java 中的错误结果 Collectors.groupingBy

Question

我有 file.txt 我想按我正在使用的 ip 对数据进行分组 Collectors.groupingBy

文件数据

333.000.000,Newyork,50
200.000.000,china,200
333.000.000,brazil,150
444.000.000,japon,40
200.000.000,icland,400

我使用 Collectors.groupingBy 对数据进行分组，所以我需要这样显示结果：

333.000.000=[Newyork,brazil]
200.000.000=[china,icland]
444.000.000=[japon]

probelme 显示结果如下：

333.000.000=[class_ip@5b6f7412,class_ip@8b6f7412]
200.000.000=[class_ip@312b1dae,class_ip@6b7f7412]
444.000.000=[class_ip@7530d0]

我的代码

class class_ip{
    private String ip;
    private String title;
    public class_ip(String ip,String title) {
        this.ip = ip;        
        this.title = title;
    }
    public String getIP() {return ip;}
    public String getTitle() {return title;}

    public void setTitle(String title) { this.title = title;}
    public void setIP(String ip) { this.ip = ip;}

    public String getAll() {
        return ip+","+title;
    }
} 

     List<class_ip> array_ip = new ArrayList<>();

    // read data from file
    while ((strLine = br.readLine()) != null)   {
                array_ip.add(new class_ip(ip,title));
    }

   Map<String,List<class_ip>> groupByIP = new HashMap<>();
   groupByIP =array_ip.stream().collect(Collectors.groupingBy(class_ip::getIP)); 

   System.out.println(groupByIP);

Answer 1

如果要显示标题而不是 class_ip 实例，请使用 Collectors.mapping 并生成 Map<String,List<String>>:

Map<String,List<String>> groupByIP = 
    array_ip.stream()
            .collect(Collectors.groupingBy(class_ip::getIP,
                                           Collectors.mapping(class_ip::getTitle,
                                                              Collectors.toList())));

Answer 2

默认返回class_ip@312b1daeObject#toString()，需要在class_ip中重写，例如：

@Override
public String toString() {
    return getTitle();
}

java 中的错误结果 Collectors.groupingBy

wrong result Collectors.groupingBy in java

java

arrays

collectors