How to make two Java maps with same keys, removing entries from the first for keys not present in the second

Question

I have two TreeMap like,

fisrtMap{
"2022": "11",
"2023": "22"
}

secondMap{
"2022": "33"
}

I want to make the firstMap like,

fisrtMap{
"2022": "11"
}

I mean, I need to remove uncommon keys from maps.

Answer 1

Use a special Set of keys from both Map objects

The solution applies to any modifiable Map class, not just the TreeMap class you mention.

As commented by Pshemo, your can do this by way of 👉 a set of keys of each map.

First set up some sample data. We make a modifiable map x by using Map.of literals syntax for convenience.

Map < String, String > x =
        new HashMap <>(
                Map.of(
                        "2022" , "11" ,
                        "2023" , "22"
                )
        );

Map < String, String > y =
        Map.of(
                "2022" , "33"
        );

Get a view on the keys of map x as a set. Then remove any entries in that map where the key is not present in a second set, a set of keys from the second map.

x.keySet().retainAll( y.keySet() );

That call to Map::keySet returns a Set view of the keys contained in the map. That set is special, in that it is connected to the map from which it came. To quote the Javadoc:

The set supports element removal, which removes the corresponding mapping from the map

Dump to console the before and after state of our map x.

System.out.println( "x = " + x );
x.keySet().retainAll( y.keySet() );
System.out.println( "x = " + x );

See this code run live at Ideone.com.

x = {2023=22, 2022=11}
x = {2022=11}

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By the way, if that first part of your data is meant to be a calendar year, you could make use of the java.time.Year class.