[四] java8 函数式编程 收集器浅析 收集器Collector常用方法 运行原理 内部实现

Collector常见用法

常用收集器概要

收集器参数列表

收集器详解

T - 输入类型 A - 在收集过程中用于累积部分结果的对象类型 R - 返回类型

收集器构成

属性特征字段

Collector  就是归约运算操作的一种抽象

Collector用 类型TAR 和四个方法将归约的过程逻辑化

Collector工厂Collectors

// 获取所有的name转换到List中
List list = people.stream().map(Person::getName).collect(Collectors.toList());

// 获取所有的name转换到Set中
Set set = people.stream().map(Person::getName).collect(Collectors.toCollection(TreeSet::new));

// 元素转换为String 并且将他们通过", " 连接起来
String joined = things.stream()
.map(Object::toString)
.collect(Collectors.joining(", "));

//计算员工薪水之和
int total = employees.stream()
.collect(Collectors.summingInt(Employee::getSalary)));

// 按照部门对员工进行分组
Map> byDept
= employees.stream()
.collect(Collectors.groupingBy(Employee::getDepartment));

// 计算部门薪资和
Map totalByDept
= employees.stream()
.collect(Collectors.groupingBy(Employee::getDepartment,
Collectors.summingInt(Employee::getSalary)));

// 按照成绩是否通过把学生分为两组
Map> passingFailing =
students.stream()
.collect(Collectors.partitioningBy(s -> s.getGrade() >= PASS_THRESHOLD));

/**
* Simple implementation class for {@code Collector}.
*
* @param  the type of elements to be collected
* @param  the type of the result
*/
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;
}
}

Collector> toList() {
return new CollectorImpl<>( (Supplier>) ArrayList::new,
List::add,
(left, right) -> { left.addAll(right); return left; },
CH_ID);
}