Notes

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.LinkedHashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Set;


public class MusicAnalyser {
static class StringPair{
private String string1;
private String string2;

public StringPair(String s1, String s2){
string1 = s1;
string2 = s2;
}

public String getString1() {
return string1;
}

public String getString2() {
return string2;
}

@Override
public int hashCode() {
return string1.hashCode() + string2.hashCode();
}

@Override
public boolean equals(Object obj) {
if(obj instanceof StringPair){
StringPair other = (StringPair)obj;
return (string1.equals(other.string1)&&string2.equals(other.string2))||(string1.equals(other.string2)&&string2.equals(other.string1));
}else{
return super.equals(obj);
}
}
}

public static final int SIZE = 50;

public static void main(String[] args) throws IOException {
Map<String, List<Integer>> artistIndex = new HashMap<String, List<Integer>>();

BufferedReader br = new BufferedReader(new FileReader("Artist_lists_small.txt"));
try {
String line = br.readLine();
Integer lineIndex = 1;
System.out.println("Indexing .. ");
while (line != null) { // making map artist_name -> [line_where_appear1, ..., line_where_appear_n]
String[] artists = line.split(",");
for(String artist : artists){
if(artistIndex.get(artist) == null){
artistIndex.put(artist, new ArrayList<Integer>());
}
artistIndex.get(artist).add(lineIndex);
}
line = br.readLine();
}

int artistId = 1;
int totalArtists = artistIndex.keySet().size();

Set<StringPair> result = new LinkedHashSet<StringPair>();
for(String artist1 : artistIndex.keySet()){ // searching for artists which have line number list intersections bigger than 50
System.out.println("Searching : " + (artistId*100.0f/totalArtists) + "% for " + artist1);
artistId++;

for(String artist2 : artistIndex.keySet()){
if(!artist1.equals(artist2)){ // ignoring same entries
int intercetionSize = 0;
List<Integer> artist1Indexes = artistIndex.get(artist1);
List<Integer> artist2Indexes = new LinkedList<Integer>(artistIndex.get(artist2));

artist2Indexes.retainAll(artist1Indexes);
if(artist2Indexes.size() > SIZE){
result.add(new StringPair(artist1, artist2));
break;
}
}
}
}
System.out.println("Total artists : " + artistIndex.keySet().size());
System.out.println("Matched artists : " + result.size());

System.out.println("Result with intercection size " + SIZE + " :");
} finally {
br.close();
}
}

}