收集器简介

  • 将流中的元素累积成一个结果
  • 作用域终端操作collect()上
  • collect/Collector/Collectors
    • collect：作为终端操作出现的，流收集的最后一个步骤，一个方法
    • Collector：是一个接口，collect方法需要接收一个实现了Collector接口的收集器才可以收集
    • Collectors：是一个工具类，已经提前封装预制了一些实现了Collector接口的收集器，可以直接哪来用

常用预定义收集器功能

  • 将流元素归约和汇总一个值
  • 将流元素分组
  • 将流元素分区

实战：预定义收集器案例

集合收集器

将流元素归约和汇总为一个值

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/**
 * 集合收集器
 */
@Test
public void toList() {
    List<Sku> skus = CartService.getCartSKuList();
    List<Sku> skuList = skus.stream()
            .filter(sku -> sku.getTotalPrice() > 1000)
            .collect(Collectors.toList());
    System.out.println(JSONUtil.parse(skuList));
}

执行后，输出的结果

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[{
	"totalPrice": 4999,
	"skuPrice": 4999,
	"skuCategory": "ELECTRONICS",
	"skuName": "无人机",
	"totalNum": 1,
	"skuId": 654032
}, {
	"totalPrice": 2299,
	"skuPrice": 2299,
	"skuCategory": "ELECTRONICS",
	"skuName": "VR一体机",
	"totalNum": 1,
	"skuId": 654033
}, {
	"totalPrice": 3999,
	"skuPrice": 3999,
	"skuCategory": "SPORTS",
	"skuName": "跑步机",
	"totalNum": 1,
	"skuId": 654221
}, {
	"totalPrice": 1999,
	"skuPrice": 1999,
	"skuCategory": "CLOTHING",
	"skuName": "牛仔裤",
	"totalNum": 1,
	"skuId": 654022
}]

分组

将流元素分组

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/**
 * 分组
 */
@Test
public void group() {
    List<Sku> skus = CartService.getCartSKuList();
    // Map<分组条件, 结果集合>
    Map<Enum<SkuCategoryEnum>, List<Sku>> collect = skus.stream()
            .collect(Collectors.groupingBy(sku -> sku.getSkuCategory()));
    System.out.println(JSONUtil.parse(collect));
}

执行后，输出的结果

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{
	"BOOKS": [{
		"totalPrice": 79.8,
		"skuPrice": 79.8,
		"skuCategory": "BOOKS",
		"skuName": "Java编程思想",
		"totalNum": 1,
		"skuId": 654121
	}, {
		"totalPrice": 149.8,
		"skuPrice": 149.8,
		"skuCategory": "BOOKS",
		"skuName": "Java核心技术",
		"totalNum": 1,
		"skuId": 654122
	}],
	"CLOTHING": [{
		"totalPrice": 299,
		"skuPrice": 299,
		"skuCategory": "CLOTHING",
		"skuName": "睡衣套装",
		"totalNum": 1,
		"skuId": 654021
	}, {
		"totalPrice": 1999,
		"skuPrice": 1999,
		"skuCategory": "CLOTHING",
		"skuName": "牛仔裤",
		"totalNum": 1,
		"skuId": 654022
	}],
	"SPORTS": [{
		"totalPrice": 3999,
		"skuPrice": 3999,
		"skuCategory": "SPORTS",
		"skuName": "跑步机",
		"totalNum": 1,
		"skuId": 654221
	}],
	"ELECTRONICS": [{
		"totalPrice": 4999,
		"skuPrice": 4999,
		"skuCategory": "ELECTRONICS",
		"skuName": "无人机",
		"totalNum": 1,
		"skuId": 654032
	}, {
		"totalPrice": 2299,
		"skuPrice": 2299,
		"skuCategory": "ELECTRONICS",
		"skuName": "VR一体机",
		"totalNum": 1,
		"skuId": 654033
	}]
}

分区

将流元素分区

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/**
 * 分区（分组的一个特例）
 */
@Test
public void partition() {
    List<Sku> skus = CartService.getCartSKuList();
    Map<Boolean, List<Sku>> collect = skus.stream()
            .collect(Collectors.partitioningBy(sku -> sku.getTotalPrice() > 1000));
    System.out.println(JSONUtil.parse(collect));
}

执行后，输出的结果

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{
	"false": [{
		"totalPrice": 79.8,
		"skuPrice": 79.8,
		"skuCategory": "BOOKS",
		"skuName": "Java编程思想",
		"totalNum": 1,
		"skuId": 654121
	}, {
		"totalPrice": 149.8,
		"skuPrice": 149.8,
		"skuCategory": "BOOKS",
		"skuName": "Java核心技术",
		"totalNum": 1,
		"skuId": 654122
	}, {
		"totalPrice": 299,
		"skuPrice": 299,
		"skuCategory": "CLOTHING",
		"skuName": "睡衣套装",
		"totalNum": 1,
		"skuId": 654021
	}],
	"true": [{
		"totalPrice": 4999,
		"skuPrice": 4999,
		"skuCategory": "ELECTRONICS",
		"skuName": "无人机",
		"totalNum": 1,
		"skuId": 654032
	}, {
		"totalPrice": 2299,
		"skuPrice": 2299,
		"skuCategory": "ELECTRONICS",
		"skuName": "VR一体机",
		"totalNum": 1,
		"skuId": 654033
	}, {
		"totalPrice": 3999,
		"skuPrice": 3999,
		"skuCategory": "SPORTS",
		"skuName": "跑步机",
		"totalNum": 1,
		"skuId": 654221
	}, {
		"totalPrice": 1999,
		"skuPrice": 1999,
		"skuCategory": "CLOTHING",
		"skuName": "牛仔裤",
		"totalNum": 1,
		"skuId": 654022
	}]
}