我想在此塊中轉換為Java 8流嗎？

Question

如何在此塊中轉換為Java 8流語法？

List<Product> tmpList = new ArrayList<>();
tmpList.add(new Product("prod1", "cat2", "t1", 100.23, 50.23));
tmpList.add(new Product("prod2", "cat1", "t2", 50.23, 50.23));
tmpList.add(new Product("prod1", "cat1", "t3", 200.23, 100.23));
tmpList.add(new Product("prod3", "cat2", "t1", 150.23, 50.23));
tmpList.add(new Product("prod1", "cat2", "t1", 100.23, 10.23));
Map<String, List<Product>> proMap = tmpList.stream().collect(Collectors.groupingBy(Product::getName));

//start 
List<Product> productList = new ArrayList<>(); // 

for(String productName : proMap.keySet()) {
    double totalCostIn = proMap.get(productName).stream().mapToDouble(Product::getCostIn).sum();
    double totalCostOut = proMap.get(productName).stream().mapToDouble(Product::getCostOut).sum();
    productList.add(new Product(productName,totalCostIn,totalCostOut));
}

// how to convert to java 8 stream grammer in this block ?
List<Product> productList = proMap.entrySet().stream()...

Answer 1

你可以這樣做

Collection<Product> productsByName = tmpList.stream()
    .collect(Collectors.toMap(Product::getName, 
        Function.identity(), 
        (p1, p2) -> new Product(p1.getName(),
            p1.getCostIn() + p2.getCostIn(), p1.getCostOut() + p2.getCostOut())))
    .values();

Answer 2

您可以在流式傳輸時map和collect

List<Product> productList = proMap.keySet().stream()
        .map(productName -> new Product(productName,
                proMap.get(productName).stream().mapToDouble(Product::getCostIn).sum(),
                proMap.get(productName).stream().mapToDouble(Product::getCostOut).sum()))
        .collect(Collectors.toList());

在另一方面，如果你要查找海東青/ costOut給定的產品名稱，你可能已經直接存儲這些的總和與特定的產品名稱，而groupingBy ，如

Map<String, Double> costIn = tmpList.stream()
        .collect(Collectors.groupingBy(Product::getName,
                Collectors.summingDouble(Product::getCostIn)));

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