I have a HashMap<Integer, List<String>> as input, e.g :
3 : a, b, c
6 : b, c
9 : a, c
12 : b ,c
I want to convert it to HashMap<String, HashSet<Integer>> for e.g.
a : 3, 9
b : 3, 6, 12
c : 3, 6, 9, 12
How can it be done ?
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Similar tasks have been covered on Stack Overflow before, with good answers, but they are hard to search for (I didn’t find an example readily). I still encourage you to try your search skills and luck.Anonymous– Anonymous2017-05-30 13:41:35 +00:00Commented May 30, 2017 at 13:41
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1Possible duplicate of Java8 streams : Transpose map with values as listAnonymous– Anonymous2017-05-30 13:43:39 +00:00Commented May 30, 2017 at 13:43
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@OleV.V. Thanks for the link. This answer did help me reaching the final solution but still required some tweak. Map Response value is a HashSet instead of List. I think it can help a beginner like me. Please Let me know if you disagree.saurabh agarwal– saurabh agarwal2017-05-30 20:22:33 +00:00Commented May 30, 2017 at 20:22
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I noticed that you answered your own question. I was happy about that and hurried to upvote your answer. I believe it deserves to be here. Very glad that my link helped.Anonymous– Anonymous2017-05-30 20:25:48 +00:00Commented May 30, 2017 at 20:25
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4 Answers
This should help
Map<Integer, List<String>> initMap = new HashMap<>();
initMap.put(3, new ArrayList<String>(Arrays.asList("a", "b", "c")));
initMap.put(6, new ArrayList<String>(Arrays.asList("b", "c")));
initMap.put(9, new ArrayList<String>(Arrays.asList("a", "c")));
initMap.put(12, new ArrayList<String>(Arrays.asList("b", "c")));
Map<String, Set<Integer>> finalMap = new HashMap<>();
initMap.forEach((key, values) -> {
values.forEach(val -> {
Object o = finalMap.containsKey(val) ?
finalMap.get(val).add(key) :
finalMap.put(val, new HashSet<Integer>(Arrays.asList(key)));
});
});
Comments
Here's an example, along with tests:
import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.stream.Collectors;
import org.junit.Assert;
import org.junit.Test;
public class SoTests {
@Test
public void so01() {
HashMap<Integer, List<String>> input = new HashMap<>(4);
input.put(3, Arrays.asList("a", "b", "c"));
input.put(6, Arrays.asList("b", "c"));
input.put(9, Arrays.asList("a", "c"));
input.put(12, Arrays.asList("b", "c"));
HashMap<String, HashSet<Integer>> output = process(input);
HashMap<String, HashSet<Integer>> expectedOutput = new HashMap<>(3);
expectedOutput.put("a", new HashSet<>(Arrays.asList(3, 9)));
expectedOutput.put("b", new HashSet<>(Arrays.asList(3, 6, 12)));
expectedOutput.put("c", new HashSet<>(Arrays.asList(3, 6, 9, 12)));
Assert.assertEquals(expectedOutput, output);
}
private HashMap<String, HashSet<Integer>> process(HashMap<Integer, List<String>> input) {
return input.entrySet().stream() //
.flatMap(entry -> entry.getValue().stream().map(s -> new IntegerAndString(entry.getKey(), s))) //
.collect(Collectors.groupingBy(IntegerAndString::getString, HashMap::new, //
Collectors.mapping(IntegerAndString::getInteger, Collectors.toCollection(HashSet::new))));
}
private static class IntegerAndString {
private final Integer integer;
private final String string;
IntegerAndString(Integer integer, String string) {
this.integer = integer;
this.string = string;
}
Integer getInteger() {
return integer;
}
String getString() {
return string;
}
}
}
The actual logic is in the process method. The ugly IntegerAndString class is due to a lack of tuple types in Java. You can use some library like javatuples instead.
1 Comment
saurabh agarwal
thanks for the code snippet. Can you add some explanation to it. ? (The streaming part)
Just looking at Duplicate link.
This is the final solution I have
private HashMap<String, HashSet<Integer>> process(HashMap<Integer, List<String>> input) {
return input.entrySet().stream()
.flatMap(entry -> entry.getValue().stream().map(s -> new SimpleEntry<>(entry.getKey(), s)))
.collect(Collectors.groupingBy(SimpleEntry::getValue, HashMap::new,
Collectors.mapping(SimpleEntry::getKey, Collectors.toCollection(HashSet::new))));
}
1 Comment
Anonymous
This is a fine tweak of the answer from the linked question to fit your particular requirements (the requirement to have sets as values in the result map in particular).
Map<Integer, List<String>> intHM = new HashMap<Integer, List<String>>();
intHM.put(3, new ArrayList<String>(Arrays.asList("a","b","c")));
intHM.put(6, new ArrayList<String>(Arrays.asList("b","c")));
intHM.put(9, new ArrayList<String>(Arrays.asList("a","c")));
intHM.put(12, new ArrayList<String>(Arrays.asList("b","c")));
Map<String,List<Integer>> StringHM = new HashMap<String, List<Integer>>();
Iterator<Entry<Integer,List<String>>> iterator = intHM.entrySet().iterator();
while (iterator.hasNext()) {
Map.Entry<Integer,List<String>> entry = (Map.Entry<Integer,List<String>>) iterator.next();
for(String str: entry.getValue())
{
if (StringHM.containsKey(str))
StringHM.get(str).add(entry.getKey());
else
StringHM.put(str, new ArrayList<Integer>(Arrays.asList(entry.getKey())));
}
}
Iterator<Entry<String,List<Integer>>> StringIterator = StringHM.entrySet().iterator();
while (StringIterator.hasNext()) {
Map.Entry<String,List<Integer>> entry = (Map.Entry<String,List<Integer>>) StringIterator.next();
System.out.print(entry.getKey()+" ");
for(Integer i: entry.getValue())
System.out.print(i+" ");
System.out.println();
}
