java list<>循环_Java 7和Java 8对list的双层循环处理

一、准备

Java 处理 List 的双层循环，一般都是当两个 List 某个值满足某条件时候，进行相应的处理。需求：两个 List 对象当 id 相同的时候就组建成美好的家庭，Husband 对象与 Wife 对象组建成 Family 对象。

Husband 对象如下：

import io.swagger.annotations.ApiModel;

import io.swagger.annotations.ApiModelProperty;

import lombok.AllArgsConstructor;

import lombok.Data;

import lombok.NoArgsConstructor;

@Data

@NoArgsConstructor

@AllArgsConstructor

@ApiModel(value = "丈夫")

public class Husband {

@ApiModelProperty(value = "家庭ID")

private int familyId;

@ApiModelProperty(value = "丈夫名称")

private String husbandName;

@ApiModelProperty(value = "妻子名称")

private String wifeName;

}

Wife 对象如下：

import io.swagger.annotations.ApiModel;

import io.swagger.annotations.ApiModelProperty;

import lombok.AllArgsConstructor;

import lombok.Data;

import lombok.NoArgsConstructor;

@Data

@NoArgsConstructor

@AllArgsConstructor

@ApiModel(value = "妻子")

public class Wife {

@ApiModelProperty(value = "家庭ID")

private int familyId;

@ApiModelProperty(value = "丈夫名称")

private String husbandName;

@ApiModelProperty(value = "妻子名称")

private String wifeName;

}

Family 对象如下：

import io.swagger.annotations.ApiModel;

import io.swagger.annotations.ApiModelProperty;

import lombok.AllArgsConstructor;

import lombok.Data;

import lombok.NoArgsConstructor;

@Data

@NoArgsConstructor

@AllArgsConstructor

@ApiModel(value = "家庭")

public class Family {

@ApiModelProperty(value = "家庭ID")

private int familyId;

@ApiModelProperty(value = "丈夫名称")

private String husbandName;

@ApiModelProperty(value = "妻子名称")

private String wifeName;

}

二、对比

import java.util.ArrayList;

import java.util.List;

import java.util.Map;

import java.util.stream.Collectors;

public class FamilyDemo {

public static void main(String[] args) {

List wives = new ArrayList<>();

List husbands = new ArrayList<>();

for (int i = 0; i < 10000; i++) {

wives.add(new Wife(i, i + "的妻子", "000" + i));

}

for (int i = 0; i < 10000; i++) {

husbands.add(new Husband(i, "我是" + i, "000" + i));

}

FamilyDemo familyDemo = new FamilyDemo();

familyDemo.testJava7(wives, husbands);

familyDemo.testJava8(wives, husbands);

}

public void testJava7(List wives, List husbands) {

List families = new ArrayList<>();

for (int i = 0; i < wives.size(); i++) {

Wife wife = wives.get(i);

for (int j = 0; j < husbands.size(); j++) {

Husband husband = husbands.get(j);

if (wife.getFamilyId() == husband.getFamilyId()) {

Family family = new Family();

family.setFamilyId(husband.getFamilyId());

family.setHusbandName(husband.getHusbandName());

family.setWifeName(husband.getWifeName());

families.add(family);

}

}

}

System.out.println("Java7结合家庭数量：" + families.size());

System.out.println(families);

}

public void testJava8(List wives, List husbands) {

List families = new ArrayList<>();

//将list转为Map，这里key必须是唯一的，即familyId

Map wifeMap = wives.stream().collect(Collectors.toMap(w -> w.getFamilyId(), w -> w));

families = husbands.stream().map(h -> {

Family family = new Family();

Wife wife = wifeMap.get(h.getFamilyId());

family.setFamilyId(wife.getFamilyId());

family.setHusbandName(wife.getHusbandName());

family.setWifeName(wife.getWifeName());

return family;

}).collect(Collectors.toList());

System.out.println("Java8完美结合家庭数量：" + families.size());

families.stream().forEach(family -> {

System.out.println("家庭ID：" + family.getFamilyId());

});

}

}

对比下来，stream 效率明显提升了很多，尤其数据量大的时候效率提升得越高。