哈哈，當我還是學生時自學PS披的第一張圖就拿來發布第一篇文章

在日常工作中我們都會遇到將數據進行分組的需求

那麼在Java 中如何將簡單又方便的將list集合進行分組呢？
在java8特性中我們可以使用stream流很方便的對集合進行操作
下面將以兩種方式進行分組操作（java8中還有其他操作可以分組，這裡只介紹兩種，其他方式大同小異）

list 分組歸類代碼

import com.alibaba.fastjson.JSON;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

/**
 * @author caizw
 */
public class Foo {
 private String name;
 private String type;
 private Double typeValue;
 private Integer count;

 public Foo(String name, String type, Double typeValue, Integer count) {
 this.name = name;
 this.type = type; 

 this.typeValue = typeValue;
 this.count = count;
 }

 public String getName() {
 return name;
 }

 public void setName(String name) {
 this.name = name;
 }

 public String getType() {
 return type;
 }

 public void setType(String type) {
 this.type = type;
 }

 public Double getTypeValue() {
 return typeValue;
 }

 public void setTypeValue(Double typeValue) {
 this.typeValue = typeValue;
 }

 public Integer getCount() {
 return count;
 }

 public void setCount(Integer count) {
 this.count = count;
 }

 @Override
 public String toString() {
 return "Foo{" +
 "name='" + name + '\\'' +
 ", type='" + type + '\\'' +
 ", typeValue=" + typeValue +
 ", count=" + count +
 '}';
 }

 public static void main(String[] args) {
 List fooList = new ArrayList<>();
 fooList.add(new Foo("A", "san", 1.0, 2)); 

 fooList.add(new Foo("A", "nas", 13.0, 1));
 fooList.add(new Foo("B", "san", 112.0, 3));
 fooList.add(new Foo("C", "san", 43.0, 5));
 fooList.add(new Foo("B", "nas", 77.0, 7));
 List<list>> groupList = new ArrayList<>();

 // 分組方式一
 fooList.stream()
 .collect(Collectors.groupingBy(Foo::getName, Collectors.toList()))
 .forEach((name, fooListByName) -> {
 groupList.add(fooListByName);
 });
\t
// 分組方式二
 Map<string>> groupMap = fooList.stream().collect(Collectors.groupingBy(Foo::getName));

 System.out.println(JSON.toJSONString(groupList));

 System.out.println(JSON.toJSONString(groupMap));
 }
}/<string>/<list>

輸出結果：

groupList

[
 [
 {
 "count": 2,
 "name": "A",
 "type": "san",
 "typeValue": 1
 },
 {
 "count": 1,
 "name": "A",
 "type": "nas",
 "typeValue": 13
 }
 ],
 [
 {
 "count": 3, 

 "name": "B",
 "type": "san",
 "typeValue": 112
 },
 {
 "count": 7,
 "name": "B",
 "type": "nas",
 "typeValue": 77
 }
 ],
 [
 {
 "count": 5,
 "name": "C",
 "type": "san",
 "typeValue": 43
 }
 ]
]

groupMap

{
 "A": [
 {
 "count": 2,
 "name": "A",
 "type": "san",
 "typeValue": 1
 },
 {
 "count": 1,
 "name": "A",
 "type": "nas",
 "typeValue": 13
 }
 ],
 "B": [
 {
 "count": 3,
 "name": "B",
 "type": "san",
 "typeValue": 112
 },
 {
 "count": 7,
 "name": "B", 

 "type": "nas",
 "typeValue": 77
 }
 ],
 "C": [
 {
 "count": 5,
 "name": "C",
 "type": "san",
 "typeValue": 43
 }
 ]
}