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Java8函数式编程之Lambda与传统方式对集合操作的案例对比

本章节中，我们通过一个实战案例对比原始集合操作与Stream集合操作具体有哪些不同，直观地展示Stream集合操作对编程效率的提升。

案例：对比原始集合操作与Stream集合操作

需求场景：

针对上面的购物车，我们想要

全局查看购物车中都有哪些商品
将购物车中的图书类商品进行过滤（删除图书类商品）
在其余商品中挑选两件最贵的
打印出上述两件商品的名称和总价

原始集合操作：

@Test
public void traditionalWay() {
    // 1. 打印所有商品
    List<Sku> skus = CartService.getCartSkuList();
    for (Sku sku : skus) {
        System.out.println(JSON.toJSONString(sku, true));
    }
    // 2. 过滤图书类商品
    List<Sku> notIncludeBooksList = new ArrayList<>();
    for (Sku sku : skus) {
        if (!sku.getSkuCategory().equals(SkuCategoryEnum.BOOKS)) {
            notIncludeBooksList.add(sku);
        }
    }
    // 3. 其余商品中挑选两件最贵的 价格倒排序，取top2
    // 3.1 先排序
    notIncludeBooksList.sort(new Comparator<Sku>() {
        @Override
        public int compare(Sku sku0, Sku sku1) {
            if (sku0.getTotalPrice() > sku1.getTotalPrice()) {
                return -1;
            }
            if (sku0.getTotalPrice() < sku1.getTotalPrice()) {
                return 1;
            }
            return 0;
        }
    });

    // 3.2 取top2
    List<Sku> top2SkuList = new ArrayList<>();
    for (int i = 0; i < 2; i++) {
        top2SkuList.add(notIncludeBooksList.get(i));
    }
    // 4. 打印出上述两件商品的名称和总价
    // 4.1 求两件商品总价
    double totalMoney = 0.0;
    for (Sku sku : top2SkuList) {
        totalMoney += sku.getTotalPrice();
    }
    // 4.2 获取两件商品名称
    List<String> resultSkuNameList = new ArrayList<>();
    for (Sku sku : top2SkuList) {
        resultSkuNameList.add(sku.getSkuName());
    }
    // 打印输出结果
    System.out.println("结果商品名称: " + JSON.toJSONString(resultSkuNameList, true));
    System.out.println("商品总价:" + totalMoney);
}

运行结果：

{"skuCategory":"ELECTRONICS","skuId":2,"skuName":"无人机","skuPrice":1000.0,"totalNum":10,"totalPrice":1000.0}
{"skuCategory":"ELECTRONICS","skuId":1,"skuName":"VR一体机","skuPrice":2100.0,"totalNum":10,"totalPrice":2100.0}
{"skuCategory":"CLOTHING","skuId":4,"skuName":"牛仔裤","skuPrice":60.0,"totalNum":10,"totalPrice":60.0}
{"skuCategory":"CLOTHING","skuId":13,"skuName":"衬衫","skuPrice":120.0,"totalNum":10,"totalPrice":120.0}
{"skuCategory":"BOOKS","skuId":121,"skuName":"Java编程思想","skuPrice":100.0,"totalNum":10,"totalPrice":100.0}
{"skuCategory":"BOOKS","skuId":3,"skuName":"程序化广告","skuPrice":80.0,"totalNum":10,"totalPrice":80.0}
结果商品名称: [
    "VR一体机",
    "无人机"
]
商品总价:3100.0

我们可以看到传统的集合操作还是写了比较多的代码，而且在编码过程中为了满足各种要求，我们通过声明新的容器来接受过程中的操作结果，这带来了内存使用量的增加。

接下来看一下Stream方式下如何编码实现我们的需求：

Stream集合操作：

@Test
public void streamWay() {
    AtomicReference<Double> money = new AtomicReference<>(Double.valueOf(0.0));
    List<String> resultSkuNameList = CartService.getCartSkuList()
                    // 获取集合流
            .stream()
            /**1. 打印商品信息*/
            .peek(sku -> System.out.println(JSON.toJSONString(sku)))
            /**2. 过滤掉所有的图书类商品*/
            .filter(sku -> !SkuCategoryEnum.BOOKS.equals(sku.getSkuCategory()))
            /**3. 价格进行排序，默认是从小到大，调用reversed进行翻转排序即从大到小*/
            .sorted(Comparator.comparing(Sku::getTotalPrice).reversed())
            /**4. 取top2*/
            .limit(2)
            /**累加金额*/
            .peek(sku -> money.set(money.get() + sku.getTotalPrice()))
            /**获取商品名称*/
            .map(sku -> sku.getSkuName())
            .collect(Collectors.toList());
    System.out.println("商品总价:" + money.get());
    System.out.println("商品名列表:" + JSON.toJSONString(resultSkuNameList));
}

运行结果：

{"skuCategory":"ELECTRONICS","skuId":2,"skuName":"无人机","skuPrice":1000.0,"totalNum":10,"totalPrice":1000.0}
{"skuCategory":"ELECTRONICS","skuId":1,"skuName":"VR一体机","skuPrice":2100.0,"totalNum":10,"totalPrice":2100.0}
{"skuCategory":"CLOTHING","skuId":4,"skuName":"牛仔裤","skuPrice":60.0,"totalNum":10,"totalPrice":60.0}
{"skuCategory":"CLOTHING","skuId":13,"skuName":"衬衫","skuPrice":120.0,"totalNum":10,"totalPrice":120.0}
{"skuCategory":"BOOKS","skuId":121,"skuName":"Java编程思想","skuPrice":100.0,"totalNum":10,"totalPrice":100.0}
{"skuCategory":"BOOKS","skuId":3,"skuName":"程序化广告","skuPrice":80.0,"totalNum":10,"totalPrice":80.0}
商品总价:3100.0
商品名列表:["VR一体机","无人机"]

我们可以看到，通过Stream集合操作，运行结果与传统集合操作完全一致。但是编码量却能够显著减少。

辩证的分析一下，如果对Stream操作没有一个较为明确的了解，阅读这段代码确实有些难度，但是只要有一点了解，Stream集合操作代码带来的无论是编码量显著降低还是可读性提升，亦或是内存空间的节约都是可观的。