导入jar包 <dependency> <groupId>com.alibaba</groupId> <artifactId>fastjson</artifactId> <version>1.2.62</version> </dependency>
<dependency> <groupId>com.google.code.gson</groupId> <artifactId>gson</artifactId> <version>2.3.1</version> </dependency> model类
代码语言：javascript复制import java.math.BigDecimal;
public class Product {
Long id;
Integer num;
BigDecimal price;
String name;
String category;
public Product(Long id, Integer num, BigDecimal price, String name, String category) {
this.id = id;
this.num = num;
this.price = price;
this.name = name;
this.category = category;
}
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public Integer getNum() {
return num;
}
public void setNum(Integer num) {
this.num = num;
}
public BigDecimal getPrice() {
return price;
}
public void setPrice(BigDecimal price) {
this.price = price;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getCategory() {
return category;
}
public void setCategory(String category) {
this.category = category;
}
}import com.fasterxml.jackson.databind.util.JSONPObject;
import com.google.gson.Gson;
import org.junit.Test;
import org.springframework.boot.autoconfigure.gson.GsonBuilderCustomizer;
import org.springframework.boot.json.GsonJsonParser;
import org.springframework.http.converter.json.GsonBuilderUtils;
import java.math.BigDecimal;
import java.util.*;
import java.util.stream.Collectors;
/**
* https://blog.csdn.net/u014231523/article/details/102535902
*/
public class GroupingByTest {
@Test
public void testGroupingByTest(){
Product prod1 = new Product(1L, 1, new BigDecimal("15.5"), "面包", "零食");
Product prod2 = new Product(2L, 2, new BigDecimal("20"), "饼干", "零食");
Product prod3 = new Product(3L, 3, new BigDecimal("30"), "月饼", "零食");
Product prod4 = new Product(4L, 3, new BigDecimal("10"), "青岛啤酒", "啤酒");
Product prod5 = new Product(5L, 10, new BigDecimal("15"), "百威啤酒", "啤酒");
// List<Product> prodList = Lists.newArrayList(prod1, prod2, prod3, prod4, prod5);
List<Product> prodList = new ArrayList<Product>();
prodList.add(prod1);
prodList.add(prod2);
prodList.add(prod3);
prodList.add(prod4);
prodList.add(prod5);
Map<String, List<Product>> prodMap= prodList.stream().collect(Collectors.groupingBy(Product::getCategory));
System.out.println("prodMap=" new Gson().toJson(prodMap));
/**
* prodMap={"啤酒":[{"id":4,"num":3,"price":10,"name":"青岛啤酒","category":"啤酒"},{"id":5,"num":10,"price":15,"name":"百威啤酒","category":"啤酒"}],
* "零食":[{"id":1,"num":1,"price":15.5,"name":"面包","category":"零食"},{"id":2,"num":2,"price":20,"name":"饼干","category":"零食"},{"id":3,"num":3,"price":30,"name":"月饼","category":"零食"}]}
*/
Map<String, List<Product>> prodMap2 = prodList.stream().collect(Collectors.groupingBy(item -> item.getCategory() "_" item.getName()));
System.out.println("prodMap2=" new Gson().toJson(prodMap2));
/**
* prodMap2={"零食_月饼":[{"id":3,"num":3,"price":30,"name":"月饼","category":"零食"}],
* "零食_面包":[{"id":1,"num":1,"price":15.5,"name":"面包","category":"零食"}],
* "啤酒_百威啤酒":[{"id":5,"num":10,"price":15,"name":"百威啤酒","category":"啤酒"}],
* "啤酒_青岛啤酒":[{"id":4,"num":3,"price":10,"name":"青岛啤酒","category":"啤酒"}],
* "零食_饼干":[{"id":2,"num":2,"price":20,"name":"饼干","category":"零食"}]}
*/
Map<String, List<Product>> prodMap3= prodList.stream().collect(Collectors.groupingBy(item -> {
if(item.getNum() < 3) {
return "3";
}else {
return "other";
}
}));
System.out.println("prodMap3=" new Gson().toJson(prodMap3));
/**
* prodMap3={"other":[{"id":3,"num":3,"price":30,"name":"月饼","category":"零食"},
* {"id":4,"num":3,"price":10,"name":"青岛啤酒","category":"啤酒"},
* {"id":5,"num":10,"price":15,"name":"百威啤酒","category":"啤酒"}],
* "3":[{"id":1,"num":1,"price":15.5,"name":"面包","category":"零食"},{"id":2,"num":2,"price":20,"name":"饼干","category":"零食"}]}
*/
Map<String, Map<String, List<Product>>> prodMap4= prodList.stream().collect(Collectors.groupingBy(Product::getCategory, Collectors.groupingBy(item -> {
if(item.getNum() < 3) {
return "3";
}else {
return "other";
}
})));
System.out.println("prodMap4=" new Gson().toJson(prodMap4));
/**
* prodMap4={"啤酒":{"other":[{"id":4,"num":3,"price":10,"name":"青岛啤酒","category":"啤酒"},{"id":5,"num":10,"price":15,"name":"百威啤酒","category":"啤酒"}]},
* "零食":{"other":[{"id":3,"num":3,"price":30,"name":"月饼","category":"零食"}],
* "3":[{"id":1,"num":1,"price":15.5,"name":"面包","category":"零食"},{"id":2,"num":2,"price":20,"name":"饼干","category":"零食"}]}}
*/
Map<String, Long> prodMap5 = prodList.stream().collect(Collectors.groupingBy(Product::getCategory, Collectors.counting()));
System.out.println("prodMap5=" new Gson().toJson(prodMap5));
/**
* prodMap5={"啤酒":2,"零食":3}
*/
Map<String, Integer> prodMap6 = prodList.stream().collect(Collectors.groupingBy(Product::getCategory, Collectors.summingInt(Product::getNum)));
System.out.println("prodMap6=" new Gson().toJson(prodMap6));
/**
* prodMap6={"啤酒":13,"零食":6}
*/
Map<String, Product> prodMap7 = prodList.stream().collect(Collectors.groupingBy(Product::getCategory, Collectors.collectingAndThen(Collectors.maxBy(Comparator.comparingInt(Product::getNum)), Optional::get)));
System.out.println("prodMap7=" new Gson().toJson(prodMap7));
/**
* prodMap7={"啤酒":{"id":5,"num":10,"price":15,"name":"百威啤酒","category":"啤酒"},
* "零食":{"id":3,"num":3,"price":30,"name":"月饼","category":"零食"}}
*/
Map<String, Set<String>> prodMap8 = prodList.stream().collect(Collectors.groupingBy(Product::getCategory, Collectors.mapping(Product::getName, Collectors.toSet())));
System.out.println("prodMap8=" new Gson().toJson(prodMap8));
/**
* prodMap8={"啤酒":["青岛啤酒","百威啤酒"],"零食":["面包","饼干","月饼"]}
*/
}
}
