wrong result Collectors.groupingBy in java

Question

i have file.txt i would to group data by ip i'm using Collectors.groupingBy

file data

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

i use Collectors.groupingBy to group data so i need to show result like this :

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

the probelme is show result like this :

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

My code

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

If you want to display the titles instead of the class_ip instances, use Collectors.mapping and generate a 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())));