Creating Hashmap using existing hashmap

Question

I created a hashmap as shown below:

Map<String, String> streetno = new HashMap<String, String>();

streetno.put("3", "Sachin");
streetno.put("2", "Dravid");
streetno.put("1", "Sehwag");
streetno.put("5", "Laxman");
streetno.put("4", "Kohli");

Now I want to create a new hashmap where key of the above hashmap becomes value and value becomes key as shown below:

Map<String, String> streetname = new HashMap<String, String>();

streetname.put("Sachin", "3");
streetname.put("Dravid", "2");
streetname.put("Sehwag", "1");
streetname.put("Laxman", "5");
streetname.put("Kohli", "4");

I don't know how to do that.. Can anyone help me out with this..

Answer 1

Map<String, String> streetname = new HashMap<String, String>();

for (Entry<String,String> e : streetno.entrySet()) {
  streetname.put(e.getValue(), e.getKey());
}

Here, the for loop iterates over all entries (i.e. key/value pairs) in the original map, and inserts them into the second map with the key and value swapped over.

It is probably a good idea to check that put() returns null. If you get a non-null value, this means that the values in streetno are not unique. Since this is homework, I leave it to you to figure out the consequences, and how best to handle this.

Answer 2

Perfect you are almost there. Now you need to iterate the first hash map keys and simulate what you have done in those 5 lines:

streetname.put("Sachin", "3");
streetname.put("Dravid", "2");
streetname.put("Sehwag", "1");
streetname.put("Laxman", "5");
streetname.put("Kohli", "4");

Tip: iteration over map might be a bit tricky for you, but usually it is done like that:

for (String key : streetno.keySet()) {
...
}

Good luck with your homework!

Answer 3

Java 8:

Map<String, String> streetname = 
    streetno.entrySet()
            .stream()
            .collect(Collectors.toMap(Map.Entry::getValue, Map.Entry::getKey));

Note:

If you are tempted to use parellelstream() instead of stream() think twice about it. This would only be appropriate if your Map is extremely large.