Stream流中将集合转成map，重复key处理，统计最大值，获取某个属性集合等12种最常用方法

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import com.alibaba.fastjson.JSON;
import org.assertj.core.util.Lists;
​
import java.math.BigDecimal;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;
​
/**
 * @author gzh -》javaNice
 * *
 */
public class StreamDemo {
    public static void main(String[] args) {
        User user1=new User("1","zhangsan",1);
        User user2=new User("2","zhangsan",2);
        User user3=new User("3","lisi",2);
​
        List<User> userList= Lists.newArrayList();
        userList.add(user1);
        userList.add(user2);
        userList.add(user3);
​
​
        //1、list转map，指定key-value，key,value是对象中的某个属性值.
        Map<String,String> userMap1 = userList.stream().collect(Collectors.toMap(User::getId,User::getName));
        System.out.println("1->"+userMap1);
​
        //2、list转map 指定key-value，key为属性值，value是对象本身
        Map<String,User> userMap2 = userList.stream().collect(Collectors.toMap(User::getId,User->User));
        System.out.println("2->"+userMap2);
​
        //3、list转map 指定key-value，value是对象本身，Function.identity()是简洁写法，返回对象本身
        Map<String,User> userMap3 = userList.stream().collect(Collectors.toMap(User::getId, Function.identity()));
        System.out.println("3->"+userMap3);
​
        //4、指定key-value，value是对象本身，Function.identity()是简洁写法，返回对象本身，遇到key 冲突的解决办法，选择第二个key的值value2覆盖第一个key的值value1。
        Map<String,User> userMap4 = userList.stream().collect(Collectors.toMap(User::getName, Function.identity(),(value1,value2)->value2));
        System.out.println("4->"+userMap4);
​
        //5、指定key-value，按照name进行分组 value是对象集合
        Map<String, List<User>> userMap5 = userList.stream().collect(Collectors.groupingBy(User::getName));
        System.out.println("5->"+userMap5);
​
        //6.统计所有age
        Integer totalAge = userList.stream().map(User::getAge).reduce(0, Integer::sum);
        System.out.println("6总年龄->"+totalAge);
​
        //7.只对名称为zhangsan的age  进行统计汇总
        Integer totalAgeFilter = userList.stream().filter(t -> t.getName().equals("zhangsan")).map(User::getAge).reduce(0, Integer::sum);
        System.out.println("7总年龄filter->"+totalAgeFilter);
​
        //8.获取最大age
        Integer maxAge = userList.stream().map(User::getAge).max((x, y) -> x - y).get();
        System.out.println("8最大年龄->"+maxAge);
​
        //9.循环打印
        userList.stream().forEach(user -> System.out.println("9->"+user));
​
        //10.获取名称集合
        List<String> nameList = userList.stream().map(User::getName).collect(Collectors.toList());
        System.out.println("10名称集合->"+ JSON.toJSONString(nameList));
​
        //11.key为多字段拼接，value为某个属性
        Map<String, Integer> map = userList.stream()
                .collect(Collectors.toMap(p -> p.getName() + p.getAge() + p.getId(), User::getAge));
        System.out.println("11->"+ JSON.toJSONString(map));
​
        //12.按多字段分组
        Map<String, List<User>> nameAgeMap = userList.stream()
                .collect(Collectors.groupingBy(l -> l.getName() +"-" + l.getAge()));
        System.out.println("12->"+ JSON.toJSONString(nameAgeMap));
​
​
    }
}