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The above Java8 | how to initialize the Map gracefully & List2Map?” How to initialize a Map and List2Map. There are lots of good suggestions for initializing a Map. In this paper, the Group By processing of set data is likely to be used in the actual development, and various methods are practiced to solve the problem.

First, development issues

Many collections of data grouping are quite common in real development, such as the grouping of product data by type that is passed to the front end. The most common way to do this is to traverse the entire collection and then store collections of different types by determining type constructs. So is there a better way,Java8 groupingBy can help us.

Similarly, suppose we have a class Product, as shown in the following code.

@Getter
@Setter
@ToString
@Builder
class Product{
    private Long id;
    private String category;
    private String name;
    private int count;
}
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We now have data in the List format. 

static List<Product> getList(a){
    final List<Product> productList = new ArrayList<>(100);
    for(int i =1; i<=100; i++){ productList.add(Product.builder() .id((long) i)
            .name("name"+i)
            .category("category"+i%9)
            .count(i)
            .build());
    }
    return productList;
}
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Now let’s do some complicated processing with this data.

Two, the simplest single column processing

Group by category type.

final Map<String, List<Product>> maps = productList.stream()
    .collect(Collectors.groupingBy(Product::getCategory));
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For the Collectors. GroupingBy parameter Function
classifier is the Key.

3. Count the total number of specified columns after grouping

Group by category type and count the total count for each type.

Map<String, Integer> maps = productList.stream()
        .collect(Collectors.groupingBy(Product::getCategory, summingInt(Product::getCount)));
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Follow the code above to get the sum from the grouping results. Here is the aggregation operation, if you want to do filtering, for example to see the largest number of products under the type group how to do? Use maxBy(comparingInt(*)).

Map<String, Optional<Product>> maps = productList.stream()
        .collect(Collectors.groupingBy(Product::getCategory, maxBy(comparingInt(Product::getCount))));
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Using these existing JDK methods to convert data formats in memory can greatly improve development efficiency.

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