java - 将一个list按照下面字段分组后放入到一个新的list里面

Question

像按以下格式展现内容

Answer 1

I don’t know what your POJO object is called, I named it Project

@Getter
@Setter
@NoArgsConstructor
public class Project {
    private Integer uud;
    private String regCode;
    private String projectName;
    private String projectAddress;
    private String companyName;
    
    // 设置分组的key，这里就是把你想要分组的key拼起来
    public String groupKey(){
        return this.projectName + "_" + this.projectAddress + "_" + this.companyName;
    }
}

Then use the Collectors.groupingBy method to group, as follows:

        List<Project> projects = new ArrayList<>();
        // 这里的key就是，宁江大院_成都市都江堰市蒲阳镇花溪村_成都文森电梯设备股份有限公司
        Map<String, List<Project>> group = projects.stream().collect(Collectors.groupingBy(Project::groupKey));

We get a map here, the key is the grouping basis we just put together, and the value is a list, which is the collection under the grouping

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Your display is on the page... The map has been divided into groups. You can just cycle through the map according to the rules of your page... I can only process it based on the POJO objects I created.

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If there is no way to use Java8, then make a similar map classification yourself, similar to the code below

        List<Project> projects = new ArrayList<>();
        Map<String, List<Project>> map = new HashMap<>();
        for (Project project: projects){
            String key = project.groupKey();
            // 按照key取出子集合
            List<Project> subProjects = map.get(key);
            
            // 若子集合不存在，则重新创建一个新集合，并把当前Project加入，然后put到map中
            if (subProjects == null){
                subProjects = new ArrayList<>();
                subProjects.add(project);
                map.put(key, subProjects);
            }else {
                // 若子集合存在，则直接把当前Project加入即可
                subProjects.add(project);
            }
        }

Answer 2

I don’t know if these rows are not repeated after your grouping. For example, SELECT projectName FROM table name GROUP BY projectName; and then it is found to be a resultSet. You instantiate an ArrayList and then loop through it and add it to the list. I don’t know what I’m talking about. Is it what you thought?

Answer 3

Collection.sort(list,new Compararor<>(){

})