java stream reduc的一点用法

随便写个简单启动项目, springboot 或者 控制台
随便两个简单class.
​​​ReqDTO.java​​

@Data
public class ReqDTO {

    private List<ReqModel> list;

}

​​ReqModel.java​​

@Data
@AllArgsConstructor
@NoArgsConstructor
public class ReqModel {

    private Integer age;

    private String name;
}

随便弄个启动方法

package cn.codejam.demo.controller;

import cn.xuetian.demo.dto.ReqDTO;
import cn.xuetian.demo.dto.ReqModel;
import com.google.common.collect.Lists;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import java.util.List;

/**
 * @author zss
 * @date 2022/9/14 10:54

 */
@RestController
@RequestMapping("/test1")
public class Test1Controller {

    @GetMapping("/test1")
    public String test1(@RequestBody ReqDTO req) {

        List<ReqModel> list = Lists.newArrayList(
                new ReqModel(1, "aaa"), new ReqModel(2, "bbb"), new ReqModel(3, "ccc")
        );


        ReqModel result = list.stream().reduce(
                new ReqModel(100, "AA"),
                (reqModel, reqModel2) -> {
                    int i = reqModel.getAge() + reqModel2.getAge();
                    return new ReqModel(i, reqModel.getName() + reqModel2.getName());
                }
        );

        ReqModel result1 = list.stream().reduce(

                (reqModel, reqModel2) -> {
                    int i = reqModel.getAge() + reqModel2.getAge();
                    return new ReqModel(i, reqModel.getName() + reqModel2.getName());
                }
        ).orElse(null);


        //其中,   BinaryOperator<U> combiner, combiner在串行流中不起作用而在并行流中起作用：
        ReqModel result2 = list.parallelStream().reduce(
                new ReqModel(100, "AA"),
                (reqModel, reqModel2) -> {
                    int i = reqModel.getAge() + reqModel2.getAge();
                    return new ReqModel(i, reqModel.getName() + reqModel2.getName());
                },
                (reqModel, reqModel2) -> {
                    int i = reqModel.getAge() + reqModel2.getAge();
                    return new ReqModel(i, reqModel.getName() + reqModel2.getName());
                }
        );

        return com.alibaba.fastjson.JSON.toJSONString(result);
    }
}

调试一下, 看看效果就懂了.

总结

其中, 前两个比较像.
以累加为例: 区别就是前面的 identity其实就是一个额外项, 也会累加进去. 顺便保证返回总是有内容的,不为null.

第三个参数比较复杂的,
只用记住一句话就够了: ​​​BinaryOperator<U> combiner, combiner在串行流中不起作用而在并行流中起作用​​