors.CollectorImpl实例，并返回

*/

public static Collector of(Supplier supplier,

BiConsumer accumulator,

BinaryOperator combiner,

Function finisher,

Characteristics... characteristics) {

Objects.requireNonNull(supplier);

Objects.requireNonNull(accumulator);

Objects.requireNonNull(combiner);

Objects.requireNonNull(finisher);

Objects.requireNonNull(characteristics);

Set 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

}

}

Collectors.CollectorImpl，静态内部类ui

/**

* Simple implementation class for {@code Collector}.

* 1.包含几个final变量，类型同继承接口Collector构造器参数，也同其抽象方法返回值；

* 2.两个构造器，实现方式与Collector相似；

* 3.重载的父类方法，用其变量赋值；

*/

static class CollectorImpl implements Collector {

private final Supplier supplier;

private final BiConsumer accumulator;

private final BinaryOperator combiner;

private final Function finisher;

private final Set characteristics;

CollectorImpl(Supplier supplier,

BiConsumer accumulator,

BinaryOperator combiner,

Function finisher,

Set characteristics) {

this.supplier = supplier;

this.accumulator = accumulator;

this.combiner = combiner;

this.finisher = finisher;

this.characteristics = characteristics;

}

CollectorImpl(Supplier supplier,

BiConsumer accumulator,

BinaryOperator combiner,

Set characteristics) {

this(supplier, accumulator, combiner, castingIdentity(), characteristics);

}

@Override

public BiConsumer accumulator() {

return accumulator;

}

@Override

public Supplier supplier() {

return supplier;

}

@Override

public BinaryOperator combiner() {

return combiner;

}

@Override

public Function finisher() {

return finisher;

}

@Override

public Set characteristics() {

return characteristics;

}

}

TODO放一段求平均数和求和的函数代码，彻底看不懂底层实现this

/**

*求平均数

*/

public static Collector

averagingInt(ToIntFunction super T> mapper) {

return new Collectors.CollectorImpl<>(

() -> new long[2],

(a, t) -> { a[0] += mapper.applyAsInt(t); a[1]++; },

(a, b) -> { a[0] += b[0]; a[1] += b[1]; return a; },

a -> (a[1] == 0) ? 0.0d : (double) a[0] / a[1],

CH_NOID);

}

/**

*求和

*/

public static Collector

summingInt(ToIntFunction super T> mapper) {

return new Collectors.CollectorImpl<>(

() -> new int[1],

(a, t) -> { a[0] += mapper.applyAsInt(t); },

(a, b) -> { a[0] += b[0]; return a; },

a -> a[0], CH_NOID);

}